Larry Dunkle, USDA-ARS plant pathologist at Purdue University, points out that gray leaf spot is normally considered “a serious disease” in the Corn Belt, adding that minimum- and no-till production has likely added to the amount of disease pressure corn and beans can face.

“Tillage operations that reduce the amount of surface residue from the previous crop should decrease disease severity,” says Dunkle. “But spores of the cercospora fungus can be carried long distances by wind, so the inoculum doesn’t necessarily come only from the previous year’s crop.”

Carl Bradley, University of Illinois plant pathologist, adds that cercospora can cause several diseases in soybeans, including frogeye leaf spot and cercospora blight. “Fungicide products that contain a strobilurin are generally effective for control of these diseases,” he says. “In research trials we've conducted, strobilurin fungicides have reduced frogeye leaf spot severity.”

He notes that when comparing yields of treated corn or beans with a check plot or plots with a different rate of fungicide, growers should have more than one strip for each treatment so that trials are replicated. “It’s important to look at replicated trials to make sound observations,” he says. “There can be a lot of variability in a field.

“In my studies, if you have a lot of disease pressure, you can get good control of gray leaf spot with products that contain a strobilurin fungicide. We only see the 10-20-bu. differences in yields when there is pretty good disease pressure out there. For the most part, the disease has to be there before we see the big consistent yield increase (from fungicide treatments),” says Bradley.