Fusarium head blight (FHB) is considered to be one of the most devastating diseases of wheat and barley worldwide. Multiple outbreaks of FHB have affected Delaware growers over the past decade, most recently in 2013. In Delaware, FHB is caused mainly by the fungus Fusarium graminearum (also called Gibberella zeae). This fungus can infect heads of wheat and barley, resulting in significant yield loss. Toxins produced by this fungus are harmful to humans and livestock and may contaminate grain. This fact sheet describes how to identify FHB, the pathogen disease cycle, mycotoxins, and FHB management recommendations.

Disease Identification

The hallmark symptom of FHB is a partial to complete bleaching of heads that occurs shortly after flowering (Figure 1). Unaffected portions of the head remain healthy and green. Over time the fungus grows into the rachis and nearby spikelets. If the weather is warm and humid, orange to pink masses of spores are visible at the base of infected spikelets . The fungus eventually infects kernels, causing them to shrink and wrinkle. Infected kernels, called “Tombstones” are lightweight and discolored.

Disease Cycle

Fusarium graminearum grows well on decaying plant material, particularly that of wheat, barley, and corn . The fungus overwinters on crop residues and produces spores under periods of wet weather (>90% RH; 59-86⁰F) [1]. Spores are forcibly dispersed into the air, where they may travel great distances [2]. Infection occurs when spores land on susceptible wheat and barley heads. Heads are most susceptible at early flowering (Feekes 10.5.1) and infection may occur up to the soft dough stage (Feekes 11.2), although severity is greatly reduced. If the flowers are infected just after their emergence kernels will not develop. Florets that are infected later will produce tombstones. Kernels that are infected by the pathogen during late kernel development may appear healthy, but be contaminated with mycotoxins (discussed in the following section). Fusarium crown and root rot can develop in the following season if infected seed is planted.

Mycotoxins

Fusarium graminearum and other species of Fusarium can produce chemicals called mycotoxins, which are harmful to humans and livestock [3]. Mycotoxins are believed to help protect the fungus and aid in the infection process. The primary mycotoxin associated with FHB is deoxynivalenol (DON). Human consumption of wheat or wheat products contaminated with high levels of DON may result in flu –like symptoms, including fever, headaches, and vomiting. Livestock, particularly hogs, may refuse to eat contaminated grain, which reduces weight gain. DON has less of an effect on cattle and poultry. The FDA has established advisory levels for DON (Table 1). Growers attempting to sell grain exceeding the thresholds for DON contamination will often be “docked” or in severe cases, may have their grain loads rejected. It is important to note that the presence of FHB does not guarantee high test levels of DON; however, the presence of tombstones or discolored kernels in harvested grain may indicate elevated levels of DON in a load.

Table 1. Deoxynivalenol (DON) Advisory Levels established by the FDA.
Maximum Allowable DON Level	Consumer
1ppm	Humans
5ppm	Swine and all animal species (except cattle and poultry). Not to exceed 20% diet for swine and 40% for other animals.
10ppm	Ruminating beef and feedlot cattle older that 4 mounts and poultry. Not to exceed 50% of diet.

Disease Management

There are several management practices that can be helpful in reducing losses caused by FHB [4]. These practices are aimed at reducing disease risk and include: 1-Resistant cultivars 2-Cultural practices, 3-Chemical controls. Growers will experience the greatest benefits when multiple practices are used together instead of alone and should never rely on a single management practice to control FHB.

Resistant Cultivars

Universities and industry continue to conduct research on the development and use of resistant cereal cultivars for FHB management [4]. Resistance to FHB is not complete, meaning that resistant cultivars still can be infected, but disease progression is often greatly reduced as is DON accumulation. 30-50% reduction in DON is fairly common when comparing a susceptible variety to a moderately resistant variety although results vary. Thus, simply planting a cultivar with resistance does not guarantee complete protection against FHB. Currently several moderately FHB resistant cultivars are available to Delaware growers. Seed companies and dealers can provide additional information on FHB resistance for specific wheat cultivars. Information on performance in Delaware growing conditions can be found in the Delaware small Grains Variety Trials, which are conducted yearly.

Cultural Practices

Residue Management

Fusarium graminearum grows well on decaying plant residues, particularly that of corn, wheat, and barley. Tillage buries residues below the soil surface, allowing for their decomposition. In addition, burial of residues reduces the amount of inoculum available for spread [5]. The increase in no-till or minimal tillage fields is believed to be a major contributor to recent regional scab epidemics. Although residue management may reduce local inoculum levels, spores dispersed from other fields may serve as a source of pathogen spores.

Crop rotation

There is some evidence that Fusarium graminearum does not grow as well on residues of some crops (e.g. soybean) compared to corn, wheat, and barley [5]. Planting wheat after soybean may help to reduce the level of local inoculum. As with residue management, aerial dispersal of spores from residues of nearby fields may serve as a source of disease.

Chemical controls

Suppression of FHB can be achieved by the timely application of fungicides to wheat and barley [4]. Several triazole (group 3) fungicides are now available for FHB management, but vary in the amount of protection they provide. If applied properly to susceptible varieties, these fungicides provide roughly 50% FHB/DON control, although this amount increases with the use of moderately resistant cultivars. Research indicates that the most effective timing for these fungicides is when approximately 30% of wheat heads have reached early flowering (Feekes 10.5.1). Current research indicates that fungicide sprays can be made up to 5 days post-flowering and still achieve acceptable levels of disease suppression. Fungicides containing a strobilurin (group 11) active ingredient should not be used. These fungicides cannot be applied at early flowering and have been shown to increase DON levels in grain. Research has shown that the use of a moderately resistant variety combined with the use of a triazole fungicide for suppression of FHB provide significantly greater reduction in DON than either method alone.

A FHB forecasting model has been developed and is available to growers online (www.wheatscab.psu.edu). This forecasting system estimates the risk of a FHB epidemic (greater than 10% field severity) using environmental data observed 7 days prior to flowering. Tests of this model indicate that it can correctly predict FHB outbreaks 75% of the time. This information can help growers determine if a fungicide is needed. The model is only one source of information available for FHB management decisions. Factors such as cultivar resistance, cultural practices, and local weather forecasts are equally important in determining the need for a fungicide spray for control of FHB.

Date Published: 05/2014

Author(s): Nathan Kleczewski Ph.D.

Extension Plant Pathologist

University of Delaware Department of Plant and Soil Sciences

References

Parry, D.W., P. Jenkinson, and L. McLeod, Fusarium ear blight (scab) in small-grain cereals-A review. Plant Pathology, 1995. 44(2): p. 207-238.
Schmale, D.G., III, et al., Genetic structure of atmospheric populations of Gibberella zeae. Phytopathology, 2006. 96(9): p. 1021-1026.
Schmale, D.G. and G.P. Munkvold, Mycotoxins in crops: A threat to human and domestic animal health. The Plant Health Instructor, 2009.
McMullen, M., et al., A Unified Effort to Fight an Enemy of Wheat and Barley: Fusarium Head Blight. Plant Disease, 2012. 96(12): p. 1712-1728.
Dill-Macky, R. and R.K. Jones, The effect of previous crop residues and tillage on Fusarium head blight of wheat. Plant Disease, 2000. 84(1): p. 71-76.

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