Fusarium Head Blight in Wheat

Key Points

• Fusarium head blight (FHB), also known as head scab, can be found in many wheat-growing areas across the United States and Canada.
• FHB is a problem in wheat (including durum), barley, oats, and other small grains, and can also be found in wild and tame grass species.
• Several species of Fusarium fungi cause symptoms of FHB.
• Fusarium graminearum is the primary causal agent of FHB.
• Fusarium spp. can produce many types of mycotoxins, poisonous compounds produced by fungi.
• The major toxin produced by FHB is deoxynivalenol (DON), which is also called vomitoxin.

Introduction

Fusarium head blight is one of the most important fungal diseases of wheat around the world and is currently the most economically important wheat disease in the U.S. and Canada. Yield losses to FHB in wheat can often range from 10 to 70% but can reach 100% in years with a severe epidemic. Yield losses are due to sterile heads and small, shrunken, low test-weight kernels. Mycotoxin contamination is the primary quality concern associated with Fusarium head blight, as consuming mycotoxin-contaminated grain can be harmful to humans and livestock. The risk of FHB epidemics increases if many viable spores are present in the environment and if frequent rainy days with high humidity and warm temperatures occur when the crop is flowering.¹

Identifying Fusarium Head Blight

The first visible signs of Fusarium head blight infection show up three weeks after flowering (Feekes stage 10.5.1) or during grain fill (Feekes stage 11.1) as a bleached section of the head, usually in the middle of the first part of the head to start flowering. The bleached section can then move up and down the head as additional spikelets are infected. Sometimes, with close observation, pink or orange spore masses can be seen at the base of the bleached spikelet. As the crop matures the kernels in the bleached area will look chalky, shriveled, and may be white to pink in color. These shriveled, lightweight kernels are called tombstone kernels. Tombstone kernels can be highly contaminated with mycotoxins but are often blown out of the combine during harvest.²

Lifecycle

The pathogen that causes FHB overwinters in the crop residue of small grain cereals and corn. Warm, moist spring weather promotes the formation of spores on the infested crop residue, which can be carried by wind and rain to the wheat plants. Initial Fusarium head blight (FHB) infection occurs during the flowering growth stage (Feekes stage 10.5.1), when anthers appear on the outer middle third of the head. FHB risk increases when the relative humidity level is 70% or higher for 36 hours or more and air temperatures are between 68 and 77 °F when the wheat crop is flowering. Infections can also be caused by planting infested seed, as the pathogen can infect the emerging seedlings. This type of seed infection can cause seedling blight but does not usually lead to FHB. However, infested seedlings can result in infested residue that can cause FHB in subsequent wheat crops.³

Mycotoxins

While poisonous mycotoxins are often found in wheat that has been infected with Fusarium head blight, not all FHB infected wheat will contain mycotoxins since favorable environmental conditions are necessary for these toxins to be produced.

Many different mycotoxins are associated with wheat. These mycotoxins can be very toxic to humans and livestock, so it is important to keep all mycotoxin-contaminated grain out of the food chain for both humans and livestock. For example, humans who consume FHB-infected wheat can develop symptoms such as headaches, food poisoning, abdominal pain, and diarrhea. Livestock can experience inhibited weight gain—which shows up as an absence of body fat and muscle wasting—along with hindered conception and abortion. It is important to know that more than one mycotoxin can be found in a wheat source and these mycotoxins can have a pre- and post-harvest source. The only way to know if mycotoxins are present is to test for them.⁴

There are many laboratories that can test for mycotoxins across the U.S. and Canada, and costs range from $25 to $100 per sample depending on the mycotoxin panel requested and the turnaround time. Rush services will add to the cost of the sample. There are also test kits available for on-farm testing for specific mycotoxins. When sending a sample to a testing facility, follow the lab’s sampling instructions.

The following advisory levels for vomitoxin or DON are quoted directly from the FDA’s 2018 guidance document, Guidance for industry and FDA: Advisory levels for deoxynivalenol (DON) in finished wheat products for human consumption and grains and grain by-products used for animal feed.

1. 1 ppm vomitoxin (DON) on finished wheat products, e.g. flour, bran, and germ, that may potentially be consumed by humans. FDA is not stating an advisory level for wheat intended for milling because normal manufacturing practices and additional technology available to millers can substantially reduce DON levels in the finished wheat product from those found in the original raw wheat. Because there is significant variability in manufacturing processes, an advisory level for raw wheat is not practical.
2. 10 ppm DON on grains and grain by-products (on an 88% dry matter basis) and 30 ppm in distillers grains, brewers grains, and gluten feeds and gluten meals derived from grains (on an 88% dry matter basis) destined for ruminating beef and feedlot cattle older than 4 months and ruminating dairy cattle older than 4 months, with the added recommendations that the total ration for ruminating beef and feedlot cattle older than 4 months not exceed 10 ppm DON, and the total ration for ruminating dairy cattle older than 4 months not exceed 5 ppm DON. For chickens, 10 ppm DON on grains and grain by-products with the added recommendation that these ingredients not exceed 50% of the diet of chickens.
3. 5 ppm DON on grains and grain by-products destined for swine with the added recommendation that these ingredients not exceed 20% of their diet.
4. 5 ppm DON on grains and grain by-products destined for all other animals with the added recommendation that these ingredients not exceed 40% of their diet.

This guidance has been prepared by the Division of Plant and Dairy Food Safety in the Center for Food Safety and Applied Nutrition and the Office of Surveillance and Compliance in the Center for Veterinary Medicine at the Food and Drug Administration (FDA).

The total ration includes grains, all grain by-products including distillers and brewers grains, hay, silage, and roughage.⁵

Management

It is too late to make management decisions to control FHB once the bleached heads appear in a wheat field. The infection occurred during the flowering growth stage and the damage is done but harvesting and grain storage decisions can still be made. While harvesting the grain, special attention should be given to combine settings—including using enough air to remove the light, FHB-infected tombstone kernels from the heavy, healthy grain—can help reduce the potential for mycotoxin contamination in the harvested grain. Store grain from FHB-infected wheat separately from grain produced in uninfected fields. Finally, test all grain for mycotoxins so that FHB-damaged grain can be kept out of markets where high levels of mycotoxins can cause problems.

More management options are available to deal with crops in the following years in areas where FHB has been identified, including:

• Plant FHB-tolerant wheat varieties (moderately resistant varieties are available).
• Fungicide application at early flowering (Feekes stage 10.5.1), but applications up to 7 days after initiation of flowering can reduce FHB and mycotoxin contamination in the grain.
• Application of triazoles (FRAC group 3) fungicides. Always follow the label for specific application instructions including pre-harvest intervals.
• Prosaro® fungicide provides excellent control of head diseases such as scab (Fusarium head blight).
• Do not apply overhead irrigation to wheat that is flowering.
• Rotate away from small grain, corn, and sorghum crops for several years.
• Do not plant wheat after corn.
• Use tillage to decompose small grain, corn, and sorghum residue.
• Plant high-quality, FHB disease free seed.³

Additionally, the Fusarium Risk Assessment Tool can be used to conduct a Fusarium head blight risk assessment for regions across the U.S.

Sources

¹Kamran Khan, M., Pandey, A., Athar, T., et al. 2020. Fusarium head blight in wheat: contemporary status and molecular approaches. Biotech. 10(4): 172. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7080935/

²2019. Fusarium head blight of wheat. Crop Protection Network. https://cropprotectionnetwork.org/encyclopedia/fusarium-head-blight-of-wheat

³Fusarium head blight – Overview. Albert.ca. https://www.alberta.ca/fusarium-head-blight-overview

⁴Ji, X., Jin, C., Xiao, Y., et al. 2023. Natural occurrence of regulated and emerging mycotoxins in wheat grains and assessment of the risks from dietary mycotoxins exposure in China. Toxins. 15(6): 389. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10300805

⁵2018. FDA. Guidance for industry and FDA: Advisory levels for deoxynivalenol (DON) in finished wheat products for human consumption and grains and grain by-products used for animal feed. Guidance Document. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/guidance-industry-and-fda-advisory-levels-deoxynivalenol-don-finished-wheat-products-human

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