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Growers need to weigh the cost of treatment against benefits PDF Print E-mail
Thursday, January 24, 2013 3:06 PM

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.

“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.” According to Wise, soybeans and corn need a level of disease greater than 5 percent for it to be economical for fungicide application.

So the same logic applies to corn. “Unless a corn crop is at risk of developing fungal diseases, farmers would be smart to skip fungicide treatments that promise increased yields,” Says Dr. Wise, “Fungicides used in fields where conditions were optimal for fungal diseases improved yields and paid for themselves. In fields where fungal diseases are unlikely to develop, however, applying a fungicide is likely a waste of money.

“About five years ago, we never used fungicides in hybrid corn. Then there was this push to use fungicides for yield enhancement, even without disease problems,” said Wise, “We found you would have to get a substantial yield increase for a fungicide treatment to pay for itself. We didn’t see that yield increase on a consistent basis, and it wasn’t predictable.”

Wise and her collaborators reviewed published data from foliar fungicide tests in 14 states, as well as data from their own research, to determine which circumstances led to yield increases when using fungicides. They found that a combination of several factors would contribute to fungal disease - no-till fields, fungus-susceptible hybrids, continuous corn, wet field conditions, etc. - needed to be present for yields to increase with fungicide applications.

Growers need to weigh the cost of the treatments - usually $32-$34 per acre - against the possible benefits and the price of corn. At low corn prices, it takes a larger yield increase to recoup the cost of a fungicide application. Wise said the general rule at today’s prices is that growers need to see a yield increase of about five to six bushels per acre to recover their investment. The probability of losing money on a fungicide application was 60 to 98 percent of the time if the disease level was less than 5 percent.

For more information see Wise’s article “Fungicide Applications in Soybean - Risk vs. Reward” at http://extension.entm.purdue.edu/pestcrop/2012/issue19/index.html#fungicide . The University of Kentucky also has a good fact sheet: “Foliar Fungicide Use in Corn and Soybeans” at: http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-MISC-5.pdf

 

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