Fusarium Head Blight

Fusarium head blight (FHB), also known as scab, is a fungal disease of small grain cereals including wheat, barley, oats, rye, corn, triticale, canary seed and some forage grasses. In Saskatchewan, durum wheat, spring wheat, and barley are most affected by this disease.

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.

Fusarium sporulation and cereal head infection is favoured by moist, warm conditions during flowering. Symptoms may occur over the entire head or on just a few spikelets and can result in a poor yield and the formation of fusarium damaged kernels (FDK), also known as "tombstone kernels". These kernels are light weight and may be lost during combining or seed cleaning. Further economic losses occur when the percentage of FDK in a sample exceeds grading tolerances.

Fusarium head blight in Saskatchewan

Annual disease surveys are conducted in Saskatchewan to determine problem areas and to monitor its spread. Survey results indicate that FHB is present in most regions of the province. The most common species detected in the survey are:

• F. avenaceum,
• F. poae; and, unfortunately,
• F. graminearum , which poses a greater risk.

Within the F. graminearum population, there are two chemotypes known as 15-ADON and 3-ADON, the latter of which is more aggressive and produces a mycotoxin called deoxynivalenol (DON). Warming weather patterns, more intensive farming practices and the movement of infected seed create the potential for increases in FHB. Changes in the pathogen population also play a role in the incidence of disease. Multiple studies in Canada and the United States indicate that while 15-ADON used to be the dominant chemotype, 3-ADON, has been increasing for the last 20 years.

Disease Cycle and Conditions Required for Infection

Fusarium fungi can survive as saprophytes or act as pathogens on various crops. The fungi overwinter as spores or mycelium on seed and crop residue. Some Fusarium species can also survive on roots of other crops, including pulses (as in field pea in the photo below) 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 indicates that disease inoculum on crop residue may serve as a source for head infections later in the season.

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. 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 spores to come into contact with the floret. The FHB fungus can also enter through wounds caused by hail, birds or insects. If conditions remain warm and moist after seed set, the pathogen may continue to spread to other kernels or heads and produce high levels of mycotoxins.

Fusarium graminearum has the ability to form a sexual spore 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 long distances compared to those spores produced during the season.

Symptoms

Symptoms usually become apparent by the end of July or early August. Symptoms first begin as water-soaked brownish spots at the base of the glumes and ultimately glumes become bleached in colour. In wheat, FHB is recognized as premature bleaching of one or more of the spikelets in the head and may result in unfilled spikelets above the point of infection. This symptom is quite striking on green heads.

In barley, symptoms may be tan or dark in colour, with individual spikelets affected. 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 are moist and warm.

Severity of damage to the kernel is dependent on the time of infection. Infection at early flowering will likely result in kernel abortion, infection later in flowering will lead to FDK, whereas later infections (up to the soft dough stage) may not exhibit symptoms but are still capable of harbouring the fungus. In wheat, rye and triticale, severe FDK are typically shrunken and chalky white. 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.

Mycotoxins

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 (DON) is the most common mycotoxin associated with FHB in Canada. DON concentration can be determined from a crushed grain sample in a laboratory and is measured as parts per million (ppm).

Mycotoxins in Harvested Grain

Tolerance for DON in grain marketed as food and feed is low. Grain marketed as fuel may also need to meet low DON specifications because the by-product distiller’s grains are used as feed. The industry monitors delivered grain to ensure it does not exceed allowable tolerances for destination markets.

There is 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.

Livestock vary in their tolerance to DON so it is important to have infected feed grain tested for the concentration of DON. However, DON is not considered a problem in straw used for feed or bedding. For more information, see our webpage Fusarium Head Blight and Mycotoxins.

Fusarium graminearum and F. culmorum are both capable of producing DON as well as the related mycotoxins T-2 and HT-2. However, F. graminearum is more common and aggressive as a pathogen and produces higher concentrations of DON. Other Fusarium species may produce other toxins, including nivalenol (F. poae and F. sporotrichioides), moniliformin (F. avenaceum), and zearalenone (F. oxysporum, F. moniliforme, and F. equiseti).

According to North Dakota State University, after FHB-infected grain dries to about 22% moisture content, fungal growth and DON production stops. However, damaged wheat should be stored at or below 12% moisture, as the risk of deterioration is higher than with healthy kernels.

Management Strategies for Fusarium Head Blight

An integrated approach is needed for controlling the spread and development of FHB.

There is no relationship between FDK and per cent Fusarium infection, so the grade does not necessarily determine suitability for planting. Commercial seed laboratories can test cereals to determine the percentage of kernels infected and the species of Fusarium in seed intended for planting.

Because infected seed can lead to poor emergence, seedling blight, and reduced tillering, it is important not to use seed with high levels of Fusarium infection (even if it is 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.

Since F. graminearum is more aggressive as a pathogen for the development of FHB, seed infected with more that 5% F. graminearum is not recommended to be used in areas where this pathogen is not yet established. The best way to determine if F. graminearum is established is to look at your records and the records of producers in your area (for example, ask your neighbours). If seed harvested on your farm or within your area has not been infested with F. graminearum over the last few years you may want to consider F. graminearum infestation levels separate from total Fusarium infection when deciding whether to treat or use a particular seed lot (see the decision tree below).

It is also important to take seed quality and the presence of other seed and soil borne pathogens into consideration when deciding whether or not to apply a registered seed treatment.

*See Guide to Crop Protection for registered seed treatments for specific diseases.

Crop Rotation - Diversity in the rotation allows time for crop residues and the pathogens that survive on them to break down, reducing the risk to subsequent susceptible crops. When it comes to Fusarium, diseases such as root rots in other crops may decrease the benefit of the crop rotation to FHB. Although crop residues from pulses breakdown quicker than canola and mustard, both tend to persist and support pathogens longer than cereal crop residues. If FHB is the major disease issue, practice a rotation away from cereal crops for at least one year, and preferably two years. Avoid seeding cereals into wheat or barley stubble and avoid planting cereals adjacent to fields which had significant FHB infection in the previous year. Canola in rotation with small cereal grains has been shown to increase FHB severity. Control grassy weeds, which may harbour the disease, between cereal crops.

Stubble Management - Cereal residue will enable the fungus to overwinter and provide disease inoculum for the following year. Effective straw chopping and chaff spreading will encourage decomposition. Burying the residue through tillage will also speed 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. Although Fusarium species overwinter on crop residue, eliminating surface debris through extensive tillage or burning is not recommended, as it may have little effect on disease prevention and can leave soils vulnerable to erosion.

Variety Selection - Cereals vary in their susceptibility to FHB. Durum and CPS wheat are the most susceptible to FHB. Winter wheat is susceptible, but often escapes infection because it flowers before Fusarium spores are present. 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 low tolerance for FDK. Within some wheat classes there are varieties that have improved resistance and should be considered in FHB-infected regions of the province. Breeders are focusing research on unravelling the complexities of FHB resistance and there are now a few varieties with improved resistance ratings. Refer to the Varieties of Grain Crops for FHB ratings.

New tool to help producers manage fusarium!

Sask Wheat launched weather-based fusarium head blight (FHB) risk maps in 2015. The purpose of the risk maps are to keep producers updated, in near-real-time, about the risk of FHB in wheat in their respective areas of the province so that they may best plan their management strategies. The map runs from mid-June until mid-July (throughout the fusarium-susceptible periods for winter and spring wheat). WIN has access to more than 400 weather stations throughout Saskatchewan and uses internal resources for modelling and mapping, relying on its team of plant pathologists, agrologists, modellers, and IT/web specialists. The map uses near-real-time data from WIN's weather stations and is updated on a daily basis.

Please visit the Saskatchewan Wheat Development Commission to access the map.

Foliar Fungicides - Foliar fungicides are registered for the suppression of FHB, however, results are not consistent or may not be economical. Several factors should be considered when deciding whether to spray.

Consider the following checklist along with tools such as the FHB risk map when making management decisions.

Fungicide Resistance: Fungicide resistance has not been observed in Fusarium populations in Western Canada. Until 2020, the triazole group of fungicides (Group 3) was the only fungicide group registered for use on FHB. A new product became available in 2020 that is a pre-packaged mix of two active ingredients from Group 3 and Group 7. Over-reliance on one fungicide mode of action and unnecessary prophylactic use of fungicides can increase the risk of fungicide resistance. Applying fungicides at recommended rates, at recommended times and only when necessary for disease prevention will delay fungicide resistance development.

If you decide to spray: Note that foliar fungicides are registered for the suppression of FHB on wheat, rather than control. It is too late to apply fungicides once symptoms are observed. Instead, fungicide must be applied at early flowering to protect the opening florets. Flowering may be variable across the crop, but aim for the crop stage when at least 75% of the heads on the main stem are fully emerged to 50% of the heads on the main stem are in flower.

Research is continuing to develop effective fungicides and improve sprayer technology for the control of FHB. Vertical targets such as wheat heads are problematic when applying a fungicide to suppress fusarium head blight (FHB). Awns on durum and some hexaploid wheat varieties can further disrupt spray droplets before they reach their destination. Higher water volumes can help achieve adequate coverage when the part of the plant that needs the most protection is hard to reach. Always follow the minimum recommended water volume on the product label. Consider at least 10 to 20 gallons (45 litres to 90 litres) per acre for fungicide applications.

Dr. Tom Wolf, Application Specialist with AgriMetrix Research and Training, recommends the following spraying tips for FHB management:

1. Angle nozzles forward, or use double nozzles
2. The greater the angle of the nozzle the better
3. Use coarse sprays
4. Maintain low boom heights
5. Slow speeds are always recommended to ensure fungicides are applied correctly, especially if the spray needs to go deep into the canopy. However faster travel speeds are not as detrimental with FHB because the more horizontally the spray moves, the more droplets will stick to the heads.
6. Maintain over 10 to 20 gpa (45 to 90 litres) for fungicide applications

Refer to the Guide to Crop Protection for current information on the fungicides registered for FHB.

Harvest Management

When harvesting cereals, adjust combine air velocities and openings to blow out lightweight kernels. If this practice is used, 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% grain moisture content. Grain with known levels of FDK should be stored in a separate bin.

History

On the Canadian Prairies, FHB was first identified in Manitoba in 1923, but did not cause concern until the mid-1980s. The first major outbreak in Manitoba was in 1993, when severe yield and quality losses occurred. In Alberta the pathogen has been isolated with increasing frequency since 2001, especially in southern Alberta.

In Saskatchewan, Fusarium graminearum was first detected in samples from a few wheat fields grown near the Manitoba border in 1993.