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Fusarium head blight: Assessing risk before spraying?

By Mitchell Japp MSc, PAg, Provincial Specialist, Cereal Crops; and Barb Ziesman PhD, AAg, Provincial Specialist, Plant Disease

Wheat crops are most susceptible to fusarium head blight at flowering. Learn how to assess FHB risk and optimize the effectiveness of fungicides in your crop.

Management of fusarium head blight (FHB) does not come down to one decision or action. It is a cumulative effect of many different decisions and actions taken throughout the year. Using a good crop rotation, growing resistant varieties, and chopping and spreading straw effectively are excellent tools, but they cannot be effectively implemented at this time of year. In-crop FHB management comes down to making a good decision on whether or not to apply a fungicide for FHB suppression and how to effectively apply it.

Why bother with the decision at all? Why not just spray and not worry about it? Spraying fungicides is costly and will not provide a return on investment if the conditions are not suitable for FHB. And, because all FHB fungicide options are in the same group, good resistance management practices are to only use those products when necessary.

What makes a good decision? FHB is a mono-cyclic disease, which means that there is only one infection cycle per season and disease management needs to occur prior to infection and the presence of symptoms. Scouting for symptoms can be helpful for harvest management, but it will not be helpful for in-crop management as fungicides need to be applied preventatively before infection occurs. Susceptibility starts at head emergence from the boot and continues until the soft dough stage, but most losses occur when infection is earlier and no fungicides are registered for application after anthesis. Though the infection window is quite long, wheat is most susceptible during anthesis (flowering); infection at this stage will result in the highest levels of fusarium-damaged kernels (FDK). When deciding whether or not to apply a fungicide, it is important to monitor the environmental conditions when the crop is most susceptible. FHB is favoured by warm, moist (including high relative humidity) conditions. Weather-based risk assessment models, such as the FHB risk maps from Sask Wheat, are valuable tools for assessing FHB risk and can be used to guide fungicide application decisions. When using these maps, it is important to accurately stage your crops and monitor the maps as crops are approaching the susceptible stage.

Weather-based risk assessment maps can help identify risk, but are regional and will not account for field-to-field differences. To make the most informed risk assessment for your farm/field, it is important to account for local conditions. To do this, monitor for local rain showers and high humidity. The "wet pants test" is a simple way to do this. Walk through your field; if your pants get wet, 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 help guide fungicide application decisions. Once crop stage and risk are established, balance these with the cost of application and the profitability of the crop. If this information is not enough to make a decision, consider test strips to aid in future decision-making.

Risk assessment chart for fusarium head blight.

Staging will be difficult in many areas due to early dry conditions followed by late rains that may lead to later tillers. Focus on the main heads, as the later tillers may not reach maturity and generally do not contribute as much to yield. Ideal timing in wheat is just as anthesis begins, but can begin as soon as 75 per cent of the heads (on the main stem) have emerged and can continue until 50 per cent of the heads on the main stems are in flower.

Fusarium head blight timing in wheat.

For barley, time your fungicide application for when the majority of the heads have emerged from the boot. The goal is to apply the fungicide in a way that protects as many of the heads as possible to protect from infections as they emerge.

Fungicides are registered for suppression of FHB, not control. Heads are a vertical target and harder to get full coverage compared to applying herbicide on a leaf that is perpendicular to the spray application. Considering those challenges, and the potentially devastating effect of FHB on a crop, taking the time to get the application done right is a good investment.

Tips from Sprayers 101 include:

  • Keep the water volume up – at least 10 gallons per acre.
  • Use twin nozzles (face forward and backward) to improve coverage on both sides of the head. If not twin nozzles, at least use angled spray nozzles.
  • Travel slow (too fast and those twin nozzles become less effective).
  • Keep boom heights low (again, too high and the effect of the twin nozzles is reduced).
  • Use coarse spray. Larger droplets maintain their trajectory longer than fine droplets, making the twin nozzles more effective. Also, awns intercept small droplets.

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