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To Spray or Not Spray: Assessing FHB Risk

By Mitchell Japp, PAg, Provincial Specialist Cereal Crops, Regina and Treena Lake, University of Manitoba Summer Student, Regina

Fusarium head blight (FHB) is still present in Saskatchewan. Although it was not the most significant downgrading factor in 2019, it consistently shows up on the annual field surveys and is detected on seed samples sent to testing labs. It takes more than just presence of the pathogen to lead to infection. Infection will occur when the three sides of the disease triangle are complete:

  • Presence of a virulent pathogen
  • Presence of a susceptible host
  • Presence of suitable environmental conditions

While all cereals are susceptible, durum wheat and spring wheat are the most susceptible. Some wheat varieties are more susceptible than others, but none are immune. Durum has very little resistance compared to other wheats, but it has improved over the past decade or so.

Two sides of the triangle are complete. What about the third? Environmental conditions are the tough one to evaluate. FHB is not like leaf spots or rusts where we can wait until we observe symptoms. FHB is a mono-cyclic disease, which means that it does not have multiple cycles of infection in the same year. Once the symptoms appear it is too late to treat and cannot be reversed, so scouting for symptoms isn't effective. FHB is favoured by warm and moist conditions including high relative humidity. Fungicide management for FHB must be based on whether or not environmental conditions are conducive to the disease.

Fungicides alone cannot manage FHB. FHB management must be part of an integrated pest management plan that includes crop rotation, the use of the best available genetic resistance, and the timely application of fungicide.

Earlier FHB outbreaks led Sask Wheat to work with Weather Innovation Network (WIN) to generate FHB risk maps. These risk maps are based on validated models that show the risk of FHB developing in a region, based on weather variables such as temperature, precipitation and relative humidity.

Fusarium Heat Blind Risk Map
Fusarium head blight risk map

The risk maps show current and future risk for both winter and spring wheats that are at or approaching heading (the models are different for winter and spring wheat). Farmers and agronomists can check the risk maps daily leading up to head emergence to assess their crop risks and decide whether or not applying a fungicide is warranted.

Wheat and durum are susceptible from the time the head emerges from the boot until soft dough stage, but losses are highest when infection occurs earlier, with anthesis most susceptible. No fungicides are registered for use after anthesis.

The risk maps alone should not determine if a fungicide should be applied. Scouting in your own fields may lead to a different decision than the risk maps suggest. The "wet pants test" is a simple way to do this. Walk through your field, if your pants get went, the conditions in your field may be favourable for disease development. Using the wet pants test along with the FHB risk maps from Sask Wheat and the history of disease in your field will help accurately assess your FHB risk and guide fungicide application decisions.

Consider other sides of the disease triangle, then, estimate the potential value of the crop you're working to protect. In addition, other diseases, such as leaf spots, that may be effectively controlled by the application of a fungicide at FHB timing.

Fusarium Risk Table

Sounds complicated? Why not just spray it to be sure? Unfortunately, like herbicide resistance, fungicide resistance is a possibility. Until 2020, there has only been one register and effective fungicide group (Group 3 – triazoles) registered for and effective on FHB. A new product, Miravis Ace, with active ingredients pydiflumetofen and propiconazole from Groups 7 and 3, has been introduced to the market. This new product will help prevent building fungicide resistance, if it is used appropriately. Prophylactic fungicide applications that are not necessary will lead to fungicide resistance. Additionally, fungicides are expensive and will not generate return unless the conditions warrant their use.

Once the decision to spray has been made, check out the article Considerations When Spraying Fungicide for Fusarium Head Blight to get the most out of your fungicide.

The University of Manitoba and collaborators are working on a project, entitled "Developing a Risk Model to Mitigate Fusarium Head Blight in Western Canadian Cereal Production," which was introduced in the first issue of CPN. This future risk model by predicts both FHB and mycotoxin levels for several cereal crops with a range in FHB resistance levels that is consistent across the Prairies. Producers will be able to enter their own information, including crop variety, seeding date, and location to produce a customized FHB risk based on information provided by the nearest weather station.

Each year, cereal crops in Saskatchewan, Alberta, and Manitoba are monitored for symptoms of FHB throughout the growing season. Small check strips are left unsprayed by fungicide so the full severity of the pathogen can be observed. The FHB severity that is recorded in the field then can be compared to the predicted FHB severity generated by the weather model. This data will help verify the model's accuracy.

This five-year project is funded by the Western Grains Research Foundation, and supported by producers and collaborators across Western Canada. This new risk model is currently in its second year of development and should be publicly available within the next four years.

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