Frogeye Leaf Spot
Frogeye leaf spot is a common problem in the southern and central United States and can occur in some areas of the upper Midwest. It has been reported as far north as southern Wisconsin. The disease may cause severe defoliation during warm, humid weather. Frogeye leaf spot can be distinguished from other soybean foliar diseases by the reddish-brown or purple ring surrounding the round leaf spots.
Symptoms of frogeye leaf spot are most visible and typically seen on leaves, but can also occur on stems, pods, and seeds. Lesions on leaves begin as small, dark, water-soaked spots. They develop into brown spots surrounded by a darker reddish-brown or purple ring. The centers of the lesions turn light brown or light gray as they age. The center of spots may turn white with black specks visible (fungal fruiting structures) or the centers may fall away leaving a ‘shot hole’ appearance. The lesions may eventually merge, covering large areas of the leaves and resulting in defoliation.
Conditions and Timing that Favor Disease:
Frogeye leaf spot can occur at any time during the growing season, but seems to typically occur after flowering. Young leaves on the tops of plants tend to be most susceptible. Infection and disease development is favored by warm, humid weather.
Frogeye leaf spot is caused by the fungus Cercospora sojina. More than five races are known in the U.S. The pathogen overwinters in soybean residue and seeds.
To manage frogeye leaf spot, use resistant soybean varieties and pathogen-free seed. Rotate soybeans with a non-bean crop. Bury infested residue where feasible and where disease is severe. Foliar and seed treatment fungicides may provide some control.