An Overview of Fusarium Head Blight

Introduction

Fusarium head blight (FHB), also called scab, is a major disease affecting small grain crops, such as wheat and barley grown in the United States and Canada. This disease is most frequently caused by the fungus Fusarium graminearum (sometimes referred to as Gibberella zeae) and occasionally by other species of Fusarium. Yield losses because of FHB can exceed 50 percent when conditions favor the disease; however, it poses a more significant threat to grain quality and animal and human health. Fusarium head blight can reduce test weight, and the FHB fungus produces mycotoxins (such as deoxynivalenol; aka DON, vomitoxin, VOM) which contaminate grain, increasing the likelihood for discounts or rejection of entire loads of grain at the point of sale.  

Historically, FHB epidemics were infrequent and limited to small, isolated areas in the United States and Canada. Since the mid-1990s, FHB has emerged as one of the most significant diseases affecting small grain production, with epidemics occurring at higher frequencies and over a larger area than in the past. Proposed reasons for this change include increased corn acreage, use of no-till agriculture practices, and expansion of small grain crops into areas that are more conducive for disease. The FHB fungus survives well on grasses and grass-crops, and causes Gibberella ear rot and stalk rot of corn. Consequently, increased prevalence of corn residue in the region can result in elevated inoculum levels and greater FHB potential.  

This publication describes the symptomology and disease cycle of FHB, diseases, and disorders with similar symptoms and provides management strategies to reduce FHB in small grain crops.

Symptoms and Signs

Symptoms of FHB on wheat spikes (heads; Figure 1) are most often visible after flower initiation (anthesis; Feekes 10.51; Zadoks 61). Infected spikelets (segments of head) prematurely die, becoming bleached (brown or straw-colored) while non-infected spikelets remain green (Figure 2). Entire heads may be killed prematurely as the fungus colonizes tissues within the head. As FHB progresses, brown to gray areas may develop along the stem beneath the heads (peduncle). Seed will become shriveled, discolored or may not develop at all. Signs of the fungus (sporodochia) develop when wet, humid conditions persist late in the growing season. Orange to salmon-pink spore masses may be first observed at the attachment of the small grain head to the stem, spikelet attachment to the rachis, or sometimes along the awns (Figure 3). Severely infected Fusarium infected kernels (scabby; tombstones) will be lightweight, chalky, pink to red, and often blown out with chaff during harvest. However, “later” infections by the FHB pathogen may result in little to no symptom development of the kernels, yet still have elevated concentrations of DON (Figure 4).  

In barley, FHB symptoms begin as brown water-soaked lesions on the spikelets and may expand to adjoining spikelets (Figure 5). Susceptible varieties may have entire heads infected but spread of the fungus within the heads of barley occurs less frequently than in wheat. Signs of the fungi causing infection in barley are similar to wheat, with salmon-pink to orange spore masses (sporodochia) occurring on the head. Symptoms on rye often begin as premature bleaching of spikelets, while non-infected parts of the head remain green (Figure 6). Symptoms on oat result in premature bleaching of spikelets on the panicle branch (Figure 6). Salmon-pink spore masses can be observed on infected tissue, especially during periods of high humidity.

Under extremely favorable conditions, black fungal fruiting bodies (perithecia) may be observed on both wheat and barley and are easily observed on corn residue (Figure 7). These black structures usually develop later in the infection cycle, becoming more visible as harvest approaches.

Disease Cycle

Residue from small grain crops and corn are important sources of inoculum (spores) for initial infection. Infections by the FHB fungus take place during prolonged periods of warm, wet, and humid weather (48 to 72 hours) during the flowering growth stages in wheat. Two spore sources can infect and cause FHB. Sexual spores (ascospores) from residue are the primary source of inoculum and are wind and rain-dispersed to open flowers in small grain crops. Asexual spores (conidia) from host residue do not travel long distances and are dispersed by rain splash. Regardless of the source, spores land on the flowers and germinate; subsequently, the fungus grows into the developing kernel. Depending on the level of resistance in the variety of the small grain crop, the fungus may continue to colonize other kernels in the head. If the environment remains conducive, the fungus continues to grow and sporulate resulting in pale pink or salmon-colored masses (sporodochia). The fungus will cause kernels to shrivel and eventually be under-developed, or the FHB-fungus can colonize the outside of the kernel with no obvious symptoms yet result in the production of DON.

Conditions that Favor Disease

Warm, humid, and wet conditions favor the development and spread of FHB in small grain crops. When these weather conditions are present prior to and during the anthesis period, risk of infection by the FHB-pathogen can be high. If weather is dry and humidity low, risk for infection during anthesis is reduced. Once infected, moist conditions can promote spread of FHB within the head. 

Yield Losses and Impact

Yield losses as a result of FHB primarily occurs when kernels do not develop or are severely shrunken and lightweight. The yield losses vary depending on variety susceptibility, small grain market class, and disease intensity. Often the greater concern is economic reductions due to quality loss, including reduced grain test weight and contamination of small grain lots with DON and other mycotoxins. Elevated concentrations of DON greater than 1 or 2 ppm can lead to price discounts, rejection, or may impose additional costs (i.e., use of cleaning equipment). 

Diseases, Disorders, and Injury with Similar Symptoms

Management

Variety Resistance

Although no varieties have complete resistance to FHB, many moderately resistant varieties are available in some wheat classes. Planting moderately resistant wheat varieties is the most important practice in reducing losses due to FHB and DON. Resistance in barley is often described based on differences in DON accumulation. Please check with your local land grant university Extension office for information on FHB resistance in small grain varieties.

Cultural Practices

The fungus that causes FHB also infects other grass crops such as corn. For this reason, the risk of FHB is greater when small grain crops follow each other or a corn crop. The fungus is able to survive longer on corn residue when compared to small grain residue. This is especially true if corn had noticeable Gibberella stalk and/or ear rot. Longer rotations between small grain and corn crops are encouraged to promote residue decomposition. Lower residue levels can result in lower levels of inoculum of the FHB fungus present in the field. 

Fungicides

Fungicides are available for management of FHB in small grains. Although some fungicides have good efficacy against FHB and DON, an adequate level of disease control may not occur when susceptible varieties are grown and conditions favor an epidemic. Therefore, it is important to use fungicides as part of an integrated FHB management strategy (e.g., paired with variety resistance, cultural practices, etc.). Application method and timing impact control. Fungicide application should be directed to the head at the timing of flowering (anthesis) using appropriate nozzle types and water volumes. 

Fungicide timing

In wheat, the window of opportunity for effective control of FHB is between the start of flowering (Feekes 10.51; Zadoks 61) and up to about seven days after the start of flowering. Keep in mind to follow information on the label and to follow pre-harvest interval requirements.

Applications of fungicides for control of FHB in barley may vary depending on spring barley vs. winter barley, specific variety planted, and geographical region.  In general, a fungicide application to barley for FHB management should be made at complete head emergence (entire head visible) or up to 7 days after full head. 

Fungicide class

Fungicides primarily in the demethylation inhibitor (DMI; aka triazole; Fungicide Resistance Action Group (FRAC) group 3) class and one in the succinate dehydrogenase inhibitor (SDHI; FRAC group 7) have been the most effective against FHB and DON. From university research trials conducted across multiple states, the SDHI fungicide is as effective as some of the best DMI fungicides for management of FHB. Quinone outside inhibitor (QoI; aka strobilurin fungicides; FRAC group 11) applied at the boot stage or beyond should be avoided for FHB control as they can increase the concentration of DON in finished grain. 

The “Fungicide Efficacy for Control of Wheat Diseases” publication is updated annually and can help guide decisions on which fungicides may provide the best control of FHB (CPN-3002-W; available on the Crop Protection Network). A US web-based FHB risk assessment tool (http://www.wheatscab.psu.edu) and a Canadian DON forecasting tool (https://www.weatherinnovations.com/doncast.html) are available to help make decisions about fungicide applications. 

If a seed-lot was confirmed to be infected with F. graminearum, the use of a fungicide seed treatment is encouraged to reduce the likelihood of seedling blight and Fusarium foot and root rot caused by the same pathogen.  

Harvest and Post-Harvest Management

At harvest, a properly adjusted combine can help reduce the amount of scabby kernels that end up in the harvested grain. Adjusting a combine’s fan speed has been reported to help remove moldy and lightweight kernels from the grain load and subsequently reduce DON concentrations to limit discounts at the grain elevator. Segregate any poor quality grain from good quality grain. 

Grain affected by FHB must be stored properly to prevent additional deterioration on quality. Grain should be cooled and dry before being stored. Temperatures of less than 50 F (10C) and less than 15% moisture are ideal for short-term storage. For long-term storage grain moisture should be below 13%. If grain is known to be moldy and/or affected by FHB, it should be tested for mycotoxins before being used for food products or fed to animals. 

Find Out More

Other publications in the Small Grains Disease Management series are available on the Crop Protection Network website (cropprotectionnetwork.org).

Acknowledgments

Authors

Andrew Friskop, North Dakota State University; Gary Bergstrom, Cornell University; Carl Bradley, University of Kentucky; Nathan Kleczewski, University of Illinois, Urbana-Champaign; Juliet Marshall, University of Idaho; Damon Smith, University of Wisconsin-Madison; Albert Tenuta, Ontario Ministry of Agriculture, Food, and Rural Affairs; and Kiersten Wise, University of Kentucky.

Reviewers

Tom Allen, Mississippi State University; Mary Burrows, Montana State University; Martin Chilvers, Michigan State University; Travis Faske, University of Arkansas; Darcy Telenko, Purdue University; Bob Hunger, Oklahoma State University; Trey Price, LSU AgCenter; Daren Mueller, Iowa State University; Emmanuel Byamukama, South Dakota State University; Kaitlyn Bissonnette, University of Missouri; Pierce Paul, Ohio State University; Alyssa Kohler, University of Delaware; Paul Esker, Pennsylvania State University; Stephen Wegulo, University of Nebraska-Lincoln; and Alfredo Martinez-Espinoza, University of Georgia.

Photo Credits

All photos were provided by and are the property of the authors and contributors.

Sponsors

This project was funded in part through Growing Forward 2 (GF2), a federal-provincial-territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario. 

The authors thank the United States Wheat and Barley Scab Initiative and the Grain Farmers of Ontario for their support. Design and production by Purdue Agricultural Communication.