Last updated: August 24. 2013 6:59PM - 134 Views

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WEST LAFAYETTE, Ind. - Applying fungicides to soybeans free of foliar disease problems isnít likely to help alleviate drought stress, and could contribute to fungicide-resistant diseases, says a Purdue Extension plant pathologist.

Soybean growers likely 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.

"Weíve done research on fungicides in the absence of disease for several years now at Purdue. What weíve found is that when we donít have disease pressure there - foliar diseases such as frogeye leaf spot or Cercospora leaf blight - we donít often see an economic benefit from a fungicide application," Kiersten Wise said.

"We know that with soybean prices what they are, that benefit would be something to really capitalize on this year. But we just donít 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 yearís excessive heat and drought have kept most at bay in fields that arenít irrigated and havenít had much rainfall.

In addition to the lack of profitability, 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.

"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 said. "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 Extensionís Aug. 3 Pest and Crop Newsletter in Wiseís article "Fungicide Applications in Soybean - Risk vs. Reward" at http://extension.entm.purdue.edu/pestcrop/2012/issue19/index.html#fungicide.

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