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Frogeye leaf spot. photo by OSU Extension.

Frogeye leaf spot

By Matt Hutcheson, CCA, Product Manager, Seed Consultants, Inc.

Frogeye leaf spot is a disease that can impact soybean yields across this eastern Corn Belt.. Typically, more prevalent in the southern growing regions, the disease can occur farther north as a result of weather favorable to its development.

The fungus that causes Frogeye leaf spot (Cercospora sojina) survives in infected plant debris and can cause infections in growing plants when weather conditions are favorable. Frogeye leaf spot lesions produce spores that are easily transported by wind, acting as inoculum for leaf infections on other plants. The disease is promoted by warm, humid weather and will continue to develop on infected plants during patterns of favorable weather. With the warm and wet weather patterns that have existed in the eastern Corn Belt during 2017, it is expected that frogeye would be observed in some fields.

Frogeye leaf spot symptoms begin as small yellow spots that become larger lesions with gray centers and dark reddish-purple or brown borders. Younger leaves are more susceptible to growth of lesions than older leaves, therefore, new plant growth will be impacted by disease development. In some cases, lesions can develop on the soybean stem and pods.

Eastern Corn Belt soybean growers have several options for managing frogeye leaf spot. Because residue is a significant source of inoculum, burying infected residue with tillage can reduce the amount of inoculum present in a field. Crop rotation can also help in minimizing the amount of inoculum present. In this Purdue Fact Sheet, university experts recommend rotation away from soybeans at least 2 years in fields where infections occurred. Starting the growing season by planting quality, pathogen-free seed. Although residue is believed to be the most significant source of inoculum, seed infected by the pathogen will lead to infections of the plant during the growing season. Growers should work with seed company sales staff and agronomists to choose varieties with strong resistance to frogeye leaf spot. Finally, if disease infections become severe and have potential to greatly reduce yield, growers may need to apply a fungicide at the R3 stage of growth to minimize yield losses.

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