Frogeye leaf spot (Cercospora sojina)

Frogeye leaf spot: biology

This fungal disease is sporadic but may be severe in wet seasons or under hot, humid conditions – particularly in susceptible varieties. Young, expanding leaves are most susceptible, however, lesions are not seen on these leaves because visible lesions develop nearly two weeks after infection. Leaves that expand during dry weather remain relatively free of this disease.

The fungal spores are dispersed by wind and splashing water. When a high percentage of leaves are heavily infected, serious yield loss can occur.

Frogeye leaf spot can be difficult to diagnose correctly in the field, because it is easily mistaken for other diseases and disorders (such as herbicide injury).

The leaf lesions measure up to 5 mm in diameter, and they may merge to form larger spots. Young lesions have a grey to brown centre with a distinct purplish to reddish brown margin. As lesions age, the centre turns tan to nearly white and the margin darkens.

Disease symptoms can also occur on stems and pods late in the season. These symptoms are more difficult to identify; stem lesions appear elongated, while pod lesions appear oblong and resemble foliar symptoms. If pods are severely diseased, seeds can also become diseased and present light purple to grey discolouration; it is also possible that infected seed may not show any visible symptoms at all.