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Fusarium Head Blight (FHB) Risk and Fungicide Questions

Last week’s widespread rains provided much-needed relief across much of North Dakota and Minnesota. It immediately increased questions about the risk for Fusarium head blight (scab). A one or two-day rain event does not automatically increase our chances of scab in small grains. We are recovering from multiple weeks of hot temperatures, sporadic rainfall, low relative humidity, and very infrequent dews. Like most fungal pathogens, the fungus that causes scab too needs rain and/or high relative humidity for the development of fruiting bodies and, eventually spore releases that can cause initial infections of scab in wheat and barley. The previous weeks greatly reduced the probability of these events occurring in most areas in both states. Therefore, we think of last weekend’s rain as priming the pump that started the scab disease cycle by providing the moisture needed to initiate the development of fruiting bodies that eventually will lead to spore releases. 
If you watched the scab risk models over the weekend and in the first half of the week you have probably noticed that models have already started to trend higher in many locations. Understand that the models are solely predicting to risk of initial infections occurring and do not predict the development of the fruiting bodies. Without those spores being released by the fruiting bodies those infections cannot occur. So what does this all mean? It means that we will likely see the risk of scab infections increase this weekend and into the beginning of next week. The risk will be moderate to high for the most susceptibility varieties (FHB score of 6 to 9), while the moderately resistant to resistant varieties (FHB score of 3 to 5) will likely remain at low risk. The fields that flowered last week, this past weekend, and at the beginning of this week more than likely escaped the elevated risk window. The fields that will be heading and/or flowering at the end of the week into next week will have a higher risk of scab infections now that the pump has been primed, especially varieties that are more susceptible. 

Leaf Disease Management - Applying a fungicide for FHB will also present an opportunity to protect the flag leaf from fungal leaf spots, particularly tan spot and/or Stagonospora nodorum blotch. To date, the fungal leaf spot pressure had been very low for the same reasons the scab risk was low. It is unlikely these diseases can reach economically damaging levels before the end of the grainfill period. The only disease that is likely to increase and reach economically damaging levels after these weather patterns is bacterial leaf streak. Unfortunately, this disease cannot be controlled with a fungicide. 

Yield Response - A recent PhD graduate (LeAnn Lux – now an Extension Plant Pathologist at North Carolina State University) in the plant pathology department conducted research from 2019-2021 and explored the response of fungicides on reducing FHB and protecting yield in dryland environments. This North Dakota Wheat Commission-funded research provided us a great opportunity to understand the yield response on two hard red spring wheat varieties (ND-VitPro and WB-Mayville) with very similar yield potential, but that differed drastically in their resistance to scab. A total of seven field trials were conducted and were categorized into three FHB disease environments (Very High, Low, and Very Low). The yield data from the research are summarized below (Table 1). The yield response of fungicide depended on the variety's susceptibility and the level of scab risk. The largest yield responses were observed in the very high FHB environment in the susceptible variety WB-Mayville. In the very low FHB environment, no differences were observed among any of the fungicides and the non-treated check on WB-Mayville. For the moderately resistant variety ND-VitPro, there were no statistical differences but a similar trend in the data with a greater risk for scab resulting in a greater yield response when a fungicide was used. 

Table 1- Grain yield of ND-VitPro and WB-Mayville in a North Dakota Wheat Commission Funded trial exploring the yield response among commonly used fungicides for management of scab. 

The Bottom Line – The risk of scab will be increasing over the next week. The decision to spray is more difficult this year than most years for two reasons. First, much of the region needs additional rain to alleviate the drought stress the crop will be under and that will limit the yield potential (this is the period that the crop uses more than a ¼” of water per day). Secondly, the fungicides will likely not provide the yield bump that we see most years from controlling the leaf diseases during the grainfill period. If you are so lucky to have adequate soil moisture and have caught a few more showers and feel confident that you still have yield potential, base your decision on the susceptibility of the variety to scab and the risk level indicated by the risk model. Use Prosaro, Prosaro Pro, Sphaerex, or Miravis Ace if the risk is high or very high and the variety is rated susceptible or very susceptible. Consider using generic tebuconazole if the variety is rated moderately resistant or better and the risk models indicate low to moderate risk

Authors: Andrew Friskop, NDSU Extension Plant Pathologist, and Jochum Wiersma

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