Assistant Professor OSU Extension Putnam County

After 2005 and the threat of Asian soybean rust, many companies and producers began regularly applying fungicides as an insurance policy. Applying fungicides when no disease symptoms are present is not a good best management practice. Not only is money wasted but some fungi are now becoming resistant to the general broad spectrum fungicides that were applied.

Fungicide application to soybeans as they enter R3 (reproductive growth stage) were promoted because companies claim their product increases yields by reducing drought stress and increasing plant photosynthetic activity. Dr. Karen Wise, a Purdue University plant pathologist has been investigating these claims. Dr. Wise says, “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.” “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,” Dr. Wise adds, “Many foliar diseases struggle to develop in hot, dry weather, so this year’s excessive heat and drought have kept disease pressure low.” Wise said 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.