August 2007

Fusarium head blight (FHB), also known as scab, is a fungal disease of small grain cereals that has become of increasing international importance in recent years. FHB can affect wheat, barley, oats, rye, corn, triticale, canary seed and some forage grasses. In Saskatchewan, durum, spring wheat, and barley are most affected by this disease. FHB is caused by several species of the fungal pathogen, Fusarium. The most important species is Fusarium graminearum, since it can result in the most yield loss and is responsible for the production of toxin in the grain.

FHB results in a reduction in yield, grade and end-use quality. Further losses to producers have occurred because of restricted crop rotations, limited variety selection, cost of control measures, as well as reduced marketing opportunities. It is important that producers are familiar with this potentially damaging disease and incorporate management practices to reduce FHB development in their crops.

Cereal head infection by fungal spores is favoured by moist, warm conditions during flowering. Symptoms may occur on the entire head or on just a few spikelets and can result in the formation of fusarium damaged kernels (FDK). FDK are typically shrivelled, light in weight, and white or pink in colour. It is because of these attributes they are named "tombstone kernels".

FHB causes reduction in yield due to shrunken, lightweight kernels that are lost during combining or seed cleaning. More importantly, grade losses occur when the percentage of FDK in a sample exceeds grading tolerances.

The Canadian Grain Commission allows only very low levels of FDK in food and feed grains and zero levels in malting barley. The tolerances were set to account for the presence of potentially harmful fungal toxins, called mycotoxins, produced in diseased grain.

The most common mycotoxin found in grain affected by FHB is deoxynivalenol or DON. DON can reduce the feed intake by livestock and adversely affect the baking quality of wheat and the malting and brewing qualities of malt barley.

In addition to mycotoxin concerns, FHB-infected seed often has reduced levels of germination and seedling vigour. Before using cereal seed for planting, it is recommended to have seed tested for percentage infection and to determine which Fusarium species are present in the seed. Infected seed may lead to the spread of F. graminearum into new regions. 

On the Canadian Prairies, FHB was first identified in Manitoba in 1923 but did not cause concern until the mid-1980’s. The first major outbreak in Manitoba was in 1993 when severe yield and quality losses occurred. In Saskatchewan, Fusarium graminearum was first detected in samples from a few wheat fields grown near the Manitoba border in 1993. In Alberta, FHB has been detected in irrigated wheat fields, but only at trace levels elsewhere.

Annual disease surveys are being conducted in Saskatchewan to determine where FHB is a problem and to monitor its spread. Survey results indicate that FHB is present in most regions of the province. Although F. graminearum is found every year, it has not become the dominant species causing FHB. F. graminearum is mostly confined to the south-eastern region of the province.

Surveys conducted by the Canadian Grain Commission provide evidence that the more aggressive F. graminearum is moving westward through the Prairie Provinces Scientists do not fully understand why F. graminearum is becoming the dominant species, but it is likely a result of several factors including warming weather patterns, a change in the aggressiveness of the pathogen, more intensive farming practices and the movement of infected seed.

Although FHB is not expected to increase to epidemic proportions in Saskatchewan, cereal producers need to be aware of its impending spread and the potential loss to quality and yield. Effective control of the spread of FHB is possible if management strategies are implemented and care is taken to use good quality seed, especially in areas considered free of FHB.

Fusarium fungi can over-winter as spores or mycelium on seed and crop residue. Seed infected with the Fusarium fungi may have reduced vigour and become infected upon emergence, resulting in seedling blight. Some Fusarium species can also survive on roots of other crops, including pulses and oilseeds.

Studies have shown that many of the Fusarium fungi found in kernels from diseased heads of wheat and barley were also found in the subcrown internodes of cereal plants collected from the same field. This confirms that disease inoculum on crop residue may serve as a source for head infections later in the season.

Development of Fusarium spores is favoured by warm and moist conditions. Spores are spread by rain-splash and wind and can come in contact with above ground parts of cereals. FHB infection is most likely to occur during July when the florets are open during flowering, allowing the spore to come in contact with the floret. The FHB fungus can also enter through wounds caused by hail, birds or insects.

Precipitation or high humidity for at least 12 hours is required for spore germination and infection. Temperatures favouring infection range from 16 to 30°C, with the optimum range for F. graminearum being 25 to 28°C. If conditions remain warm and moist after seed set, the pathogen may continue to spread to other kernels or heads and produce higher levels of mycotoxins.

F. graminearum has the ability to form a sexual spore stage on over-wintering cereal residue. New fungal strains with increased pathogenicity may develop as a result of the sexual reproductive cycle. In addition, the sexual spores can become air-borne and disperse with the wind over greater distances compared to those spores produced during the season.

Symptoms in the field usually become apparent by the end of July or early August. FHB is recognized as premature bleaching of one or more of the spikelets on the head. This symptom is quite striking on green heads.

Symptoms first begin as water-soaked brownish spots at the base of the glumes and ultimately glumes become bleached in colour. If humid conditions prevail, masses of white, pink or orange spores may form along the base of the glumes or over the infected head.

FHB symptoms can develop within three days of infection if conditions remain moist and warm. Heads infected at or shortly after flowering will display the greatest damage, although infection can still occur on the mature head if conditions favour sporulation.

Severity of damage to the kernel is dependent on the time of infection. Early infection will likely result in kernel loss, whereas later infections may not exhibit symptoms but are still capable of harbouring the fungus. FDK in wheat are the most obvious, whereas, those in barley and oats are often more difficult to detect.

In wheat, rye and triticale, severe FDK are typically shrunken and chalky white. However, in hulled barley and oats, symptoms are less apparent since the kernels may not be shrunken, but orange or black fungal growth may develop on the hull. In hulless barley, both types of symptoms can occur.

If FDK are suspected, it is recommended to have seed tested for F. graminearum before planting or tested for levels of DON before feeding to livestock. FDK can be confused with similar symptoms caused by other diseases, saprophytic growth, midge damage, or environmental injury.

In wheat, rye and triticale, severe FDK are typically shrunken and chalky white. However, in hulled barley and oats, symptoms are less apparent since the kernels may not be shrunken, but orange or black fungal growth may develop on the hull. In hulless barley, both types of symptoms can occur.

If FDK are suspected, it is recommended to have seed tested for F. graminearum before planting or tested for levels of DON before feeding to livestock. FDK can be confused with similar symptoms caused by other diseases, saprophytic growth, midge damage, or environmental injury.

Mycotoxins are toxic, secondary metabolites produced during the fungal infection process. Mycotoxins are not destroyed during processing such as milling, baking, malting, or ethanol production. Deoxynivalenol or DON is the most common mycotoxin associated with FHB in Canada and is produced in high amounts by F. graminearum. Compared to other mycotoxins, DON is one of the least toxic but can still result in reduced feed consumption or feed refusal, especially when fed to non-ruminants. DON concentration can be determined from a crushed grain sample in a laboratory and is measured as parts per million (ppm).

Livestock vary in their tolerance to DON so it is important to have infected feed grain tested for levels of DON. Agriculture and Agri-Food Canada has developed guidelines for DON intake in livestock. The DON limit for swine, dairy cattle and horses is one ppm and the limit for beef cattle, sheep and poultry is five ppm. DON is not considered a problem in straw used for feed or bedding.

There is a zero tolerance for DON in malting barley. Producers should consult their nearest grain company and ask for the latest Canadian Grain Commission’s grading tolerances and for laboratories capable of testing for DON.

Seed Quality: Plant clean, vigorous seed. Commercial seed laboratories can test cereals to determine the percentage and species of Fusarium in seed intended for planting. It is recommended that producers not plant F. graminearum infected seed in regions where F. graminearum is not already established. Seed infected with less than five per cent F. graminearum can be planted back into regions where this species is established as there will be disease inoculum present in the region, e.g. infected cereal residue in neighbouring fields. 

Studies have shown that FDK have decreased seedling emergence and tillering. Hence it is important to not use seed with high levels of Fusarium (even if its not F. graminearum) and/or to use a seed treatment.

Seed Treatment: The use of seed treatments will limit seedling blights caused by seed and soil-borne pathogens, including Fusarium species. Seed treatments will not prevent FHB from developing later in the season from stubble-borne disease inoculum in regions where Fusarium species are already established.

For more information on seed quality and seed treatments, refer to the factsheet Guidelines for Seed-Borne Diseases of Cereals.

Crop Rotation: Practice a rotation away from cereal crops for at least one year, preferably two years. Avoid seeding cereals into wheat or barley stubble and avoid planting adjacent to fields which had known levels of FHB in the previous year.

For those producers growing corn, be aware that corn in rotation with small cereal grains has been shown to increase FHB. Control grassy weeds, which may harbour the disease, between cereal crops.

Stubble Management: Cereal residue will enable the fungus to over-winter and act as a source of disease inoculum for the following year. Effective chopping and spreading of straw and chaff will encourage decomposition. Burying the residue through tillage will also speed up its decomposition but may not completely eliminate the risk since all residue is not buried or may be brought to the surface again in subsequent tillage operations.

Variety Selection: There are currently no varieties with true resistance to FHB; however, cereals vary in their susceptibility to FHB. Overall, durum and CPS wheat are the most susceptible to FHB. Winter wheat is susceptible, but often escapes infection because it is flowering before Fusarium spores are present. Within the wheat class there are varieties that have improved resistance and these varieties should be considered in FHB-infected regions of the province. Barley is less susceptible than wheat, but barley can still develop significant levels of FHB. Overall, oats are the least susceptible to FHB, but because they are often used for food processing, there is a very lower tolerance for FDK.

Refer to the Varieties of Grain Crops for FHB ratings.

When harvesting cereals, adjust combine air velocities and openings to blow out lightweight kernels. If this practice is done, do not plant a cereal crop for two years in that field as F. graminearum has been found to survive on seed over-wintering in the soil.

Cleaning the grain after harvest to remove shrunken lightweight kernels may also reduce grade losses and DON concentration. However, even after cleaning out the severely infected kernels, there may still be some DON present in the grain. Prevent additional grading losses from occurring in the bin by storing at less than 14 per cent grain moisture content. Grain with known levels of FDK should be stored in a separate bin.

Foliar Fungicides: Foliar fungicides are registered for the suppression of FHB, however, results are not always consistent or economical. It is too late to apply fungicides once symptoms are observed. Instead, fungicide must be applied at early flowering to protect the opening florets. There is no forecasting system for FHB in cereals in Saskatchewan.

A fungicide application may be warranted if:

• The pathogen is established in a region. This includes: (1) If the grower has experienced losses due to FHB in the past two years, or (2) F. graminearum has been isolated from seed samples at greater than five per cent, and (3) If planting next to infected residue from the year before or in a field that had durum or wheat in rotation within the last two years; AND,
• The conditions have been wet and warm at the crop heading stage and the forecast is for continued favourable conditions during cereal flowering; AND,
• The expected yield return justifies the cost of application. 

Research is continuing to develop effective fungicides and improve sprayer technology for the control of FHB. Studies have indicated that FHB control is improved with high water volumes and sprays directed both forward and backward (double nozzles). Refer to the Guide to Crop Protection for current information on the fungicides registered for FHB.

   Related publications:

• Guidelines for Seed-borne Diseases of Cereal Crops
• Variety of Grain Crops
• Guide to Crop Protection

• For more information, contact:  Agriculture Knowledge Centre: 1-866-457-2377
• Provincial Crop Protection Laboratory: (306) 787-8130

Canadian Grain Commission: http://www.grainscanada.gc.ca/information/fhb-e.htm

Crop Diseases Council: http://www.cropdiseasescouncil.ca/

Diseases of Field Crops in Canada, 3^rd Edition. Editors: K.L. Bailey, B.D. Gossen, R.K. Gugel, and R.A.A. Morrall.  University Extension Press, Saskatoon, SK., ISBN 0-9691627-6-6 (http://www.cps-scp.ca/dfccfrontcover.htm)