If the wet weather that forced many growers to plant soybeans later than planned continues, frogeye leaf spot may be a significant problem this year, Koenning adds.

Frogeye has caused yield losses of 30 percent in some fields, so the general recommendation for susceptible varieties is the application of a strobilurin type fungicide, especially if continued wet and/or humid weather is expected. 

“We do not have a threshold for number of spots or percent leaf area affected to justify fungicide application. 

“If wet and/or humid weather persists as plants start to senesce, older leaves become susceptible again, and the plant may defoliate early,” Koenning says. 

“Early defoliation can result in smaller seeds which will translate into yield loss.  Also, pod infection can cause a reduction in seed quality or contribute to seed rot,” he adds.

Another tricky disease that many growers in the Upper Southeast will likely deal with this year is cercospora leaf blight. Whether it is actually a disease or an endophyte needed to help fight insects is not entirely clear, Koenning says.

“It’s hard to get a handle on cercospora,” Koenning says.

In some states, like Louisiana, researchers contend it is their No. 1 disease in soybeans. Closer to home Virginia Tech Plant Pathologist Pat Phipps says it may be Virginia’s No. 1 disease pest of soybeans.

Cercospora leaf blight is caused by a pathogen closely related to a pathogen that causes frogeye leaf spot. Cercospora leaf blight of soybeans is caused by the fungus Cercospora kukuchii, a close relative of Cercospora sojina that causes frogeye.

This late-season leaf and seed disease shows up in during pod-filling stages. It is relatively easy to identify by a light purple to bronze discoloration that resembles a sunburn on the uppermost leaves.

The coloration deepens and takes on a leathery appearance as the plants approach maturity.

Seeds can also be infected, resulting in a seed disease called purple seed stain. If infected seeds are planted, the fungus grows through the seed coat to the cotyledons and into the stem.

The key to managing cersospora blight in soybeans is to detect the disease very early, Koenning says. If growers wait until the R3 stage or later to spray, fungicides typically won’t keep the disease in check, he adds.


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