Fungicide Use in Field Crops: Web Book

1.2: Fungicides as Part of Integrated Pest Management (IPM)

1.2: Fungicides as Part of Integrated Pest Management (IPM)

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.

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.  

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1.2: Fungicides as Part of Integrated Pest Management (IPM)