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Soybean disease shows resistance to fungicides in Iowa

Photo Courtesy of Iowa State University Extension and Outreach

AUDIO: Daren Mueller, ISU Extension

Researchers confirm isolates of frogeye leaf spot pulled from Iowa fields show resistance to a specific class of fungicides.

Daren Mueller serves as plant pathologist at Iowa State University Extension and Outreach. Mueller said researchers found frogeye leaf spot in several fields across in Iowa last year.

“In several of our locations, especially in the central to eastern half of the state, we had frogeye leaf spot show up,” Mueller said. “Not at high levels, but at levels high enough we took notice. We grabbed leaves from those fields and brought them back to the lab. We were able to tease out the fungus which causes the disease and collect several different isolates from these locations and tested them.”

Mueller adds researchers, from Iowa State University and the University of Kentucky, detected resistance to quinone outside inhibitor (Qol, stobilurin) fungicides within the tested isolates. He says the genetic diversity of frogeye leaf spot influenced this resistant tendency.

“We’re finding it in frogeye leaf spot pathogens,” Mueller said. “This pathogen is very genetically diverse. It’s sort of the canary. This is a warning or at least early indication of how we use these products.”

Mueller says growers must ask themselves, “Is this necessary?” before applying fungicides.

“Qol fungicides are one of the more effective classes of fungicides for most fungi,” Mueller said. “They’re usually part of premixes for a lot of different products. Having resistant strains in Iowa means people need to do a little more homework on what fungicide products they are selecting, if they are going to be spraying to control foliar diseases.”

Mueller notes other options available to growers facing frogeye leaf spot resistant to fungicides.

“Soybean breeders have done a great job of incorporating resistance to many of the soybean diseases including frogeye leaf spot. I would say start with a good variety which has high levels of resistance. We don’t usually have to worry about rotation. Rotating away from soybeans is a good option which will reduce the amount of inoculants, but most people are going to do that anyways,” Mueller said.

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