Applying fungicides to soybeans free of foliar disease problems isn't likely to help alleviate drought stress and could contribute to fungicide-resistant diseases, says Kiersten Wise, a Purdue Extension plant pathologist. Soybean growers probably are feeling pressure to apply fungicides as the crop enters the R3 growth stage, regardless of disease presence, based on claims that the products can reduce drought stress, increase photosynthesis and, ultimately, increase yields.
But a series of Purdue University research trials has been unable to confirm those claims.
"We've done research on fungicides in the absence of disease for several years now at Purdue. What we've found is that when we don't have disease pressure there – foliar diseases such as frogeye leaf spot or Cercospora leaf blight – we don't often see an economic benefit from a fungicide application," Kiersten Wise says.
"We know that with soybean prices what they are, that benefit would be something to really capitalize on this year. But we just don't see a consistent response, so it makes it very hard to recommend those fungicides in the absence of disease."
Many foliar diseases struggle to develop in hot, dry weather, so this year's excessive heat and drought have kept most at bay in fields that aren't irrigated and haven't had much rainfall.
In addition to the lack of profitability, Wise says applying unnecessary fungicides also could lead to fungicide-resistant diseases. One example is frogeye leaf spot, a major disease of soybeans that already has resistant populations in five Midwestern and Southern states.
"One of the big drawbacks to using fungicides for these plant-health benefits is that when we use the same mode of action over and over again, we select for fungicide-resistant strains of the fungus," Wise says. "Our standard recommendation is that fungicides should be applied only when foliar disease pressure is potentially yield-limiting."
More information and research results are available via Purdue Extension's Aug. 3 Pest and Crop Newsletter in Wise's article Fungicide Applications in Soybean - Risk vs. Reward.