Andrew Friskop, Extension Plant Pathologist at NDSU, shared my concern that the risk models were underestimating the risk of FHB last week. Since the weekend the models have started to trend towards moderate to high risk across much of North Dakota and Northwest Minnesota. Below are his thoughts and my two cents about the current risk situation, a summary of the efficacy of the different fungicides, and some thoughts about if and when to spray fungicides in lodged grain.
Risk Models
This past weekend’s weather for most of the state presented conditions of high humidity, rain, and prolonged dews. These three factors greatly contribute to scab risk and now an elevated scab risk exists for small grains (click here to see the most recent maps). According to the NDSU Small Grain Disease Forecasting Model, the greatest scab risk exists for spring wheat varieties that are rated as very susceptible and susceptible to Fusarium head blight varieties. This includes varieties like AP Murdock, SY Longmire, WB 9479, and WB9590. However, there is still a moderate risk for the spring wheat varieties that are rated as moderately susceptible and moderately resistant in parts of North Dakota (and Minnesota).
When looking at the immediate forecast, high humidity levels will be sporadic amongst the days, yet prolonged morning dews are still likely to occur at least a couple times this week. Given this past weekend’s weather and the forecast, scab risk will likely remain elevated for this week.
Fungicide Efficacy
There are several labeled fungicides that provide “good”
scab suppression of scab. These include
Caramba®, Proline®, Prosaro®, Prosaro Pro®, Miravis Ace®, and Sphaerex®. Fungicides
with the sole active ingredient of tebuconazole are rated as “fair”. The active ingredient propiconazole
has “poor” efficacy on scab. To put this in terms of percentage reduction of
scab and deoxynivalenol (DON/VOM), “good” fungicides provide about 45-60%
suppression, “fair” fungicides may only offer 20-25% suppression, and “poor” fungicides
provide only about 10% suppression. Understand that tank mixing fungicides does not mean that the efficacy of the tank mix is the sum of both products. In other
words, adding tebuconazole to Caramba does not increase the level suppression
of scab to 80% or more. Finally, remember that the premixes that do contain
tebuconazole are to create a product that provides good to excellent control of
other fungal pathogens, including tan spot and leaf rust.
Lodged small grain and fungicide effectiveness
The weekend storms unfortunately caused some lodging in small grain fields. This raises the question of if and how to spray fungicides on lodged grain. Small grains, including spring wheat, generally adhere to the ‘three strikes and you’re out' rule. Once the grain lodges - especially if it lodges immediately after stem elongation - individual stems will erect themselves by pumping water into the cells that are on the shaded side of the nodes. After two lodging events or as the plant approaches physiological maturity, the cells in those nodes lose the ability to stretch any further, and the grain will remain lodged.
Fungicides do not move great distances on plant tissue and coverage of the spike is key to suppressing scab and DON. Therefore, the best approach is to wait for the grain to partially straighten up. Luckily, recent studies have shown that the window of application is a bit wider than previously thought. While the optimum timing remains Feekes 10.5 for barley and Feekes 10.51 for spring wheat, the labeled fungicides rated good for scab can be applied up to 7 days after the optimum timing while maintaining adequate efficacy.
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