Most of the currently grown soybean varieties are resistant to frogeye leaf spot, and the use of resistant varieties is the preferred method of control.
Although frogeye leaf spot is seedborne, it tends to be worse in fields of continuous soybeans.
Frogeye leaf spot is caused by the fungus Cercospora sojina.
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Only newly formed leaves are susceptible to this disease, and fully expanded leaves are resistant until they start to senesce.
Immature leaves become infected with periods of rain or high humidity, but infection will be limited by dry weather. So, as the soybean plants put on new layers of leaves, frogeye may be present or absent depending on weather conditions during leaf expansion.
This can lead to a situation where frogeye is layered in the canopy at different levels.
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.
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.
For more information on frogeye leaf spot, see http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy003/soy003.htm.
Soybean growers in Tennessee can find comments on the current frogeye leaf spot situation at http://news.utcrops.com/.