Fungicide Use in Field Crops Web Book
CPN 4008. Published April 21, 2021. DOI: Doi.org/10.31274/cpn-20210329-0
Daren Mueller, Iowa State University; Kiersten Wise, University of Kentucky; Carl Bradley, University of Kentucky; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin-Madison; Erin Hodgson, Iowa State University; Albert Tenuta, Ontario Ministry of Agriculture, Food, and Rural Affairs; Andrew Friskop, North Dakota State University; Shawn Conley, University of Wisconsin-Madison; Travis Faske, University of Arkansas; Edward Sikora, Auburn University; Loren Giesler, University of Nebraska-Lincoln; and Martin Chilvers, Michigan State University.
In 1755, M. M. Tillet discovered the application of lime to plant seed could be used to manage seedborne fungi such as Tilletia tritici, the cause of wheat bunt. Although use of fungicides occurred in field crops as early as the 1600s, Tillet’s discovery started what could be called the fungicide revolution. Scientific knowledge and fungicide use has increased dramatically since that time, and fungicides, along with other pesticides, have become an extremely important crop protection tool. New discoveries continue to refine and improve fungicide products and disease management. However, the prolific and diverse nature of plant pathogens, coupled with the misuse of fungicide products, has led to fungicide-resistant pathogens. This “resistance treadmill,” where new fungicide products eventually decrease in usefulness due to pathogen resistance, is a driver not only for the discovery of new fungicide products, but also educational efforts aimed at preserving the fungicide tools currently available.
The primary goal of this Crop Protection Network publication is to help protect field crops from plant disease through increasing knowledge of fungicides and promoting best practices. Farmers, agronomists, and others must deal with these crop protection issues on a daily basis. Supporting goals include mitigating fungicide resistance, increasing clarity of communication when discussing fungicide-related topics, and integrating fungicide use within the larger, whole-farm management plan.
With these goals in mind, Fungicide Use in Field Crops addresses current fungicide issues by considering the most recent advances in fungicide science while building upon past discovery. The publication will help equip fungicide applicators, educators, agribusiness employees, students, and others with a baseline of information needed to effectively manage diseases of field crops using fungicides. Importantly, it will also help determine when it is best not to apply a fungicide. The authors wish to highlight the importance of fungicides as tools for yield protection, while at the same time considering economics, human health, environmental responsibility, and preservation of fungicide effectiveness.
Use the Table of Contents on this page to navigate between chapters and resources available as part of this text. If tables are not fully visible, adjust browser window size or view settings.
Earn four Certified Crop Advisor CEUs after reading this web book. Successfully complete a quiz for each chapter to earn up to four CEUs total. See the Crop Protection Network CCA CEU page or access quizzes directly for Chapter 1, Chapter 2, Chapter 3, and Chapter 4.
The Crop Protection Network is a multi-state and international partnership of university and provincial Extension specialists, and public and private professionals that provides unbiased, research-based information. The editors and authors of this publication are members of the Crop Protection Network, and strive to provide the best regional, national, and international resources to crop protection practitioners.
This educational resource was made possible by contributions from Iowa State University Integrated Pest Management; the Grain Farmers of Ontario; and the United States Department of Agriculture - National Institute of Food and Agriculture (USDA-NIFA).
© 2021 Crop Protection Network unless otherwise noted. All rights reserved.
What is a fungicide? A fungicide is a chemical used for killing or limiting the development of fungi or organisms similar to fungi (i.e., oomycetes). Fungicides are used to preserve yield and quality in field crops by mitigating the potential impact of disease on plants. Fungicides in field crops are applied in a variety of ways that include soil applications, seed treatments, or foliar sprays.
Fungicides are an important group of tools used in modern agricultural production. Proper use of these chemicals decreases risk of fungal diseases that can cause economic and yield variability in crop production. The organisms that cause fungal diseases threaten crop production every year, and can be particularly destructive during some seasons. Fungal disease development is strongly impacted by the environment and it can be difficult to predict which disease will be problematic in any given year. For this reason, having fungicide management tools ready to use is of great importance. Fungal organisms are continually changing through genetic mutation, and spread from region to region by the activities of humans or fluctuations in the environment. Sometimes, this can result in situations where fungicides may be the only way to preserve a crop. An example of this occurred when the pathogen that causes soybean rust began to move into U.S. soybean producing regions (Gallery 1.1). Soybeans grown in these areas did not have genetic resistance to this pathogen, thus fungicide application was the only option for control of this disease. Emergency labeling of certain fungicides to manage soybean rust occurred, but fortunately the threat to soybeans, at least in the primary soybean growing regions of the U.S., did not materialize. Without fungicides, an outbreak of this or another disease may have disastrous consequences for crops and those who manage them.
Soybean rust on soybean leaf. Image: Daren Mueller
Soybean rust visible in the soybean canopy. Image: Albert Tenuta
Aerial view of field with soybean rust. Image: Tom Allen
Soybean rust pustules viewed through a hand lens. Image: Albert Tenuta
Fungicide treated soybean rust research plots next to untreated soybean. Image: Tristan Mueller
Gallery 1.1. Soybean rust in U.S. soybean producing regions.
Fungicide use is not a cure-all, and overuse will often cause them to become ineffective. Fungicides are only one group of tools for effective disease management and are best used as part of an overall strategy that includes field scouting, variety selection, crop rotation, and many other strategies. There is no doubt that fungicides have become an important part of modern farming to preserve yield potential, protect seed and grain quality, and reduce toxins in food and feed.
The foliar application of fungicides protects aboveground plant parts from infection from fungal pathogens. In general, foliar fungicides are not capable of eliminating a pathogen once it invades plant tissue, and are most effective when applied prior to infection. Multiple factors should be considered before a fungicide is applied, such as disease presence or risk, field history, past and predicted environmental conditions, susceptibility of the crop to a particular disease, crop phenological stage, cost to apply a fungicide, yield potential, and crop value. Foliar fungicides are common in the production of several field crops including corn, dry bean, peanut, potato, rice, soybean, sugarbeet, and wheat.
Seed and soil-applied fungicides protect from fungi and fungal-like organisms that cause damping-off, seedling blight, and root rots. Applied to seed or soil at or before planting, these fungicides can reduce disease risk when favorable environmental conditions exist for disease development during and after germination, or if poor quality seed are planted. Seed and soil-applied fungicides only protect against disease, they will not improve or increase germination of poor-quality seed.
Over the last two decades, fungicide use on several important field crops in the U.S. has increased (Table 1.1). Multiple reasons exist for the increased application of fungicides in field crops including market prices, the threat of new and emerging diseases, robust fungicide marketing, and product availability. Fungicide use is expected to continue to increase on these crops in the coming years.
Table 1.1. Fungicide treatment (type not specified) by percent total acres planted as reported in chemical use surveys conducted by the USDA for selected states (USDA-NASS).
Crop | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2009 | 2010 | 2012 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 |
Corn | NR¹ | NR | NR | . | NR | . | . | 8 | . | 12 | . | 12 | . | 17 | . | . |
Potato | 85 | . | 91 | . | 90 | . | . | 96 | . | 96 | . | 97 | . | . | . | . |
Soybean | NR | <1 | . | 1 | 2 | 4 | . | . | 11 | . | 11 | . | 14 | 15 | . | 22 |
Wheat- Winter | .² | NR | . | 2 | . | 2 | 7 | . | 19 | . | 19 | . | 25 | . | 22 | . |
Wheat- Spring³ | . | NR | . | 20 | . | 15 | 36 | . | 49 | . | 51 | . | 45 | . | 46 | . |
Wheat- Durum | . | NR | . | NR | . | 5 | 23 | . | 39 | . | 55 | . | 36 | . | 35 | . |
¹Not reported, although chemical use data was collected that year for that crop. ²Survey not conducted or completed at time of publication. ³Excludes durum wheat.
Using fungicides in field crops comes with a price. Application equipment, fungicide product, and time spent to mix and apply chemicals are all costs that must be considered, along with expected crop yield and market value. Farmers should carefully weigh the costs and benefits of a fungicide application. In general, a fungicide application when disease risk is high is much more likely to be profitable than an application that occurs when disease risk is low.
The use of fungicides may also have environmental concerns. Certain fungicides are toxic to several aquatic organisms. For example, azoxystrobin (FRAC code 11; QoI), a commonly used fungicide registered for use on many field crops, inhibits mitochondrial respiration, causes increased ROS (reactive oxygen species) production, and can affect aquatic plants and animals. Fungicides may also cause unintended effects on other organisms in a field crop production setting, like non-target fungi developing resistance or reducing populations of endophytic fungi that keep insect populations in check. The potential side effects of fungicide applications on the environment should be considered, and caution used to prevent disruption of natural systems due to misuse or overuse of fungicides.
Fungicide use is one of several ways in which field crop diseases can be managed. Foliar fungicides can provide a rapid response when the threat of a foliar disease develops during the growing season, and seed and soil-applied fungicides protect seed and emerging seedlings early in crop development. However, there are other important aspects of disease management that take place before seed are planted and after crops are harvested. These include selecting disease-resistant varieties, rotating crops, managing plant residue, proper fertility, and planting pathogen-free seed. Fungicides are often used in conjunction with other management strategies, all of which are tools used as part of an integrated pest management (IPM) plan. IPM seeks to achieve economical disease control while minimizing environmental hazards. IPM plans are informed through field scouting, weather forecasts, and understanding diseases and other agronomic information, and can use decision-making tools such as forecasting systems. Information from these sources is augmented through careful recordkeeping. A carefully thought out IPM plan can include multiple tools and considers how various aspects of disease and other crop management factors are interrelated.
The disease triangle is a helpful concept in understanding factors necessary for disease development, and how these factors can be modified in an IPM plan to manage disease. The three sides of the disease triangle represent the factors that are absolutely necessary for disease development: 1) presence of a disease-causing organism, 2) a susceptible host, and 3) a disease-conducive environment (Figure 1.1). If one of these three factors is absent, a disease will not develop. Plant host, environmental conditions, and/or management factors including the use of fungicides can affect the interactions of these factors and influence disease development. A fourth dimension that can be added to the disease triangle is time, as disease development does not occur instantaneously, but by a combination of the necessary factors over time.
Figure 1.1. The disease triangle is composed of three factors that must exist simultaneously for development of disease to occur. These factors are the susceptible host, the disease-causing pathogen, and a disease-conducive environment.
Iowa State University Integrated Pest Management
Using disease-resistant crops is an extremely useful tool to reduce yield and quality losses caused by disease. Even if the pathogen is present and the environment is conducive to disease development, the plant’s genetics will reduce or potentially eliminate the impact of disease on the crop. Plant breeders aim to increase resistance in commercial varieties to common diseases, but complete resistance to most diseases rarely, if ever, exists. Knowledge of variety susceptibility to a disease can help determine if a fungicide should be used, and to select the most effective product if needed. Varieties susceptible to disease can benefit from a fungicide application. However, fungicide use on a disease susceptible variety may only decrease the damage caused by the disease. Pairing disease resistant varieties and appropriately timed fungicide applications when needed may provide greater levels of disease management, but may not always result in a positive return on fungicide investment (Table 1.2 and Figure 1.2).
Table 1.2. Pairing disease-resistant varieties with fungicide application can increase the likelihood of fungicide return on investment and effective disease management. These data illustrate the impact of this practice for white mold of soybean. Data courtesy D. Smith, University of Wisconsin-Madison.
White mold DIX¹ % | Yield bu/a | ||||
Variety² | Population seeds/a | Non-treated | Endura at R2 | Non-treated | Endura at R2 |
52-82B | 120,000 | 5.2 | 5.3 | 40 | 44.9 |
52-82B | 160,000 | 8.9 | 4.8 | 48.6 | 50.8 |
Dwight | 120,000 | 39.9 | 18.9 | 39.1 | 40.1 |
Dwight | 160,000 | 46 | 21.9 | 34.3 | 44.4 |
¹ DIX is a measure of disease severity. ²52-82B is moderately resistant to white mold; Dwight is susceptible to white mold.
Figure 1.2. Pairing disease-resistant varieties with fungicide application can increase the likelihood of fungicide return on investment and effective disease management. These data illustrate the impact of this practice for white mold of soybean (sometimes referred to as Sclerotinia stem rot). DIX is a measure of disease severity. Figure courtesy D. Smith, University of Wisconsin-Madison.
Fungal and fungal-like pathogens are more likely to thrive when optimal environmental conditions for development exist. Understanding the conditions that are beneficial for pathogen development will help determine the likelihood of disease occurrence and disease risk. Development of several foliar pathogens of field crops are favored when prolonged periods of high humidity and rain occur and there are several soil pathogens that benefit from excess soil moisture. Dry conditions can also lead to the development of specific diseases (e.g., charcoal rot of multiple crops). Environmental conditions can also vary within a field. Low lying areas will likely have increased dew periods and soil moisture levels that often favor the development of specific diseases. Additionally, areas of a field located near trees may have reduced air flow, increasing disease risk.
Both agronomic and environmental factors can have a significant influence on disease development. Many pathogens survive unfavorable periods by persisting in crop residue. Thus, residue management and crop rotation to a non-host are components of IPM. Modern minimum and no-till practices greatly aid in soil and moisture conservation, but also increase crop residue and the potential for some pathogens to overwinter, which may increase disease pressure. For pathogens that are not able to overwinter in a growing region (such as some rust pathogens), tillage and crop rotation practices will have little to no effect on inoculum levels in a field. Environment is a strong driver of disease development, and agronomic factors such as row spacing, plant population and planting date can influence the crop microclimate and conditions for disease development. Altering these practices can increase or decrease the risk of disease. A dense canopy will likely hold moisture, favoring conditions for pathogen infection. Likewise, irrigated fields can be at greater risk for disease development due to prolonged periods of plant surface wetness.
Planting pathogen free seed stock is useful to prevent introduction of disease-causing pathogens into a field or geographic area. Pathogen-free seed stock can also improve plant stand since pathogen-infected seed may be less likely to produce healthy plants.
There are several important concepts to consider with fungicides including nomenclature, classification criteria, and fungicide terminology. Understanding these concepts helps provide clarity among educators, applicators, and other agricultural professionals. Also, fungicide labels use specific terminology to convey information to users, highlighting the importance of understanding the meanings of these words.
Fungicides are required by law to have a product label, which becomes a legal document. It is a violation of Federal law to use a product in a manner inconsistent with its labeling. A fungicide label provides details about the fungicide and how to properly apply the product. Information as it relates to the label includes the crops a fungicide may be used on, diseases managed, application rate, applicator safety, and numerous other important statements regarding the product or products contained within the fungicide. Fungicide labels are organized in a similar fashion, making it easy to quickly identify the most important information. Although there is a lot of information present on a label, there are a few especially important parts to carefully consider (Figure 1.3).
Figure 1.3. All fungicide labels contain the same key information. The specific information is important for proper application, user safety, resistance management, and many other things. Vital information includes: 1) chemical name, 2) common name, 3) trade name, and 4) Fungicide Resistance Action Committee (FRAC) code.
Iowa State University Integrated Pest Management
Fungicide Nomenclature
Three terms are used to differentiate fungicides: the chemical name, common name, and trade name(s). These names help farmers, scientists, and applicators to reference fungicide products for ease of identification and classification, depending on the specific audience. These names are found on the fungicide product label (Figure 1.3).
Chemical name: designates the fungicide active ingredient (e.g., carbamic acid, [2-[[[1-(4-chlorophenyl)-1H-pyrazol-3-yl]oxy]methyl]phenyl]methoxy-, methyl ester).
Common name: less technical reference for fungicide active ingredient (e.g., pyraclostrobin).
Trade name(s): registered reference to a commercially available product containing the fungicide active ingredient (e.g., Headline®). Several different trade names may exist for a single active ingredient.
Characterizing Fungicides
Different criteria are used to characterize fungicides including mobility within the plant (phytomobility), mode of action, chemical class, FRAC (Fungicide Resistance Action Committee) code, metabolic activity, role in plant protection, and selectivity. Other methods of fungicide characterization also exist, such as bioavailability.
Phytomobility
After application, fungicides that come into contact with plant tissue either remain in place or have the ability to move within plant tissue or between plant parts. Potential movement of a fungicide within plant tissue is termed “phytomobility,” and should not be confused with mode of action (see below). Fungicides can be broadly classified into two groups based on phytomobility: contacts or penetrants/systemics.
Contact, or non-systemic fungicides do not move into plant tissues. Products consisting of or containing contact fungicides stay on the surface of the plant that was sprayed and are not absorbed. Contact fungicides may re-distribute over the target area (i.e., leaf) due to wetting events such as irrigation, rain, or dew. Contact fungicides are more prone to being washed off the plant by rain compared to penetrant fungicides. Additional fungicide applications may be required if protection of new plant growth is needed later in the season.
Penetrant/Systemic fungicides are absorbed into plant tissues. However, these fungicides are not all equally mobile within the plant. Systemic fungicides can be further classified into three subgroups, based on their mobility within the plant (phytomobility): acropetally mobile fungicides, ambimobile fungicides, and locally systemic fungicides.
Acropetally mobile fungicides move along a water potential gradient and are distributed between cells (Figure 1.4, top). These types of fungicides are mobile in the xylem (water-conducting vessels in the plant) and are moved upwards toward leaf tips.
Ambimobile fungicides have the ability to move upward AND downward in plants (Figure 1.4, bottom). Ambimobile fungicides can move through live protoplasts and cells along a sugar gradient. These gradients run from high concentration in fully expanded leaves to low concentration in roots and new leaves. Ambimobile fungicides, such as phosphorous acid fungicides, are rare in field crops.
Locally systemic fungicides have more limited mobility compared to acropetal or systemic penetrants. They are typically attracted closely to waxy compounds, thus diffusing mostly in the waxy cuticle of the plant surface. Some fungicides can move from the cuticle of the upper leaf surface through the leaf to the cuticle of the lower leaf surface, or vice versa (translaminar movement; Figure 1.5). However, most of the active ingredient stays near where it is applied.
Figure 1.4. Acropetally mobile fungicides (top) move upwards in the plant while ambimobile fungicides (bottom) can move up and down within the plant.
Iowa State University Integrated Pest Management
Figure 1.5. Translaminar movement of fungicides occurs when the chemical moves through the leaf from the side of application to the opposite side.
Iowa State University Integrated Pest Management
Mode of Action
The mode of action is defined as the target and process the fungicide possesses to inhibit or reduce growth of fungal or fungal-like pathogens. Mode of action is different than phytomobility. Fungicide modes of action include interfering with fungal respiration and energy production, impairing cell membranes, and inactivating important proteins and enzymes. Details of specific modes of action of various fungicides are included in Section 2.1. Mode of action also helps define several other characteristics of a fungicide including chemical class and FRAC group, metabolic activity, role in plant protection, and selectivity. These characteristics are further defined below.
Chemical Class and FRAC Code
Fungicides with the same biochemical mode of action belong to specific chemical classes. Chemical structure may differ among fungicides within the same group or class. In an effort to understand the important issue of chemical classification and the development of fungicide resistance based on class, the Fungicide Resistance Action Committee (FRAC) was developed. The committee created the FRAC code to help fungicide applicators easily distinguish products with different modes of action groups. FRAC codes are composed of letters and/or numbers such as 11 (pyraclostrobin), M3 (mancozeb), or M5 (chlorothalonil). See Section 1.4 for additional details on FRAC codes.
Target Site (Biochemical Mode of Action)
Fungal and fungal-like pathogens, like other living organisms, have metabolic pathways essential for life. Metabolic pathways consist of multiple chemical reactions that can be disrupted by fungicides to cause harm to the fungal pathogen.
A single-site fungicide disrupts one function or point of a single metabolic pathway. The fungicides contained within this particular metabolic group may also be active against a single protein or enzyme critical to development. Fungicides with single-site metabolic activity are at increased risk of fungicide resistance development in the target fungus (Figure 1.6, left).
A multi-site fungicide disrupts more than one fungal metabolic site. These types of fungicides typically make it harder for a fungus to develop resistance, as it is more difficult to overcome disruption to multiple sites versus one target (Figure 1.6, right)
Figure 1.6. Single-site fungicides disrupt one function within the plant (left droplet) while multi-site fungicides disrupt more than one plant function (right droplet).
Iowa State University Integrated Pest Management
Role in Plant Protection
Fungicides protect plants by preventing infection, inhibiting early pathogen development, and preventing spore production (antisporulant). Some fungicide products exhibit more than one of these types of protection (mixed chemical class products).
Fungicides with preventive activity act as a barrier to fungal infection, and must be present before pathogen arrival or initiation of disease development. Preventive activity is also referred to as protective activity.
Some fungicides have the ability to inhibit early fungal development. Depending on the fungicide, this type of activity is effective from 24-72 hours post-infection. This type of activity is also called curative or kickback activity, although fungicides do not “cure” the plant once infection has occurred. Fungicides that have this ability usually also have preventive activity, and effectiveness is maximized if applied prior to infection.
Antisporulant activity fungicides prevent spore germination. Spores often contain essential lipids and carbohydrates in stored forms. Thus, fungicides that inhibit the formation of these compounds in fungi effectively inhibit spore germination.
Selectivity
Selectivity refers to the ability of a fungicide to be active on a specific targeted pathogen without having a detrimental effect on the host, non-target animals, or the environment. Mode of action and fate determine the selectivity of a fungicide. Some fungicides are active on a specific biochemical pathway that only exists in some organisms and not others. The level of selectivity often leads to influencing the spectrum of activity of a fungicide. That is to say some fungicides are effective on a wide range of pathogens, while others have an extremely narrow spectrum of activity, only affecting a limited number of specific organisms within a particular fungal (or oomycete) family.
Bioavailability
Fungicides can be categorized in other ways, including their bioavailability, which affects how they are absorbed into and translocated within the plant and their half-life.
Uptake and translocation within the plant are influenced by lipophilicity (logP) of the active ingredient, which is the ability of an organic compound to dissolve in fats, oils, lipids, and non-polar solvents. The greater the logP, the less movement within the leaf (Zhange et al., 2018). Translocation within the plant is a passive process and is affected by the active ingredient’s polarity. The logP and polarity of active ingredients can be used to calculate the “translocation stream concentration factor”, which is an indication as to how readily fungicides can move within the plant (Briggs et al., 1982). The half-life of fungicides and logP values, as well as many other properties, can be found on the University of Hertfordshire Pesticide Properties DataBase (PPDB). The properties for fungicide active ingredients can vary widely (Table 1.3), which can affect how well products work in different situations.
Table 1.3. Translocation stream concentration factor, typical half-life, and lipophilicity (logP) of some common fungicides used on field crops.
Fungicide | TSCF¹ | Typical half life (days)² | LogP² |
|---|---|---|---|
Mefentrifluconazole | 0.267 | 268 | 3.40 |
Tetraconazole | 0.214 | 61 | 3.56 |
Difenconazole | 0.051 | 130 | 4.36 |
Tebuconazole | 0.173 | 63 | 3.70 |
Cyproconazole | 0.388 | 142 | 3.09 |
Metconazole | 0.135 | 142 | 3.85 |
Prothioconazole | 0.142 | 14 | 3.82 |
Propiconazole | 0.168 | 72 | 3.72 |
Flutriafol | 0.701 | 1358 | 2.3 |
Fluxapyroxad | 0.371 | 183 | 3.13 |
Bixafen | 0.304 | 500 | 3.3 |
Fluopyram | 0.304 | 309 | 3.3 |
Boscalid | 0.443 | 200 | 2.96 |
Penthiopyrad | 0.106 | 32 | 3.99 |
Benzovindiflupyr | 0.026 | 121 | 4.62 |
Pydiflumetofen | 0.12 | 2416 | 3.8 |
Pyraclastrobin | 0.099 | 32 | 3.99 |
Trifloxystrobin | 0.038 | <1 | 4.5 |
Picostrobin | 0.202 | 24 | 3.6 |
Fluoxastrobin | 0.486 | 59 | 2.86 |
Azoxystrobin | 0.634 | 78 | 2.5 |
Kresoxim-methyl | 0.267 | 16 | 3.4 |
¹Translocation stream concentration factors were calculated using the Briggs, Bromilow, and Evans equation (Briggs et al., 1982). ²Values for typical half life and logP can be found on the University of Hertfordshire Pesticide Properties Database (PPDB).
The Fungicide Resistance Action Committee (FRAC) is an international consortium of fungicide manufacturers that provides information regarding fungicide resistance mitigation. The organization has created a code useful for easy classification of fungicides based on their cross-resistance behavior. Every fungicide label displays a FRAC code which consists of numbers and letters (Table 1.4).
Table 1.4. Examples of fungicide classification and resistance risk for fungicides available for use on field crops.
FRAC code | Mode of action | Target site of action | Group name | Active ingredient common name | Risk of resistance |
|---|---|---|---|---|---|
1 | Cytoskeleton and motor protein | Mitosis and cell division (ß-tubulin assembly) | Methyl Benzimidazole Carbamates (MBC) | Thiabendazole | High |
Thiophanate-methyl | |||||
2 | Signal transduction | MAP/Histidine-kinase in osmotic signal transduction in os-1, Daf1 genes | Dicarboximides | Iprodione | Medium to high |
Vinclozolin | |||||
3 | Membrane sterol biosynthesis | C14 - demethylation in sterol biosynthesis | Demethylation Inhibitors (DMI)¹ | Cyproconazole | Medium |
Difenoconazole | |||||
Fenbuconazole | |||||
Flutriafol | |||||
Imazalil | |||||
Metconazole | |||||
Myclobutanil | |||||
Propiconazole | |||||
Prothioconazole | |||||
Tebuconazole | |||||
Tetraconazole | |||||
Triticonazole | |||||
4 | Nucleic acid metabolism | RNA polymerase 1 | Phenylamides | Mefenoxam | High |
Metalaxyl | |||||
7 | Respiration inhibitor | Complex II: succinate-dehydrogenase | Succinate Dehydrogenase Inhibitors (SDHI)² | Benzovindiflupyr | Medium to high |
Boscalid | |||||
Carboxin | |||||
Fluopyram | |||||
Flutolanil | |||||
Fluxapyroxad | |||||
Penthiopyrad | |||||
Pydiflumetofen | |||||
Sedaxane | |||||
9 | Amino acid and protein synthesis | Methionine | Anilino-pyrimidines (AP) | Cyprodinil | Medium |
Biosynthesis (proposed) | Pyrimethanil | ||||
11 | Respiration inhibitor | Complex III: cytochrome bc1, Qo site | Quinone Outside Inhibitors (QoI)³ | Azoxystrobin | High |
Famoxadone | |||||
Fenamidone | |||||
Fluoxastrobin | |||||
Picoxystrobin | |||||
Pyraclostrobin | |||||
Trifloxystrobin | |||||
12 | Signal transduction | MAP/Histidine-kinase in osmotic signal transduction in os-2, HOG1 genes | Phenylpyrroles (PP) | Fludioxonil | Low to medium |
14 | Lipid synthesis, transport, or membrane function | Cell peroxidation (proposed) | Aromatic Hydrocarbons | Chloroneb | Low to medium |
Quintozene (PCNB) | |||||
21 | Respiration inhibitor | Complex III: cytochrome bc1, Qi site | Quinone Inside Inhibitors (QiI) | Cyazofamid | Medium to high |
22 | Cytoskeleton and motor protein | Mitosis (Beta-tubulin assembly) | Thiazole carboxamide | Ethaboxam | Low to medium |
27 | Unknown | Unknown | Cyanoacetamide-oximes | Cymoxanil | Low to medium |
28 | Lipid synthesis, transport or membrane function | Cell membrane permeability, fatty acids (proposed) | Carbamates | Propamocarb | Low to medium |
29 | Respiration inhibitor | Uncoupler of oxidative phosphorylation | Oxidative Phosphorylation Uncouplers | Fluazinam | Low |
30 | Respiration inhibitor | Inhibits oxidative phosphorylation, ATP synthase | Organo Tin Compounds | Fentin hydroxide | Low to medium |
(Triphenyltin hydroxide) | |||||
32 | Nucleic acid metabolism | DNA/RNA synthesis (proposed) | Heteroaromatics | Hymexazol | Not known |
40 | Cell wall biosynthesis | Cellulose synthase | Carboxylic Acid Amides (CAA) | Dimethomorph | Low to medium |
Mandipropamid | |||||
44 | Lipid synthesis, transport, or membrane function | Microbial disrupters of pathogen cell membranes | Microbial | Bacillus amyloliquefaciens (several strains) | Not known |
49 | Lipid synthesis, transport, or membrane function | Lipid homeostasis and transfer/storage | Oxysterol Binding Protein Homologue Inhibition | Oxathiapiprolin | Medium to high |
M 01 | Multi-site | Multi-site, contact | Inorganic | Copper | Low |
M 03 | Multi-site | Multi-site, contact | Dithiocarbamates and Relatives | Mancozeb | Low |
Maneb | |||||
Thiram | |||||
M 04 | Multi-site | Multi-site, contact | Phthalimides | Captan | Low |
M 05 | Multi-site | Multi-site, contact | Chloronitriles | Chlorothalonil | Low |
P 07 (33) | Host plant defense induction | Phosphonates | Phosphonates | Phosphorous acid and salts | Low |
¹Some fungicides in the DMI group are commonly referred to as "triazoles." ²Some of the succinate dehydrogenase inhibitor fungicides may be referred to as “carboxamides.” ³Some fungicides in the QoI group may be referred to as "strobilurins."
Earn a Certified Crop Advisor CEU after reading this chapter. Successfully complete the Chapter 1 quiz for one CEU. Each chapter has a corresponding quiz at Crop Protection Network CCA CEU page.
Fungicides can be characterized with different criteria including Fungicide Resistance Action Committee (FRAC) code and chemical group, mode of action, specific target site of action (if known), mobility within the plant (phytomobility), role in plant protection, biological spectrum of activity, and risk for resistance. Fungicides with the same biochemical mode of action belong to the same specific chemical class. Specific details of the major classes of fungicides used in field crops are described for FRAC codes 1-21 and FRAC codes 22-M in the following two sections of this web book.
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For FRAC codes 22-M, see the next section.
FRAC Code 1 - Methyl Benzimidazole Carbamate (MBC) Fungicides
Group Name and Chemical Group: The methyl benzimidazole carbamate (MBC) fungicide group contains the benzimidazole and thiophanate chemical groups, such as thiophanate-methyl.
Mode of Action and Target Site: Cytoskeleton and motor protein: Inhibits β-tubulin production, interfering with normal cell division in sensitive fungi.
Phytomobility: While these fungicides have systemic properties, they cannot move down in the plant, making complete plant coverage essential for control.
Role in Plant Protection: These fungicides have preventive and early-infection activity.
Spectrum: MBC fungicides are effective against a broad range of fungi that cause leaf spots, root and crown rots, stem rots, and powdery mildews, but are not effective on rust fungi.
Risk for Resistance: The modification of a single amino acid in a fungus can result in resistance. Resistance to these fungicides was first reported in 1970. Many important fungal plant pathogens have become resistant to these fungicides. The MBC fungicide risk of resistance is HIGH.
FRAC Code 2 - Dicarboximide Fungicides
Group Name and Chemical Group: The dicarboximide group contains only the dicarboximide chemical group, which includes, but is not limited to, iprodione and vinclozolin.
Mode of Action and Target Site: Signal transduction: Inhibits fungal growth by affecting osmotic regulation in fungal cells and disrupting membrane function.
Phytomobility: Fungicides in this group are locally systemic, accumulating in the waxy cuticle with translaminar movement to the other side of the leaf. These fungicides are not translocated in the water-conducting elements of the plant (xylem). Complete plant coverage is essential to maximize control.
Role in Plant Protection: Preventive and early-infection activity.
Spectrum: Dicarboximide fungicides are effective against a broad range of fungi.
Risk of Resistance: Fungicide resistance is common in many fungi for this group of fungicides. Single mutations can lead to fungicide resistance to this group, and cross-resistance is common. Dicarboximide risk of resistance is MEDIUM to HIGH.
FRAC Code 3 - Demethylation Inhibitor (DMI) Fungicides
Group Name and Chemical Group: The demethylation inhibitors (DMI) fungicide group includes important chemical groups such as the triazole and triazolinthione fungicides, but also includes the piperazines, pyridines, pyrimidines, and imidazoles. Examples of active ingredients used on field crops include prothioconazole and propiconazole.
Mode of Action and Target Site: Sterol biosynthesis in membranes: These fungicides work by inhibiting a specific enzyme that plays a role in sterol production in fungi. Sterols are necessary for the development of functional cell membranes in fungi. Application of DMIs results in abnormal fungal growth and death. However, triazoles have no effect on spore germination because spores contain enough sterol for the formation of germ tubes.
Phytomobility: DMI fungicides are acropetally mobile, meaning that they are taken up into the plant and can move short distances in the water-conducting elements (xylem) of plants.
Role in Plant Protection: DMI fungicides must be applied preventively or at early infection to be effective.
Spectrum: DMI fungicides are highly effective against powdery mildews, rusts, and many leaf spotting fungi.
Risk of Resistance: DMI fungicides have a very specific site of action, so the risk of resistance development is a concern. Reduced sensitivity to certain DMI fungicides has been reported in several U.S. states for Fusarium graminearum on wheat. Resistance management practices include avoiding repeated applications of DMI fungicides in the same season against high-risk pathogens such as those that cause powdery mildews. The DMI fungicide risk of resistance is MEDIUM.
FRAC Code 4 - Phenylamide Fungicides
Group Name and Chemical Group: The phenylamide group contains the chemical groups, acylalanines, butyrolactones, and oxazolidinones. Within the acylalanines chemical group are the widely used fungicides metalaxyl and mefenoxam.
Mode of Action and Target Site: Nucleic acids metabolism: Fungicides within this group interfere with critical enzymes for building proteins for cell structure and regulation. When these enzymes are inhibited, pathogen growth slows or stops. This group is commonly used against oomycete pathogens.
Phytomobility: Fungicides within this group are acropetally mobile and can be translocated in the xylem toward leaf tips.
Role in Plant Protection: Fungicides in this group are capable of preventing further development of existing infections (kickback) but work best as preventives.
Spectrum: In field crops the spectrum of activity is narrow, focused mostly on oomycete pathogens, such as Pythium and Phytophthora spp.
Risk of Resistance: Resistance and cross-resistance has been well documented in oomycete pathogens, but the mechanism leading to this resistance is unknown. The phenylamide fungicide risk of resistance is HIGH.
FRAC Code 7 - Succinate Dehydrogenase Inhibitors (SDHI) Fungicides
Group Name and Chemical Group: Succinate dehydrogenase inhibitors (SDHI) fungicides include several important chemical families such as the carboximides and benzamides. Within these broad chemical families are 11 different chemical groups, including but not limited to, the pyridine-carboximide, pyrazole-4-carboximides, pyridinyl-ethyl-benzamides, and phenyl-benzamides. Some common active ingredients used in field crops are boscalid, carboxin, flutolanil, fluopyram, and penthiopyrad.
Mode of Action and Target Site: Respiration: Fungicides in these chemical groups inhibit the respiration of target fungi, specifically complex II of fungal respiration.
Phytomobility: SDHI fungicides are acropetally mobile fungicides, meaning that they are taken up into the plant and can move short distances in the water-conducting elements (xylem) of plants.
Role in Plant Protection: SDHI fungicides are excellent when used preventively and can inhibit early infections.
Spectrum: Reasonably broad; although some target specific pathogens. For example, boscalid is primarily a foliar fungicide used against Botrytis, Sclerotinia, and Alternaria pathogens.
Risk of Resistance: Resistance has been documented for these fungicides. The SDHI fungicide risk of resistance is MEDIUM to HIGH.
FRAC Code 9 - Anilio-pyrimidine (AP) Fungicides
Group Name and Chemical Group: The anilio-pyrimidines (AP) chemical group contains fungicides such as cyprodinil and pyrimethanil.
Mode of Action and Target Site: Amino acids and protein synthesis: Fungicides in this group inhibit production of amino-acids in fungal pathogens and specifically inhibit fungal penetration and subsequent growth in the host plant.
Phytomobility: AP fungicides are acropetally mobile, capable of moving upward in the water-conducting elements (xylem) of the plant, toward leaf tips.
Role in Plant Protection: AP fungicides should be considered protectants and are most effective when applied before infection takes place.
Spectrum: Active against species of Botrytis and several other genera.
Risk of Resistance: Resistance to AP fungicides has been documented in species of Botrytis and Venturia. The AP fungicide risk of resistance is MEDIUM.
FRAC Code 11 - Quinone Outside Inhibitor (QoI) Fungicides
Group Name and Chemical Group: The Quinone outside inhibitor (QoI) group contains nine chemical groups including several significant ones, such as the methoxy-acrylates, methoxy-carbamates, and oximino-acetates. Examples of commonly used QoI fungicides include pyraclostrobin and azoxystrobin. Certain QoI fungicides are sometimes referred to as strobilurin fungicides, although this terminology is outdated and not encouraged.
Mode of Action and Target Site: Respiration: Fungicides in this group inhibit mitochondrial respiration, specifically complex III at the Qo site, effectively stopping energy production of the fungus, and resulting in death.
Phytomobility: QoI fungicides have varied phytomobility. Some are locally systemic, while others are acropetally mobile.
Role in Plant Protection: QoI fungicides are effective on spore germination and early growth. QoI fungicides are not effective against fungi that are growing inside the leaf tissue, so they must be applied preventively or at early infection to be effective. These fungicides have approximately 7-21 days of residual activity.
Spectrum: QoI fungicides are very effective against a broad spectrum of fungi.
Risk of Resistance: QoI fungicides have a very specific site of action, so the risk of resistance development is high. Currently there are more than 20 plant pathogens with some level of resistance to QoI fungicides. The Qol fungicide risk of resistance is HIGH.
FRAC Code 12 - Phenylpyrrole (PP) Fungicides
Group Name and Chemical Group: The phenylpyrroles (PP) contain the phenylpyrroles chemical group. The main fungicide used in field crops in the PP group is fludioxonil, which is used as a seed treatment and also as a foliar fungicide in some field crops.
Mode of Action and Target Site: Signal transduction: Fludioxonil affects osmotic regulation in some fungi. Reduced osmotic regulation results in bursting of the fungal cell, which thereby prevents fungal growth.
Phytomobility: Fludioxonil is locally systemic, accumulating in the waxy cuticle and moving from one side of the leaf to another (translaminar movement). These fungicides are not translocated in the water-conducting elements of the plant (xylem).
Role in Plant Protection: Fludioxonil should be used preventively. In addition, using spray technology that maximizes coverage on the plant may help maximize control using this fungicide.
Spectrum: Fludioxonil is effective against a broad spectrum of fungi.
Risk of Resistance: Resistance to this fungicide has been documented in a limited number of fungi. The PP fungicide risk of resistance is LOW to MEDIUM.
FRAC Code 14 - Aromatic Hydrocarbon (AH) Fungicides
Group Name and Chemical Group: The aromatic hydrocarbons (AH) group contains the aromatic hydrocarbon group and heteroaromatics. In field crops, fungicides of interest that fall in the AH chemical group are usually included on seed treatments, such as tolclofos-methyl.
Mode of Action and Target Site: Lipid synthesis or transport/membrane integrity function: The specific target site of AH fungicides such as chloroneb is uncertain. However, there is evidence to suggest that the AH fungicides cause a breakdown of certain lipids in the cell membrane.
Phytomobility: Phytomobility of AH fungicides is considered low. Chloroneb is considered a primarily contact fungicide with very limited absorption into plant tissue.
Role in Plant Protection: AH fungicides should be used as preventives as they are unlikely to be able to suppress an existing infection.
Spectrum: In field crops the spectrum of control is relatively narrow, directed toward soilborne organisms. AH fungicides are typically used as in-furrow or seed-treatments in field crops.
Risk of Resistance: Resistance is known in some fungi. Cross-resistance is complicated due to different spectra of activity within the group. The AH fungicide risk of resistance is LOW to MEDIUM.
FRAC Code 21 - Quinone Inside Inhibitor (QiI) Fungicides
Group Name and Chemical Group: The quinone inside inhibitors (QiI) group consists of three chemical groups, cyano-imidazole, sulfamoyl-triazole, and picolinamides. In field crops, the cyano-imidizole group is of importance as it contains the fungicide cyazofamid. Cyazofamid is an important fungicide for the control of late blight on potato, which is caused by an oomycete organism.
Mode of Action and Target Site: Respiration: QiI fungicides function by disrupting electron transport in mitochondrial respiration. This process is similar to that of the QoI fungicides, except that the specific target site of QiI fungicides is different than that of the QoI fungicides, on the electron transport chain (specifically on the Qi site).
Phytomobility: Cyazofamid is locally systemic and exhibits translaminar movement (see section 1.3). Cyazofamid is not translocated in the water-conducting elements of the plant (xylem).
Role in Plant Protection: QiI fungicides like cyazofamid should be used preventively. However, QiI fungicides are capable of early-infection activity.
Spectrum: The spectrum of control of QiI fungicides in field crops is fairly narrow, and targeted toward oomycete pathogens.
Risk of Resistance: No cases of QiI resistance are known. However, the QiI fungicide risk of resistance is assumed to be MEDIUM to HIGH.
For FRAC codes 1-21, see the previous section.
FRAC Code 22 - Benzamide and Thiazole Carboximide Fungicides
Group Name and Chemical Group: Both the benzamides and thiazole carboxamide groups are found in this FRAC class. Within the benzamide group is the toluamide chemical group which contains zoxamide, a fungicide used against late blight and other oomycete diseases of potato. Within the thiazole carboxamide group is the chemical group ethylamino-thiazole-carboxamide, which contains the fungicide ethaboxam. Ethaboxam is a newer fungicide used as a seed treatment in grain crops and as a foliar fungicide for some vegetable crops. Ethaboxam is active against oomycete organisms.
Mode of Action and Site of Action: Cytoskeleton and motor protein: Fungicides in this chemical group affect mitosis and cell division of the target organisms, resulting in inhibition of spore germination and growth of the organism.
Phytomobility: Zoxamide is not considered to be very phytomobile, while ethaboxam may exhibit some acropetal mobility.
Role in Plant Protection: Both zoxamide and ethaboxam should be used preventively. However, both fungicides are capable of suppressing further development of existing infections.
Spectrum: Narrow, targeting oomycete pathogens.
Risk of Resistance: The benzamide and thiazole carboxamide fungicide risk of resistance is LOW to MEDIUM.
FRAC Code 27 - Cyanoacetamide-oxime Fungicides
Group Name and Chemical Group: The cyanoacetamide-oxime group is composed of just one chemical group of the same name. Within this group exists one fungicide called cymoxanil. Cymoxanil is important as a seed treatment and foliar fungicide in potato.
Mode of Action and Site of Action: The mode and site of action for cymoxanil are unknown.
Phytomobility: Cymoxanil is locally systemic.
Role in Plant Protection: This fungicide is considered to be a preventive fungicide with some curative ability.
Spectrum: Narrow, targeting oomycete pathogens.
Risk of Resistance: The cyanoacetamide-oxime fungicide risk of resistance is LOW to MEDIUM.
FRAC Code 28 - Carbamate Fungicides
Group Name and Chemical Group: The carbamates contain one chemical group of the same name. Within this chemical group are three fungicides: iodocarb, prothiocarb, and propamocarb. Propamocarb is an important fungicide for late blight control in potato and is also labeled in turfgrass, vegetables, and ornamentals.
Mode of Action and Site of Action: Lipid synthesis or transport/membrane integrity or function: Carbamate fungicides inactivate phospholipid compounds in cell membranes of target organisms. This results in “leaky” cells and reduced fungal growth.
Phytomobility: Carbamates are acropetally mobile, moving into the water-conducting elements (xylem) of plants and moving upward toward leaf tips.
Role in Plant Protection: Carbamate fungicides should be used preventively. However, they are capable of early-infection activity.
Spectrum: Narrow, targeting oomycete pathogens.
Risk of Resistance: The carbamate fungicide risk of resistance is LOW to MEDIUM.
FRAC Code 29 - Oxidative Phosphorylation Uncoupler Fungicides
Group Name and Chemical Group: Oxidative Phosphorylation Uncoupler fungicides consist of two chemical groups: the dinitrophenyl crotonates and 2,6-dinitro-anilines. Fluazinam, in the 2,6-dinitro-aniline group, is used to control white mold or Sclerotinia stem rot in potato and other field crops and has been recently labeled for soybean.
Mode of Action and Site of Action: Respriation: Oxidative Phosphorylation Uncoupler fungicides inhibit fungal respiration by disrupting the conversion of energy to a usable form.
Phytomobility: Fluazinam is a contact fungicide and has little mobility within the plant.
Role in Plant Protection: Fluazinam inhibits the development of fungal infection structures and spore germination and should be used preventively.
Spectrum: In field crops the spectrum of control is narrow, effective for Sclerotinia-induced diseases and powdery scab of potato.
Risk of Resistance: The Oxidative Phosphorylation Uncoupler risk of fungicide resistance is LOW.
FRAC Code 30 - Organo Tin Compound Fungicides
Group Name and Chemical Group: The organo tin compounds are composed of a single chemical group called the tri-phenyl tin compounds. In field crops, the most important fungicide in this chemical group is fentin hydroxide.
Mode of Action and Site of Action: Respiration: Tri-phenyl tin compounds function by inhibiting oxidative phosphorylation in mitochondrial respiration. Thus, the target organism cannot produce energy to grow.
Phytomobility: Tri-phenyl tin compounds are considered contact fungicides and are not absorbed into the plant tissues.
Role in Plant Protection: Tri-phenyl tin fungicides should be used preventively, before infection.
Spectrum: Narrow, in field crops, fungicides in this group are used to control Cercospora leaf blight in sugar beet and early and late blight of potato.
Risk of Resistance: Resistance has been shown in some limited cases. However, the tri-phenyl tin risk of fungicide resistance is LOW to MEDIUM.
FRAC Code 32 - Heteroaromatic Fungicides
Group Name and Chemical Group: The heteroaromatics are composed of two chemical groups, the isoxazoles and isothiazolones. In field crops, the most significant group is the isoxazoles as this group contains one fungicide, hymexazole, an important seed treatment for sugar beets.
Mode of Action and Site of Action: Nucleic acid metabolism: Hymexazole functions by disrupting nucleic acid synthesis in targeted soilborne pathogens.
Phytomobility: Hymexazole exhibits acropetal mobility and can move upward in the water-conducting elements of the plant.
Role in Plant Protection: Hymexazole is used preventively as a seed treatment.
Spectrum: Narrow, targeted toward soilborne pathogens of sugar beets.
Risk of Resistance: Resistance to heteroaromatic fungicides is not known.
FRAC Code 40 - Carboxylic Acid Amide (CAA) Fungicides
Group Name and Chemical Group: The carboxylic acid amide (CAA) group is composed of the cinnamic acid amide, valinamide carbamate, and mandelic acid amide chemical groups. Fungicides important for field crops include dimethomorph (cinnamic acid amide) and mandipropamid (mandelic acid amide).
Mode of Action and Site of Action: Cell wall biosynthesis: CAA fungicides disrupt cellulose synthase, which leads to the obstruction of cell wall synthesis in the target organisms.
Phytomobility: CAA fungicides are acropetally mobile, moving into the water-conducting elements (xylem) of plants and moving upward in the plant toward the leaf tips.
Role in Plant Protection: CAA fungicides exhibit both protective and early-infection activity when applied just prior to infection or at the first signs of infection.
Spectrum: Narrow, oomycete pathogens.
Risk of Resistance: Resistance in some organisms toward CAA fungicides is known. Cross resistance exists between all members of the CAA group. The CAA risk of fungicide resistance is LOW to MEDIUM.
FRAC Code 44 - Microbial Fungicides
Group Name and Chemical Group: The microbial fungicides contain only one chemical group, which encompasses the Bacillus spp. and the fungicidal peptides they produce. This group includes various species and specific strains of Bacillus.
Mode of Action and Site of Action: Lipid synthesis or transport/membrane integrity or function: These fungicide formulations function by disrupting pathogen lipid synthesis which leads to reduction in cell membrane integrity. At least one strain in this group has also been shown to induce plant host defense as another mode of action.
Phytomobility: Unknown
Role in Plant Protection: These microbial fungicides are protective and should be applied before infection takes place.
Spectrum: Microbial fungicides are used to target a broad spectrum of fungi in field crops.
Risk of Resistance: The microbial fungicide risk of resistance is unknown.
FRAC Code 49 - Oxysterol Binding Protein Homologue Inhibitor (OBPHI) Fungicides
Group Name and Chemical Group: The oxysterol binding protein homologue inhibitors contain one chemical group; piperidinyl-thiazole-isoxazolines. Oxathiapiprolin is an example of an OBPHI fungicide that is used in field crops.
Mode of Action and Site of Action: Lipid synthesis or transport/membrane integrity or function: Fungicides in this group function by disrupting the oxysterol binding protein leading to disruption of lipid movement between cell membranes.
Phytomobility: Oxathiapiprolin has acropetal mobility in the plant.
Role in Plant Protection: Oxathiapiprolin is used preventively as a seed treatment in soybeans and sunflower.
Spectrum: This fungicide is labeled to manage oomycete diseases including Phytophthora root rot in soybeans and downy mildew of sunflower.
Risk of Resistance: Risk of resistance in MEDIUM to HIGH.
FRAC Code P 07 - Phosphonate Fungicides
Group Name and Chemical Group: The phosphonate fungicides incude two chemical groups, one called ethyl phosphonates and another unknown group. Within the unknown group are phosphorous acid and salts. These compounds are used for disease control in some field crops including potato and more recently, corn, soybeans, and small grains.
Mode of Action and Site of Action: Host defense induction: Phosphorous acid and salt trigger defense mechanisms in the plant. Some studies suggest that the phosphite ion may also inhibit energy production in fungi and oomycetes.
Phytomobility: Phosphorous acid and salts are ambimobile fungicides, meaning they are translocated in the plant via the phloem and xylem (moving upward and downward in the plant).
Role in Plant Protection: Phosphorous acid and salt fungicides work best when applied preventively. However, they can inhibit early infections if applied when symptoms are first observed.
Spectrum: Broad, targeting many foliar and soilborne pathogens of various crops.
Risk of Resistance: The phosphorous acid and salt risk of fungicide resistance is LOW.
FRAC Code M - Multi-site Contact Activity Fungicides
Group Name and Chemical Group: Fungicides in the multi-site contact activity group are numerous and diverse. There are 11 groups with an equal number of chemical groups. FRAC code numbers begin with M and are numbered sequentially from M01 - M12. The two inorganic fungicides, copper and sulfur, while in the same chemical group, have been assigned separate FRAC codes (copper M01 and sulfur M02).
Mode of Action and Site of Action: Chemicals with multi-site activity: Multi-site contact activity fungicides affect multiple biochemical sites in fungi.
Phytomobility: Multi-site activity fungicides are contact only fungicides.
Role in Plant Protection: Multi-site activity fungicides should be used preventively since they are applied to the leaf and stem surfaces prior to pathogen appearance. They do not prevent disease once the pathogen has infected the plant. These fungicides are susceptible to wash-off by rainfall and photodegradation by exposure to sunlight since they are not absorbed into the plant, generally remaining active for 7-14 days.
Spectrum: Multi-site activity fungicides have a broad spectrum of disease-control activity.
Risk of Resistance: Multi-site activity fungicides have a low risk of resistance development. Because of this, multi-site activity fungicides are an important part of fungicide resistance management. The multi-site activity fungicide risk of resistance is LOW.
2.1. This table shows the risk of fungicide resistance development. Fungicides at the top have the most resistance risk. Risk decreases lower on the chart. At the bottom are fungicides with unknown resistance risk. Fungicide risk is assessed by the Fungicide Resistance Action Committee (FRAC).
FRAC Code | Fungicide Group Name | Resistance Risk |
|---|---|---|
1 | Methyl Benzimidazole Carbamates (MBC) | High |
4 | Phenylamides | |
11 | Quinone Outside Inhibitors (QoI) | |
2 | Dicarboximides | Medium to High |
7 | Succinate Dehydrogenase Inhibitors (SDHI) | |
21 | Quinone Inside Inhibitors (QiI) | |
49 | Oxysterol Binding Protein Homologue Inhibitor (OBPHI) | |
3 | Demethylation Inhibitors (DMI) | Medium |
9 | Anilino-pyrimidines (AP) | |
12 | Phenylpyrroles (PP) | Low to Medium |
14 | Aromatic Hydrocarbons | |
22 | Benzamides | |
27 | Cyanoacetamide-oximes | |
28 | Carbamates | |
30 | Organo Tin Compounds | |
40 | Carboxylic Acid Amides (CAA) | |
29 | Oxidative Phosphorylation Uncouplers | Low |
P 07 | Phosphonates | |
M | Multi-site Activity | |
32 | Heteroaromatics | Unknown |
44 | Microbial |
Biological fungicides or biological control agents are products which contain living organisms, usually fungi/bacteria or naturally occurring materials derived from microorganisms, plants, animals, etc. that are used to control plant pathogenic fungi. With advances in production equipment such as large batch fermentation technology, as well as consumer demand and regulatory concern, there has been an increased emphasis by many groups on development of biological control products.
For more information on biopesticides, see the CPN web book Biopesticides for Crop Disease Management (CPN 4010)
How do Biofungicides Work?
Biofungicides often use one or more of the following modes of action:
Antibiosis: production of antibiotic substances or toxins which impact pathogens
Parasitism: attacks the pathogen directly
Competition: competes with the pathogen for nutrients, space, and infection sites and/or have the ability to colonize parts of the plants such as roots, leaves, reproductive structures, etc.
Induced plant host resistance: triggers the plant’s natural defense mechanism in response to a pathogen’s presence or attempt to infect which is often called Systemic Acquired Resistance (SAR).
Use and Expectations of Biofungicides
Unlike many broad-spectrum fungicide products, biofungicides often target one or a few specific pathogens. Therefore, it is important to properly diagnosis the problem and read the label to confirm the intended biocontrol organisms are effective against the pathogen of concern. Biofungicides contain living organisms and need to be stored appropriately. There are often temperature restrictions and expiration dates for storing biofungicide products. Product labels will have storage recommendations, as well as application instructions on timing, number of applications, considerations for environmental conditions, etc. In general, biofungicides work best when applied preventively, but there are many factors which could impact efficacy, many of these are discussed in Chapter 3.
Meeting expectations is often one of the most difficult challenges when using biological control agents. Planning and patience are often required since biological agent activity can be a slow process, especially when compared to other conventional pest control fungicides. The level of control can be variable as with any product and expectations need to be realistic. There is great potential, but more research is needed to reach the full potential of biocontrol.
Aflatoxin Management - Biocontrol Strategy in Action!
Aflatoxin management in corn demonstrates how a biological control strategy can be successfully adopted in a field crop system. Aspergillus ear rot, caused by the fungus Aspergillus flavus, is the most economically important corn ear rot in the United States. The fungus produces the mycotoxin known as aflatoxin, which is dangerous and regulated in food and feed by most governments due to its toxicity to humans and livestock.
Biocontrol products for aflatoxin management, known as atoxigenics, use strains of A. flavus which do not produce aflatoxin. These atoxigenics help reduce aflatoxin accumulation in corn and other at-risk crops, including peanuts, cottonseed, and pistachios. For corn, there are two atoxigenic strains of A. flavus labeled for use in the U.S. to prevent aflatoxin accumulation. They are sold under the trade names AF36 Prevail® and Afla-Guard®.
When applied to the crop, these atoxigenic strains are dormant and are carried on nonviable grain (either sterilized wheat or barley). The atoxigenic fungus is activated by moisture and begins to produce spores, relying on the grain-carrier as a food source (Figure 2.1). The spores, which are dispersed by the wind, eventually blow upward and colonize the kernels of the developing ear.
Figure 2.1. Spores of atoxigenic fungi spread via wind and colonize kernels and outcompete toxin-producing fungi.
Iowa State University Integrated Pest Management
Atoxigenics result in Aspergillus ear rot, but they have a minimal effect on kernel quality. The spores from the atoxigenic strains will outnumber the spores of native, toxin-producing A. flavus strains, and they will out-compete the native strains for the limited number of sites in the kernels where they can grow. This decreases the overall aflatoxin contamination of the crop.
Under some circumstances, applying atoxigenics increases the incidence of Aspergillus ear rot, usually at the tips of the ears. However, this damage is greatly offset by the reduction in aflatoxin.
As with other biocontrol products, there are many factors which need to be considered before using these products. Applying atoxigenics is not without risks. Before using these products, always consider these factors: application timing, moisture (low/high), storage, cost of product (return on investment), environmental conditions, etc. See Using Atoxigenics to Manage Aflatoxin (CPN 2005) for more information.
Earn a Certified Crop Advisor CEU after reading this chapter. Successfully complete the Chapter 2 quiz for one CEU. Each chapter has a corresponding quiz at Crop Protection Network CCA CEU page.
The most appropriate fungicide application type and method depends on many factors, including target disease, crop, available application equipment, cost, and other factors. Consult the fungicide label for information about specific application methods approved for the fungicide. Application factors, such as timing and coverage, greatly influence the success of a fungicide application.
Foliar fungicide application in corn.
Brandon Kleinke
Application Coverage
Fungicide coverage of the target plant is important in foliar fungicide efficacy, particularly when using fungicides with limited plant mobility. Coverage is a function of carrier (spray) volume and droplet size, which are influenced by factors such as nozzle type, ground speed, and pressure.
Carrier Volume
In general, higher carrier volumes are recommended to improve fungicide coverage as plants grow and plant density increases. Carrier volumes of 15 to 20 gallons per acre (57 to 76 liters) are recommended for ground applications, with reduced coverage observed at volumes under 15 gallons per acre (57 liters). Higher carrier volumes are needed when spraying fungicides because of xylem-limited or translaminar movement in the plant. Aerial application carrier volumes are recommended to be 3 to 5 gallons per acre (11 to 19 liters). In this case, adjuvants thought to improve coverage are often used.
Droplet Size
Droplet size also influences application coverage. Although large droplets are recommended to reduce drift from herbicide applications, smaller droplets are needed to improve coverage of fungicide applications, since many fungicides have limited foliar movement. Nozzle droplet size is categorized by the American Society of Agricultural and Biological Engineers (ASABE) and defined in terms of volume median diameter (VMD) of droplets (the midpoint droplet diameter, where half of the volume of spray is in droplets smaller, and half of the volume is in droplets larger than the midpoint) (Figure 3.1). In practice, VMD is used in conjunction with other volume diameter percentages to classify droplet size into seven categories: very fine (VF), fine (F), medium (M), coarse (C), very coarse (VC), extremely coarse (XC), and ultra-coarse (UC). Fungicide spray droplets should be in the fine to medium category for optimum plant coverage and canopy penetration.
Figure 3.1. Volume median diameter (VMD) is the midpoint droplet diameter, where half of the volume of spray is in droplets smaller, and half of the volume is in droplets larger than the midpoint. (Adapted from Mathews, 1992).
Iowa State University Integrated Pest Management
Figure 3.2. Droplet size representation charts of 105, 205, and 305 microns (top to bottom), representing those that would be found in very fine, fine, and medium droplet size categories, respectfully.
Spray Nozzle Type and Size
Selecting the right spray nozzle type and size for ground application is critical for fungicide application. Today, many nozzle types are available that influence spray pattern and also reduce drift (Figure 3.3). Application timing, crop growth stage, application method, and target disease should all influence nozzle selection. For example, nozzles that reduce herbicide drift may not produce the recommended droplet size, and ultimately optimal coverage, needed for fungicide applications. The orifice size of the spray nozzle is one factor that influences droplet size and spray pattern. Agricultural nozzles are universally color coded to identify the flow rates in gallons per minute (GPM) at a pressure of 40 pounds per square inch (PSI), as established by the International Organization for Standards. All nozzle manufacturers use this code. For example all red nozzle tips have a flow rate of 0.4 GPM at 40 PSI.
Figure 3.3. Spray patterns of various nozzle types. Adapted from Grisso et al. 2019. Nozzles: Selection and Sizing. Virginia Cooperative Extension. Pub 442-032 and Johnson and Swetnam. 1996. Sprayer Nozzles: Selection and Calibration. University of Kentucky. PAT-3.
Iowa State University Integrated Pest Management
The droplet size created by a given spray nozzle is influenced by the pressure of the application, and the spray angle of the nozzle. Increasing pressure decreases droplet size. Within a given nozzle, changing pressure can change the droplet spectrum dramatically (Table 3.1). It is very important to select nozzles that are capable of producing the correct droplet size at the desired spray angle and pressure for the application. Sprayers equipped with Pulse Width Modulation are better able to maintain droplet size with changes in pressure.
Table 3.1. Examples of nozzles that can achieve fine or medium droplets.
TeeJet Nozzle | Droplet Spectrum¹ | Pressure (PSI) to achieve required droplet size for fungicide application |
|---|---|---|
XR8002 (80) | F-M | F = 40-60 |
M = 15-30 | ||
AIXR11002 | M-XC | M = 75-90 |
TT11002 | F-VC | F = 75-90 |
M = 40-60 | ||
Pentair HyPro | ||
Even (E) Flat Fan 02 | F-M | F = 40-60 |
M = 30 | ||
ULD Ultra Lo-Drift 02 | M-XC | M = 70-115 |
¹Volume median diameter is used to classify droplet size into seven categories: very fine (VF), fine (F), medium (M), coarse (C), very coarse (VC), extremely coarse (XC), and ultra-coarse.
Nozzle Orientation
Certain diseases may require special nozzle orientations or configurations to deliver fungicide to the desired plant tissue. For example, angled or off-set nozzles may effectively deliver more fungicide to the wheat head, the target for fungicide applications to control diseases like Fusarium head blight. Nozzles that can be “dropped” into the canopy to improve coverage in the mid-to lower canopy are also available.
Adjuvants
Coverage can be enhanced through the use of a surfactant or adjuvant, which are wetting agents. Check surfactant labels and fungicide labels for directions on optimum use and mixing to avoid plant injury from these chemicals. Flowable-type fungicide formulations may already contain an adjuvant. In these types of formulations, adding additional adjuvant may reduce product efficacy.
Comparing Application Methods
There are often questions on how application method affects foliar fungicide coverage and efficacy. Fixed wing aircraft and helicopters are used to apply fungicides aerially and use lower carrier volumes than ground sprayers, so there are concerns if adequate product is being delivered to the crop. However, aerial delivery systems use different nozzle and delivery systems that are not directly comparable to ground application delivery systems. If fungicide applicators are using correctly calibrated spray systems, have adequate carrier volume, correct nozzles, and careful application techniques, all factors should be equal between fixed wing, helicopter, and ground application systems.
Center-pivot and linear irrigation systems are capable of delivering foliar fungicides, a process known as chemigation (see Section 3.3 for more information). Technologically sophisticated irrigation systems are especially effective at chemigation. There is currently limited data on how chemigation compares to other foliar fungicide delivery methods, and this area requires more research.
Choosing the optimal application method will depend on many factors, including cost, equipment availability, desired application timing, and individual preference. Check the fungicide label for approved application methods.
Seed-applied fungicides are used to protect against soilborne pathogens that cause seed rots and seedling diseases (e.g., damping-off, root rots, and seedling blight). Additionally, seed-applied fungicides protect against seedborne pathogens present on the seed surface such as those causing common bunt of wheat or downy mildew of soybean; and seedborne pathogens present in the embryo such as those causing loose smut of cereal crops.
The use of seed-applied fungicides varies greatly by crop. Nearly all sugar beet, canola, corn, and cotton seed for production have a high adoption rate of fungicidal seed treatments, while the use of seed-applied fungicides in wheat and soybean are increasing, they are often used on a field-by-field basis. Selection of a seed-applied fungicide to manage seedling diseases can be economically beneficial in locations that have a history of soilborne diseases or where environmental conditions are conducive for seedling disease development. For example, seed-applied fungicide may be beneficial when soybeans are planted in cool, wet soils that cause slow seedling emergence and promote disease development (e.g., those caused by Pythium spp.). The economics of fungicide application is also important to consider. For example, does the reduced disease risk due to seed treatment offset the cost of the treatment and improve profits? Efficacy of seed-applied fungicides is variable. Some fungicides target specific groups of pathogens. For example, sedexane specifically targets species of Rhizoctonia and metalaxyl specifically targets Pythium and Phytophthora spp. (Figure 3.4). Other factors to consider include rate, product, application method, planting method, field history, disease risk, type of soil, and soil conditions at and post planting.
Figure 3.4. Metalaxyl fungicide specifically targets Phytophthora spp., which have caused damping-off in this soybean plant.
Craig Grau
Slurry mixers and drill-box seed treatment equipment are used for on-farm application of seed-applied fungicides. Additionally, mixing seed with fungicide by applying fungicide at the base of the auger is used in some farm operations immediately before planting. Worker protection during these applications is necessary to protect applicators from pesticide exposure. Disadvantages of these systems include variable fungicide coverage on seed, risk of mechanical damage of seed, and additional time and labor needed during planting season. Though these methods are still used in some areas, they are becoming less common as seed-applied fungicides by commercial applicators comprises the majority of that applied on seed in row crop production.
Seed can be purchased with a fungicide treatment that is applied by a commercial applicator. Alternately, a customized seed treatment can be made on-site by farm operators for some crops. Commercial application has several benefits over on-site application, and is becoming the industry standard. Since the 1990s, the trend has been to use fungicides at very low rates that target specific diseases; thus, precision is required to ensure the desired amount of active ingredients are applied correctly to the seed. Commercial facilities can coat or pellet the seed using direct-injection technologies, a method superior to using slurry seed treatment equipment or on-farm seed dressing. Direct-injection methods provide more complete coverage of fungicide active ingredient that can contribute to improved seedling stand uniformity. Use of commercially-treated seed also reduces the exposure of farm workers to pesticides. A common trend in the seed-treatment industry is to provide a complete seed treatment package that includes a fungicide, insecticide, and nematicide (Figure 3.5). Some of these pesticides are restricted use products (RUPs), meaning only those who are properly certified can use them or restricted to be applied only by a commercial applicator. Additionally, seed-applied inoculants for legume crops may be affected by some fungicides, making it important to read and follow product labels.
Figure 3.5. Soybean seed can be purchased with commercial seed treatment applied.
Adam Sisson
Soil-Applied Fungicides (In-Furrow Types)
Soil-applied fungicides are used to protect seed and the developing seedling and can be used with or without seed-applied fungicides. In-furrow applications direct the fungicides to the soil opening created while planting; this technique concentrates the product to the seed and soon-to-be seedling roots. Banded applications direct fungicides in a narrow strip over the developing seedling or seed. Tip orientation can be parallel, 45 degrees, or perpendicular depending on farmer preference. Those applied on a six-inch band perpendicular to the open furrow or on covered seed on the soil surface are called t-band applications. These soil-applied fungicides are applied using a pressurized sprayer to deliver 5-7 gallons per acre (19 to 26 liters). In general, in-furrow or banded applications are most beneficial to protect emerging and developing seedlings from disease in fields where disease pressure is high. Diseases caused by Rhizoctonia are often targeted with these types of applications, but they may reduce other seedling diseases depending on fungicide efficacy. In corn, flutriafol applied in-furrow is labeled for use against certain foliar diseases. Modified equipment may be necessary to use these methods of application. Fungicides must be labeled for this type of use, as some products can be phytotoxic to seed and seedling development.
Chemigation
Chemigation or fungigation delivers fungicides through an irrigation system. Fungicides are mixed in irrigation flow through metering equipment or a chemical injector. This is often used in place of ground spray applications for foliar diseases or to distribute fungicides near the soil line to protect against soilborne diseases. While this can be effective, the choice of product and application carrier rate can be critical. For white mold control in legume crops, lower irrigation rates have been shown to be critical to maintain efficacy of the fungicides applied through irrigation. Rates around 0.1 to 0.2 inches of water have been shown to be as effective as ground applications in dry bean when using products like fluazinam for control of white mold. Read and follow the fungicide label as not all fungicides are approved for chemigation use.
Applying an effective foliar fungicide at the right time will help delay or prevent disease development and protect yield. Several factors will influence the timing of a fungicide including plant growth stage, level of disease, pathogen biology, and application logistics (i.e., environmental conditions and equipment). Below are some examples illustrating the importance of fungicide timing in field crops.
Wheat
Wheat production may include three possible foliar fungicide application timings that coincide with foliar diseases and a head disease. An application of a fungicide early in the season (tillering to jointing) will target residue-borne diseases that can develop early in the season. A fungicide at flag leaf will not only protect the most important leaves (in terms of yield), but also offset any infections from both rust pathogens and residue-borne pathogens. Arguably the most important fungicide timing in wheat pertains to suppressing Fusarium head blight (FHB; scab) and production of associated mycotoxins (Figure 3.6). Fungicide recommendations for FHB management pertain to applying a fungicide when wheat is at early-anthesis (flowering). This timing coincides with the stage of wheat that is most vulnerable to Fusarium infection. The window of this application often extends four to seven days after flowering begins in wheat.
Figure 3.6. Fungicide application for Fusarium head blight of wheat is timed to coincide with the wheat growth stage most susceptible to infection by Fusarium.
Emmanuel Byamukama
Soybean
Foliar fungicide applications in soybean are increasingly common to reduce disease caused by a number of foliar pathogens during the season. Application timing will depend on specific disease risk. Some diseases are best managed with early (R1) applications, as is the case with white mold where the fungus infects mainly through open flowers, or in the case of target spot where applications prior to canopy closure are most effective. Other foliar diseases such as frogeye leaf spot, Septoria brown spot, or soybean rust may be managed after observing initial symptoms. Therefore, it is very important to scout fields regularly. Applications at any point between R1 to R6 may be appropriate depending on the disease, variety susceptibility, prevailing environmental conditions, and economic considerations. For example, the white mold fungus infects soybean primarily through open flowers. Research has shown that the efficacy of fungicides targeting white mold can be maximized when the applications are made at the beginning of flowering (R1 growth stage) through the start of pod development (R3 growth stage). This is because flowers are present during this time. Applications of fungicides protect flowers from infection. Applying fungicide outside this window will result in sub-optimal efficacy and economic returns (i.e., the fungicide may not pay for itself).
Corn
Application timing of foliar fungicides in corn is primarily based on disease risk and growth stage. Corn may be affected by one or more of several foliar diseases such as gray leaf spot, northern corn leaf blight, and others during the growing season depending on hybrid susceptibility. Plants are most vulnerable to yield loss at tasseling through early grain fill, and less susceptible to yield loss due to foliar disease as plants approach physiological maturity. Depending on the foliar disease, hybrid planted and environmental conditions, a fungicide application at VT and up to R3 may be economically beneficial. For example, southern rust is typically a late-season disease that often is detected in corn in June or July when conditions favor disease development in the South. Corn is often tasseling or in early stages of reproduction by this time in the southern U.S., thus fungicides applied at early vegetative stages of growth do little or nothing to protect corn against southern rust (Figure 3.7). It is generally accepted that all corn hybrids are susceptible to southern rust with the disease developing slower on some hybrids. As a general rule, fungicides applied when southern rust has been detected in the field or nearby field and the growth stage is between R1 and R3 provides the best timing to control southern rust and protect corn yield potential.
Earn a Certified Crop Advisor CEU after reading this chapter. Successfully complete the Chapter 3 quiz for one CEU. Each chapter has a corresponding quiz at Crop Protection Network CCA CEU page.
There are many factors to consider before fungicide application in field crops, including economics and assessment of disease risk. This chapter also covers why fungicides may fail to manage the targeted disease, fungicide resistance, fungicides and crop physiology, and how to test fungicides on-farm.
Researchers testing foliar fungicide spray coverage obtained using different types of application systems.
Brandon Kleinke
Fungicides increase production costs, so matching specific fungicide products to management of specific diseases is essential for optimum use. Based on the crop prices and cost of the product and application, the yield increases needed to cover the cost of the fungicide application can be determined for each application (Tables 4.1, 4.2, and 4.3). While average market prices for many commodity crops have approximately doubled over the past decade compared to prices from the early 2000s (Table 4.1), the cost of production (Figure 4.1) including applying the fungicide (Table 4.2) has also increased. Higher value crops increase the amount of justifiable production expenses, including fungicide application, in order to prevent valuable crop loss. However, changes in international trade policies, climate patterns, and government policies and other issues impact farm profitability and increase market uncertainty.
Table 4.1. Average crop commodity price.
Commodity | 2000-2005 | 2006-2011 | 2012-2017 | Units |
|---|---|---|---|---|
Barley | $2.48 | $4.35 | $5.45 | bushels |
Corn | $2.10 | $4.38 | $4.22 | bushels |
Flaxseed | $5.54 | $10.96 | $10.93 | bushels |
Oats | $1.52 | $2.61 | $2.94 | bushels |
Peanut | $0.21 | $0.23 | $0.23 | pounds |
Rice | $6.24 | $13.53 | $13.32 | cwt² |
Rye | $3.04 | $5.46 | $6.63 | bushels |
Sorghum | $3.63 | $7.38 | $7.11 | cwt |
Soybean | $5.53 | $9.98 | $10.87 | bushels |
Sugarbeet¹ | $39.23 | $53.68 | $49.54 | ton |
Sunflower | $11.09 | $20.92 | $20.52 | cwt |
Tobacco | $1.90 | $1.78 | $2.06 | pounds |
Wheat | $3.20 | $5.89 | $5.67 | bushels |
¹No data from 2017 for sugar beets. ²cwt = centrum weight (100 pounds). Source: USDA-NASS
Table 4.2. Average per acre cost for custom application of corn and soybean pesticide application in Iowa (excluding cost of pesticide).
Application Type | 2005 $/ac | 2011 $/ac | 2019 $/ac |
|---|---|---|---|
Ground, broadcast, tractor | 4.95 | 6.05 | 7.25 |
Ground, broadcast, self-propelled | 5.05 | 6.80 | 7.65 |
Aerial | 5.80 | 8.90 | 10.75 |
Source: Iowa Farm Custom Rate Surveys: 2005, 2011, and 2019.
Table 4.3. Breakeven yield increases in bushels per acre needed to cover fungicide application costs based on corn price.
Fungicide Product + Application Cost ($/acre) | |||||
|---|---|---|---|---|---|
Corn Price ($/bu) | $20 | $24 | $28 | $32 | $36 |
$2.00 | 10.0 | 12.0 | 14.0 | 16.0 | 18.0 |
$3.00 | 6.7 | 8.0 | 9.3 | 10.7 | 12.0 |
$4.00 | 5.0 | 6.0 | 7.0 | 8.0 | 9.0 |
$5.00 | 4.0 | 4.8 | 5.6 | 6.4 | 7.2 |
$6.00 | 3.3 | 4.0 | 4.7 | 5.3 | 6.0 |
$7.00 | 2.9 | 3.4 | 4.0 | 4.6 | 5.1 |
$8.00 | 2.5 | 3.0 | 3.5 | 4.0 | 4.5 |
Figure 4.1. Average seed cost and yield for soybean from 1996 through 2016 in the U.S. Heartland.
Assessing Disease Risk
Disease levels in a field are influenced by the cultivar sown (host), the amount of inoculum (pathogen), and weather (environmental conditions). The combination of these factors construct the disease triangle and can explain the level of disease in a field and the prevalence of disease in different growing regions and/or years. These factors are also used to develop models to help with the decision to apply a fungicide. These include forecasting models and scoring systems to gauge disease risk (e.g., Table 4.4).
Cultivar Selection
Cultivar selection can increase or decrease disease risk in a field. Planting a highly susceptible cultivar will likely increase the risk of disease, while a highly resistant cultivar will decrease disease risk. Some diseases may require the combination of a resistant cultivar and a fungicide for sufficient management, whereas other diseases may be managed sufficiently with only a resistant cultivar. Understanding the impact of host resistance on disease development or risk will help determine the need for a fungicide.
Inoculum and Production Practices
Tillage and crop rotation are production practices that impact the amount of pathogen inoculum in a field. Many pathogens of field crops survive the winter or periods of unfavorable conditions on crop residue left on the soil surface. Conservation tillage, lack of crop rotation, or continued use of susceptible cultivars contribute to increased survival of residue-borne and soil-borne pathogens, which leads to greater disease risk. For pathogens that are unable to overwinter in a particular growing region, tillage and crop rotation practices will have little to no effect on inoculum levels in a field. As an example, the fungus that causes southern rust of corn does not overwinter on crop reside in the U.S. or Canada, and therefore tillage and rotation are not management options for this disease. This fungus can overwinter in Central America and travels in weather systems that move from overwintering areas to the U.S. Because of this, development of southern rust of corn in parts of the U.S. and Canada might be delayed depending on pathogen movement and weather conditions favorable for disease development. Understanding when diseases are likely to be observed will help with the timing of fungicide application.
Environment
Fungal and fungal-like pathogens are more likely to thrive when optimal environmental conditions for development exist. Understanding the conditions that are beneficial for pathogen development will help determine the likelihood of disease occurrence and disease risk. Several pathogens of field crops are favored when prolonged periods of humidity and rain occur. On the other hand, dry conditions can lead to the development of other diseases, such as charcoal rot. Environmental conditions can also vary within a field. Areas of a field next to a tree line or low-lying areas of a field may be areas where diseases are more likely to occur because of increased dew periods and/or soil moisture levels, which often favor pathogen infection and disease development. Other factors that influence environmental conditions include planting date, row spacing, row orientation, plant population, and irrigation practices. Altering these practices can increase or decrease risk of disease. Observing and considering environmental conditions as part of a disease management decision strategy also is important since the environment strongly influences disease risk.
Table 4.4. Example of a scoring system used to determine risk of foliar disease development in soybean. Tabulate the disease risk score in the crop risk column based on management category and scale.
Management Category | Category Scale | Crop Risk |
|---|---|---|
Rotation | None = 3 | |
1 year between soybean plantings = 2 | ||
2 or more years between soybean plantings = 1 | ||
Tillage | No-till = 3 | |
Reduced till = 2 | ||
Conventional = 1 | ||
Variety susceptibility (frogeye leaf spot and white mold) | Susceptible = 8 | |
Moderately resistant = 4 | ||
Resistant = 1 | ||
Overhead irrigation | Yes = 2 | |
No = 1 | ||
Weather 30 days prior to reproductive stages | Ten percent of more above average rainfall = 5 | |
Average rainfall = 3 | ||
Ten percent or less below average rainfall = 1 | ||
Date of planting | Not ideal = 2 | |
Ideal = 1 | ||
Total | ||
Low Risk = less than 8; Medium risk = 8-12; High risk = greater than 12 | ||
Adapted from “Know Your Disease Risk in Soybeans: What’s Your Score?” by Smith et al. (2018).
Scouting
Field scouting is an important part of determining disease risk each year. For foliar diseases, early awareness of emerging disease issues will help ensure management strategies are put in place to mitigate diseases before unacceptable yield losses occur. Regular and targeted scouting should take place, not only for disease but for other issues such as insect injury, herbicide performance, and general crop condition. It can be helpful to take identification resources into the field to help with diagnostics. Some diseases are difficult to identify, and samples can be sent to plant diagnostic laboratories for correct identification. Informed scouting considers current national, state, and local reports from Extension personnel, area agronomists, and other crop advisers. Record and keep track of scouting reports as these provide detailed field histories which may inform future decisions. Disease severity on leaves is often overestimated, and crop scouts should “calibrate” their eyes to more accurately assess disease levels in a field (Figures 4.2, 4.3, and 4.4). For soilborne disease, scouting will not help with in-season decisions, but can help with management decisions in future years, especially for diseases such as crown rot on corn or sudden death syndrome of soybean.
The following situation is an example of high disease risk for a residue-borne foliar disease:
Lack of crop rotation, or rotation with crops that are hosts of the same pathogens
High amounts of surface crop residue infested with pathogens that affect the next crop
Disease presence before or at critical stages of crop development (e.g. disease presence at tasseling for corn, early reproductive stage for soybeans, jointing for wheat). Earlier disease onset may result in greater yield loss than diseases that occur during later stages of crop development.
Susceptible cultivars
Disease conducive weather (e.g. high relative humidity, frequent rainfall)
Irrigation or other factors that increase disease conducive environments
Foliar Fungicide Efficacy Guides are available from the Crop Protection Network for corn, soybean, and wheat.
Figure 4.2. Diagram showing low severity levels of corn foliar disease. From the top, severity levels are 1, 2, 5, and 10 percent.
Iowa State University Integrated Pest Management
Figure 4.3. Diagram showing low severity levels of soybean foliar disease. From the top, severity levels are 1, 2, 5, and 10 percent.
Iowa State University Integrated Pest Management
Figure 4.4. Diagram showing low severity levels of wheat foliar disease. From the top, severity levels are 1, 2, 5, and 10 percent.
Iowa State University Integrated Pest Management
Seed-Applied Fungicides
There are many fungicides applied to seeds to manage seedling diseases in field crops. Fungicidal seed treatments can manage two different types of pathogens: seedborne fungal pathogens, such as the Phomopsis seed decay fungus (caused by Diaporthe longicolla); and soilborne pathogens that infect seedlings and roots, such as Pythium spp. In general, fungicide-treated seed provide up to approximately 20 days of protection from seed rots. Additional applications of fungicides in-furrow at planting may occasionally provide additional seedling protection and assist in stand establishment when disease severity is high, but may require specialized application equipment. A fungicide seed treatment can reduce seedling diseases and improve stand, especially in early planted soybean; however, seed treatments will not improve poor germination or vigor in poor quality seed that is not caused by pathogens. There are many products and product combinations in row crops, which can make it difficult to determine what treatments will protect against a certain disease and if seed treatments consistently provide positive returns.
The following situations increase the risk of disease, and should be considered when selecting a product or deciding to use a fungicide seed treatment:
Disease is present in field
Field history of seedling diseases
Planting into wet soils
Planting into soils below 60°F (16°C)
Planting into compacted soils
Using less than recommended seeding rates at planting
Practicing reduced tillage
History of flooded soils
Lack of crop rotation
Planting cultivars susceptible to soilborne diseases
High levels of seedborne fungal infection
A Seed-Applied Fungicide Efficacy Guide is available from the Crop Protection Network for soybean.
Disease Forecasting Systems
Multiple disease forecasting systems have been developed for field crops covering a variety of diseases. Some forecasting systems consist of informal or formal networks of farmers, extension specialists, and others who monitor local disease development and report findings in order to warn others. Sentinel plots, as well as commercial field observations, can be part of these systems. Some rust diseases, such as southern rust of corn (caused by Puccinia polysora) are monitored in this way, as disease progression moves from southern locations northward. Other warning systems rely on weather data in combination with user input to formulate risk. Sporecaster is an example of this type of warning system, where farmers provide site-specific data about a particular field (e.g., row closure and growth stage) that is then coupled with remotely-accessed weather data from the “cloud” to provide an informed prediction of the risk of yield-limiting epidemics of white mold (also known as Sclerotinia stem rot; caused by Sclerotinia sclerotiorum) in soybean. Some of these systems are highlighted in Table 4.5.
Table 4.5. Examples of disease forecasting systems and networks for field crops.
Name of system | Crop | Disease | Type |
|---|---|---|---|
Sporecaster | Soybean | ||
Blightcast | Potato | Late blight | |
Fusarium Head Blight Prediction Center | Wheat |
Every fungicide product has limitations and may not always work the way it is intended. Conversely, a user may expect a product to work in a way it was not intended on the label. A fungicide application may not be effective for a number of reasons. These include issues that arise before application occurs, such as improper storage of fungicide leading to degradation of product efficacy. Application issues, like incorrect sprayer calibration or carrier volume may also affect performance. Furthermore, post application problems such as weather immediately after application can impact efficacy. So
me fungal pathogens may be resistant to specific fungicide classes, which also may result in fungicide failure. Careful recordkeeping and crop scouting, both before and after an application, can help to identify reasons for fungicide performance issues. Possible factors for fungicide failure are discussed in this chapter.
Fungicide Product Selection
Several fungicide products often are marketed in a single crop, and the spectrum of diseases controlled by each fungicide product may differ. Also, not all fungicides may be labeled for a particular disease. For example, the active ingredient propiconazole (FRAC code 3; DMI) provides excellent control of eyespot (Aureobasidium zeae) of corn (Figure 4.5). It is, however, less effective against southern rust (Puccinia polysora) and is not effective for anthracnose leaf blight (Colletotrichum graminicola). Metalaxyl (FRAC code 4; Phenylamides) and mefenoxam (FRAC code 4; Phenylamides) have activity against seedling blights caused by Pythium and Phytophthora spp., but do not have efficacy against Fusarium and Rhizoctonia spp. Fungicides with the same FRAC code may differ in efficacy. For example, there are multiple fungicides with FRAC code 3 (DMI fungicides) that are labeled for Fusarium head blight in wheat; however, metconazole and prothioconazole both have greater efficacy than tebuconazole and propiconazole.
Researchers at land-grant universities study the efficacy of commercial fungicides on various diseases and make this information available to the public. Fungicide efficacy tables for several crops are available at the Crop Protection Network website or through local extension networks.
Foliar Fungicide Efficacy Guides are available from the Crop Protection Network for corn, soybean, and wheat. A Seed-Applied Fungicide Efficacy Guide is available from the Crop Protection Network for soybean.
Figure 4.5. Some fungicide active ingredients are more effective against certain diseases than others. For example, propiconazole is very effective for eyespot of corn (shown here), but less so for southern rust of corn.
Adam Sisson
Fungicide Rate and Mixing Factors
Make sure the correct rate of fungicide is applied for the crop and targeted disease. The rate is often provided as a range, with specific recommendations within the range. The rate on a fungicide label is based on multiple years of research occurring across diverse locations. This ensures that the rates provided are applicable over a wide range of field conditions experienced by farmers. Rates may differ based on geographic location and target disease. A lower than recommended rate may not control the targeted disease, and may lead to faster selection of fungicide resistant pathogens. Additionally, there are restrictions on the total amount of fungicide active ingredient that can be applied per acre in a given year, and fungicide rate will impact the total.
There is a proper way to mix and add formulations in the spray tank. First, be sure to correctly calculate the treatment area and corresponding amount of product to be used. Calculation errors may result in increased costs, crop injury, and/or poor disease control.
The ideal water pH for fungicide mixing is approximately 7.0. Fungicidal activity can be reduced if mixed with water with a pH that is too high (alkaline) or too low (acidic). This can especially be a problem if the pH is greater than 8.0. Use a pH buffer to correct unfavorable pH levels, adding the buffer before the fungicide.
Applying more than one pesticide at once, or mixing with liquid fertilizers, may save time and applications costs. However, some combinations may not compatible when mixed, as insoluble precipitates may form in the spray tank. Using a compatibility agent may alleviate this problem. Check product labels for compatibility information. If this information is not available, test compatibility by mixing a small volume in a glass jar. If settling or separation of product can be observed after leaving the jar to sit for 30 minutes, products are incompatible. Certain pesticides may also have antagonistic effects on the crop or on disease control when mixed. Check product labels and always follow recommendations for mixing pesticides prior to application.
Multiple products should be added to the spray tank in a specific order to ensure compatibility. The order is as follows:
Compatibility, buffering, or defoaming agents,
Wettable powders (WP) and dry flowables (DF),
Water-soluble concentrates (WSC or SC),
Emulsifiable concentrates (EC),
Soluble powders (SP), and finally,
Adjuvants or spray additives.
Fungicides should be used soon after mixing as product efficacy declines after mixing. Poor water quality (water hardness, pH, etc.) can speed up reductions in fungicide efficacy.
Fungicide Application Factors
Drift - Fungicide drift is the aerial movement and unintentional deposit of fungicide outside the target area. Drift results in wasted product, can compromise crop protection, and may adversely affect nearby sensitive environmental areas, crops, people, and wildlife. Fungicide drift may affect coverage and/or the amount of product on the targeted crop and thus the efficacy of the fungicide.
The following strategies can help reduce the risk of fungicide drift:
Do not spray when wind speeds are high or gusty. These conditions increase the potential for spray drift. Some fungicide labels may state allowable wind speeds for spraying. However, not all labels provide this specific information, and instead refer users to state-specific extension guidelines for spray drift prevention.
Constantly monitor wind conditions during spraying using a good-quality wind meter. Record the wind speed and direction. As wind conditions change, adjustments may be needed to further reduce the drift potential, such as increasing water volume, minimizing nozzle-to-target distance, changing nozzle technology, halting spraying operations until conditions improve or, if possible, moving to another field where wind conditions are acceptable for spraying.
It is not advisable to spray during periods of dead calm as it may lead to a phenomenon called temperature inversion. A temperature inversion occurs during periods of dead calm (typically winds less than 3 MPH) in early morning or late evening, at which time cooler air masses are trapped at the ground level. This can result in small or fine spray droplets being caught in the air mass. Those droplets can move unpredictably when light variable winds move the air mass, resulting in spray droplets moving away from the target area to adjacent non-target areas. Off-target drift in calm conditions can occur hours after the spray event was completed. Drift retardants can be added to help reduce drift and maximize coverage.
Calibration of the sprayer used to apply fungicide ensures proper rates of the product are delivered and that good coverage is achieved. Improper sprayer calibration is likely the most common cause of application errors. Phytotoxicity (see below) from excessive fungicide rates and lack of disease control from too little product reaching target sites are the result of poor sprayer calibration. The extra time and effort required for adjusting and calibration can increase fungicide effectiveness and save money. Sprayers should be recalibrated after nozzles, pressures, or sprayer speeds are modified.
Carrier volume is an important factor in pesticide application and is specified on the product label. Optimal application pressure may differ for fungicides and herbicides. Additionally, constant speed during application and appropriate spray pressure will help optimize coverage. If the same equipment is used for applying both fungicides and herbicides, be sure to adjust nozzles and pressure for the type of pesticide being applied. Fungicide drift can occur with small droplets less than 100 microns produced by high pressures. Also, adjust boom width, height, and sprayer drive rows to reduce spray overlap or misses.
Nozzle selection plays an important role in proper delivery of active ingredients to plant tissue and improper nozzles can reduce fungicide effectiveness. Plant tissue coverage of the lower canopy may be improved by using an angled nozzle or drop nozzle (Figure 4.6). For more information on proper nozzle selection, see Section 3.1.
Figure 4.6. This sprayer is equipped with nozzles that drop down into the crop canopy.
Brandon Kleinke
The environmental conditions during and immediately following application can greatly impact how a fungicide functions. For example, dew, rain, or irrigation occurring immediately after application can dilute fungicide product or wash it from foliage before it dries or becomes rainfast. In general, three hours is required for systemic fungicides to dry; contact fungicides are susceptible to rain removal at all times, and particularly so prior to drying. Strong winds can cause drift, reducing efficacy. Small droplets can evaporate after leaving the spray nozzle if humidity is less than 50% and temperature is more than 92°F (33ºC) during application.
For fungicide seed treatments, if the targeted planting date is early or conditions are very cool and wet, seed treatments may not be enough to protect against certain pathogens. Additionally, fungicide seed treatments may only protect seeds and seedlings for up to approximately 3 weeks after planting, depending on the product and disease. If environmental conditions conducive to disease do not occur until after that time and on a susceptible variety, a farmer may see disease and think the seed treatment has failed, despite the fact that fungicide seed treatments have a limited window of activity.
Fungicide Timing
Most fungicides have a limited period of activity after being applied, which typically is 14-21 days after application. Applying fungicides too early in the growing season may result in not enough active ingredients remaining in the plant when disease onset occurs. Some diseases cannot be adequately controlled by fungicides once disease symptoms or signs appear, and in these cases, fungicides applied at symptom or sign onset may be too late to protect against losses. Thus, it is important to apply fungicides at the most appropriate time or plant growth stage.
The most appropriate fungicide application window differs for individual crops and diseases. There may be only a small time period for optimal application when protection during a specific growth stage is necessary, as with Fusarium head blight (caused by Fusarium graminearum) of wheat or with white mold of soybean. Predictive tools can help crop managers to determine risk of these diseases and subsequent need for fungicide application (see Section 4.2). Fungicide applications outside optimal timing reduces likelihood of economic return and satisfactory disease control.
Fungicide Resistance
A fungicide may fail to perform as desired because selection of fungicide resistance has occurred in the targeted fungal pathogen. Fungicide resistance is one of the first things that may come to mind when a fungicide fails to manage a disease. Selection of fungicide-resistant strains of fungal pathogens is complex and is influenced by fungicide mode of action, pathogen biology, and other factors. The only way to be certain fungicide resistance is the cause is to isolate pathogens and examine them in a lab. Do not immediately assume that the cause of any fungicide failure is due to fungicide resistance. See Section 4.4 for detailed information regarding fungicide resistance. If you believe resistance is the reason for poor fungicide performance, consult with a local extension agent.
Other Issues
Fungicide storage conditions are important to maintain product efficacy between applications. Products stored for more than two years or subjected to poor conditions, such as exposure to extreme cold (below freezing), can have reduced efficacy. Settling can occur with flowable fungicides, making it important to mix individual products in the storage container prior to placing in the sprayer. Fungicide active ingredient dosage may not be correct if products have settled.
Be sure to store fungicides in the original containers and with easy access to the product label. Mislabeled chemical containers can result in the wrong product being applied and at worst, accidental consumption or human exposure. Having easy access to and reviewing the label minimizes the chance of mixing and application mistakes.
Misdiagnosis of diseases present may result in poor control, since an ineffective fungicide product may be applied. Accurate disease diagnosis will help differentiate economically important diseases, diseases that are not economically important (those that do not cause yield losses great enough to justify a fungicide application), and other disorders that are not caused by fungal plant pathogens, but that may cause similar symptoms to fungal plant diseases. Disease symptoms caused by bacteria that look similar to those caused by fungi will not be managed by fungicides. Besides diseases caused by bacteria, there are many other disorders that can be confused with fungal-caused diseases including environmental damage, chemical injury, insect damage, genetic flecking or striping, and root injury caused by nematodes.
Lack of disease, or low levels of disease, can result in a negative or lower than expected return on investment. Research suggests that use of fungicides is less likely to be profitable if the risk of disease is low. In 2011, university researchers compiled data on hundreds of treatments from multiple fungicide-use studies in corn (Table 4.6). When disease severity on the ear leaf was less than 5% during early dent stage, a mean yield response of 1.5 bushels per acre was observed with less than 32% of treatments breaking even. However, if disease severity was more than 5%, the mean yield response was almost 10 bushels per acre with a break-even likelihood of nearly 60%.
Table 4.6. Impact of quinone outside inhibitor (QoI; FRAC code 11) fungicides on corn yield response from multiple research studies occurring from 2000 to 2010.¹,²
Disease severity in untreated plots | Mean yield response of treated plots | Total treatments | Treatments with break-even yield response³ |
<5% | 1.5 bushels/acre | 347 | 31.60% |
(94.4 kg/ha) | |||
>5% | 9.6 bushels/acre | 266 | 59.00% |
(603.8 kg/ha) |
¹Wise and Mueller, 2011. ²Data were pooled for treatments that had a QoI fungicide applied between V15 and R3 stages of corn growth and disease severity ratings. ³A break-even yield response is 6 or more bushels per acre (603.8 kilograms per hectare).
It is also important to remember that many diseases are unlikely to cause economic losses in a crop. Common rust of corn (caused by Puccinia sorghi) and downy mildew of soybean (caused by Peronospora manshurica) are examples of diseases that rarely cause yield loss, but can be commonly observed in fields each year.
Applications made in expectation of favorable conditions for disease development can be unprofitable if the expected weather conditions do not occur. This may require additional fungicide application when disease begins and conditions favor disease development, which increases production cost and reduces profit.
Certain fungicides may cause plant injury (phytotoxicity), including leaf burning and plant stunting (Figure 4.7). This injury can occur when fungicides are applied at excessive rates, making it important to follow the recommended rates and other directions on the product label. Unfavorable environmental conditions during or after fungicide application, such as high temperatures, can also result in plant injury. Injury can occur on one variety with no or very little injury to another.
Plant injury from spray tank additives applied with a fungicide can occur and may be perceived as being caused by the fungicide. As an example, nonionic surfactants applied with fungicides to pre-tassel corn are thought to be a cause of arrested ear development. Check additive and fungicide labels to ensure proper rates and compatibility.
Figure 4.7. Foliar injury on soybean caused by fungicide application.
Daren Mueller
Variability of area treated within a field can give the appearance of fungicide failure due to differences in conditions that exist in that field. A fungicide-treated field may not appear to yield more than an untreated field, or treated areas within a field may not appear to yield more than untreated areas. This is more likely to be observed when disease severity is low or spatial variability of disease is high. Small yield responses are difficult to discern if these responses are less than existing natural field variability. For example, in low disease risk corn fields, Mueller and Wise (2011) determined a mean yield response to fungicide application of 1.5 bushels per acre (94.4 kilograms per hectare). If natural field variability is greater than this, yield response to fungicide application may be undetectable or even perceived as a loss in certain situations.
Impact of Fungicide Resistance
Pathogens on field crops have been managed with fungicides since the 18th century. Copper and sulfur-based fungicides were used in the 19th century, while mercury-based fungicides were used in the early 20th century until animal toxicity concerns were revealed. The contact fungicides mancozeb (FRAC code M3; Dithiocarbamates and relatives) and captan (FRAC code M4; Phthalimides), which were only effective if applied prior to infection, were marketed in the 1940s and 1950s. Apparent risk of fungicide resistance was low, since these fungicides have multiple sites of action.
In recent decades, fungicides with systemic properties, specific modes of action, and the ability to inhibit many different fungi have proven to be highly effective for disease control. These include the quinone outside inhibitors (FRAC code 11; QoIs). Increased reliance on these fungicides, coupled with the fact that fungi can more easily overcome a specific mode of action than multi-site activity, has resulted in the emergence of fungicide resistance. After only four years of use, populations of the causal pathogen of frogeye leaf spot of soybean, Cercospora sojina, already exhibited resistance to QoI fungicides. Conversely, it may take decades for other fungicides to lose efficacy due to pathogen resistance. Extensive use of a specific fungicide mode of action or active ingredient increases risk of resistance developing. Sunflower, soybean, sugar beet, and potato farmers have all experienced the loss of fungicide products resulting from the selection of fungicide resistant pathogens and widespread development of fungicide resistance within fungal pathogen populations.
Development of Fungicide Resistance
A fungal population becomes resistant to a fungicide through selection pressure. Over time and with repeated use of the same fungicide active ingredient, less sensitive individuals within a pathogen population will multiply and eventually make up the majority within that population (Figure 4.8). As a pathogen population exhibits less sensitivity to a fungicide product, that product becomes a less valuable, or even useless, pest management tool.
Figure 4.8. This diagram illustrates how fungicide resistance occurs. The fungicide resistant genetic variant (red hexagon) is not controlled by a fungicide application. As use of the same fungicide active continues for successive applications, the resistant genetic variant survives and multiplies, eventually reducing the effectiveness of the fungicide.
Iowa State University Integrated Pest Management
Attributes of the chemical product and pathogen determine the risk of selection for fungicide resistance. Fungal plant pathogens with a high degree of genetic variability in the population are at greater risk for fungicide resistance. This greater degree of genetic variability often is observed in plant pathogens with polycyclic life cycles (repeating spore stages) and those that reproduce sexually. Increased genetic variability increases the chances that a genetic variant with reduced sensitivity to a fungicide will occur within the pathogen population. Fungicides with multi-site activity are less likely to select for resistant pathogens than those with a single-site mode of action. Applicators cannot control the genetic variability within a pathogen population, but can take measures to reduce selection pressure related to fungicide choice.
Reducing Risk of Fungicide Resistance
The risk of fungicide resistance development can be reduced using a few standard practices such as use of integrated pest management, mixing and alternating fungicides, and following label recommendations. The best way to ensure long-term effectiveness of a fungicide is to make sure a fungicide program includes all of these fungicide resistance management practices.
Monitoring
In order to assess if a fungicide has performed as desired, field scouting should be conducted after an application. There are multiple reasons why a fungicide may not work as expected, but identification of potential resistance is important. If fungicide resistance is suspected, the first step is to contact local extension personnel or representatives from the fungicide manufacturer.
In order to monitor fungicide resistance, researchers use laboratory analyses to establish a baseline level of pathogen sensitivity to a fungicide. The baseline sensitivity level is the sensitivity to a particular fungicide of a fungal population that has not been previously exposed to that fungicide (or fungicides with the same mode of action This baseline level provides a comparison point that researchers can use when monitoring pathogen populations suspected of being resistant, or becoming less sensitive to a fungicide active ingredient as time passes. This is accomplished by periodically testing pathogen populations in a laboratory.
Integrated Pest Management (IPM)
An IPM program includes using multiple disease management methods, and regular field scouting, local and regional disease monitoring. Using recommended fungicides if disease risk is high is part of IPM. Because fungicides are important components of field crop disease management programs, it is necessary to protect the long-term effectiveness of these tools. IPM practices are implemented to help preserve this effectiveness (Figure 4.9). For more information about fungicide use as part of IPM, see Section 1.2.
Using fungicides only when warranted due to disease risk and scouting observations is a way to help ensure that fungicides will continue to be useful for as long as possible. Every time a fungicide is applied, fungal plant pathogens are exposed to the fungicide. Application of a fungicide when it is not warranted due to low disease risk or scouting observations will unnecessarily expose the fungal pathogen population to the fungicide and speed up the selection of resistant (insensitive) individuals within the population. Fungicide application when disease risk is low also decreases the probability of a positive return on investment.
The same fungicide products or active ingredients may be registered for many field crops. This is important to consider if these crops are part of the same rotation. Keep in mind the need to rotate modes of action as well as products and active ingredients, as different fungicides may have the same mode of action.
Figure 4.9. Integrated Pest Managment (IPM) includes the use of many tools to reduce disease losses and also helps to delay development of fungicide resistance. Using disease-resistant plant varieties is one such tool. The corn hybrid on the left is susceptible to northern corn leaf blight, while the hybrid on the right is resistant.
Albert Tenuta
Alternating and Mixing Fungicides
Use of fungicide products that contain a single active ingredient can increase selection pressure for fungicide resistant pathogen populations compared to using premix products or tank mixing products with active ingredients that have distinct modes of action. If a resistant fungal propagule is not killed by one fungicide mode of action in the tank mix, the other fungicide mode of action should kill it, reducing survival of propagules that can increase to become resistant populations. This only works if both fungicides have the ability to manage the target pathogen. For example, mixing two products such as flutriafol (FRAC code 3; DMI) and fluazinam (FRAC code 29; Oxidative phosphorylation uncouplers), would not be effective for resistance management when trying to control frogeye leaf spot, as only one of these exhibits activity against the causal pathogen. However, mixing flutriafol and thiophanate-methyl (FRAC code 1; MBC) would be effective as both are able to control the pathogen that causes frogeye leaf spot.
Use different modes of action when more than one fungicide application is needed during a single season. Applications should be timed to be most efficacious for disease management when fungicides are alternated.
Label Recommendations
It is important to remember that the “label is the law” regarding fungicide application. Fungicide labels may have a resistance management section which should be followed carefully. The label contains important information regarding application restrictions such as permissible number of single season applications, or if back-to-back applications are allowed.
Sublethal doses of fungicide may increase risk of pathogen desensitization to an active ingredient. This makes following label rates an important part of slowing resistance development, as well as ensuring proper disease control.
Cross-resistance occurs when a fungal organism exhibits resistance to multiple fungicides in the same FRAC code, as is often the case when a fungus exhibits resistance to one of the fungicides within that FRAC code.
Managing Fungicide Resistance
Using the same fungicide with the same mode of action/FRAC code season after season or several times within the same season could result in the target pathogen becoming resistant to the chemical family. A pathogen can develop resistance to one chemical family but still be very sensitive to another. Therefore, to reduce the risk of a pathogen developing resistance, rotate among chemical groups and/or families within the same season or during successive growing seasons for control of the same disease. Also, some types of fungicides have a higher risk of resistance development than others. See Section 2.2 for fungicide resistance risk classification.
Fungicide resistance management is an important part of field crop production. The following guidelines can help minimize the risk of resistance:
Spray when needed
Do not make unnecessary fungicide applications. There should be a tangible, defined reason for application, as opposed to emotion- or tradition-based applications.
Only use chemical control when necessary and consider implementing an integrated pest management strategy, including cultural control (e.g., crop rotation, disease- resistant varieties, scouting, use of certified seed, etc.) or biological control, which will also help reduce the risk of a pathogen developing resistance to a fungicide.
Scout fields regularly, noting incidence and severity of diseases. Use this information to develop a field history for future disease management decisions.
Be willing to accept some level of disease. A completely clean crop is not necessary to maximize yield or economic returns.
Healthy plants help
Ensure good agronomic practices are in place to minimize fungicide need.
Plant disease-resistant hybrids/varieties whenever possible.
Maintain proper soil fertility.
Utilize a crop rotation that fits your area and field history.
Avoid sites with a history of high disease pressure.
Fungicide selection is important
Use a pre-mix fungicide or tank-mix high-risk fungicides with fungicides that have different modes of action, are active against the targeted disease(s), and have similar lengths of residual activity.
Alternate fungicides with different modes of action when multiple applications are required during a season.
Do not apply the product at rates lower than the recommended rate on the label.
Be sure to follow the rates, restrictions and other application instructions on the fungicide label.
Do not exceed the total number of applications or total amount of material allowed per year for each product.
Fungicide timing is important
Apply fungicides preventively or early in the disease cycle and when disease risk is high.
Avoid fungicide applications at late stages of disease development, especially with high-risk fungicides, as these increase the risk of selecting for resistance.
Monitor after application
Monitor disease progression following fungicide application (Figure 4.10). This is a way to check for potential fungicide resistance. If the fungicide is found to be ineffective, this may be an indicator of resistance. However, other reasons can cause fungicide failure and should be ruled out first.
Figure 4.10. Check disease progression after a fungicide application, making sure to follow the fungicide label for re-entry interval of the sprayed field.
Iowa State University Integrated Pest Management
Fungicides have been reported to have physiological benefits independent of disease control. Since 2009, the Environmental Protection Agency (EPA) has approved the use of “plant health” claims on certain fungicide labels. Multiple companies now market products focusing on selling the “plant health” benefits of fungicides. Examples of these benefits include improved nitrogen use efficiency, delayed senescence or “the greening effect,” and improved harvestability.
Physiological effects may also result in unwanted consequences such as soybean green stem (Figure 4.11), or increased grain moisture in corn, leading to delayed or difficult harvest. Therefore, it is important to balance the benefits and risks of fungicide applications and use them as recommended. Physiological effects of fungicides may provide modest yield increases, but the greatest yield gains from fungicides are consistently obtained when fungicides are used to mitigate disease risk.
Figure 4.11. Green stem of soybean can be a consequence of fungicide physiological effects. Green stem can increase harvest difficulty.
Xavier Philips
On-farm trials can be used to test fungicide effectiveness and are an important aspect of determining the need for fungicide for a specific farming operation. On-farm trials are conducted by farmers, retailers and private industry, and university researchers. Trials are sometimes the result of partnerships among these groups.
Key differences exist between on-farm research trials and small plot research trials. On-farm trials often only test one or two fungicides using larger plots with a higher likelihood of variability within the trial. Conversely, small-plot research allows for more trial uniformity and the ability to test multiple fungicides applied across several variables, such as varieties, timings, etc. To decrease errors associated with on-farm trials and to gain meaningful information, it is important to consider proper trial design and the inputs needed to conduct an on-farm trial before implementing the trial.
Conducting an On-Farm Trial
Determine goals and objectives of research--identify collaborators if needed.
Decide on treatments
Include a control plot or untreated check
Choose a uniform field site and research site history
Check equipment width (planter, sprayer, combine, etc.) to determine plot layout and make sure data collection is compatible with equipment
Calibrate equipment
Establish plot layout (minimum of three replications preferred) and randomize treatments. It is important to set up field plots correctly to obtain the best data (Figure 4.12). Incorrect field plot layouts will not produce good data (Figure 4.13).
Choose what types of data to collect
Determine a data evaluation plan
Figure 4.12. Examples of good plot plans for on-farm experiment with four replications and randomized treatments.
Iowa State University Integrated Pest Management
Figure 4.13. Examples of poor plot plans that will not produce useful trial results.
Iowa State University Integrated Pest Management
Figure 4.14. Including an untreated control in fungicide experiments allows researchers to appropriately guage fungicide impact.
Interpreting Results
Using statistics to analyze trial results is important to determine if differences observed are truly due to the treatments, or just due to “chance” or other factors in the trial that may have influenced results. However, analysis of on-farm research data can be difficult without a background in statistics. Conducting trials as a partnership with university researchers provides access to statistical software and data interpretation. Many university researchers will help with trial design, plot layout, data collection and formatting, and analysis of results. Having their trained expertise will help you determine if the treatments tested had a true effect on disease control and/or yield.
Earn a Certified Crop Advisor CEU after reading this chapter. Successfully complete the Chapter 4 quiz for one CEU. Each chapter has a corresponding quiz at Crop Protection Network CCA CEU page.
Authors
Daren Mueller, Iowa State University; Kiersten Wise, University of Kentucky; Carl Bradley, University of Kentucky; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin-Madison; Erin Hodgson, Iowa State University; Albert Tenuta, Ontario Ministry of Agriculture, Food, and Rural Affairs; Andrew Friskop, North Dakota State University; Shawn Conley, University of Wisconsin-Madison; Travis Faske, University of Arkansas; Edward Sikora, Auburn University; Loren Giesler, University of Nebraska-Lincoln; and Martin Chilvers, Michigan State University.
Citation:
Mueller, D., Wise, K., Bradley, C., Sisson, A., Smith, D., Hodgson, E., Tenuta, A., Friskop, A., Conley, S., Faske, T., Sikora, E., Giesler, L., and Chilvers, M. 2021. Fungicide Use in Field Crops. Crop Protection Network. CPN 4008. Doi.org/10.31274/cpn-20210329-0
Reviewers
Entire book: Katherine Stevenson, University of Georgia; Chapter 1: Tom Allen, Mississippi State University and Darcy Telenko, Purdue University; Chapter 2: Heather Kelly, University of Tennessee and Alyssa Koehler, University of Delaware; Chapter 3: David Hooker, University of Guelph; Travis Legleiter, University of Kentucky; and Paul "Trey" Price, Louisiana State University; Chapter 4: Hillary Mehl, USDA Agricultural Research Service and Kenny Seebold, University of Kentucky; Illustrations: Andrew Penney, Iowa State University
Images
All photographers are listed alongside their images appearing throughout this work.
Illustrations
Emily Poss and Renee Tesdell, copyright Iowa State University Integrated Pest Management Program.
References
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Smith, D., Mueller, D., Kleczewski, N., Wise, K., and Bradley, C. 2018. Know Your Disease Risk in Soybeans:What’s Your Score? United Soybean Board. Article
United States Department of Agriculture - National Agricultural Statistics Service (USDA-NASS). Accessed: April 2021. Database
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Sponsors
This educational resource was made possible by contributions from Iowa State University Integrated Pest Management; the Grain Farmers of Ontario; and the United States Department of Agriculture - National Institute of Food and Agriculture (USDA-NIFA).
This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
Earn Certified Crop Advisor CEUs after reading this book. Successfully complete the Chapter 1, Chapter 2, Chapter 3, and Chapter 4 quizzes for four CEUs. Each chapter has a corresponding quiz at Crop Protection Network CCA CEU page.
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