Frogeye leaf spot has become more prevalent in Iowa. It is especially problematic in continuous soybean fields. Diseased plants are usually widespread within a field. Early season infections from infected seed result in stunted seedlings. On leaves, lesions are small, irregular to circular and gray with reddish-brown borders that most commonly occur on the upper leaf surface. Lesions start as dark, water-soaked spots that vary in size, and as lesions age, the central area becomes gray to light brown with dark, red-brown margins. In severe cases, disease can cause premature leaf drop and will spread to stems and pods.
Symptoms on the stems are no as common or distinctive as foliar symptoms and appear as narrow, red-brown lesions that turn light gray with dark margins as they mature. Lesions on pods are circular or oval shaped and are initially red-brown and turn light to gray with a dark brown margin. Seed close to lesions on pod can be infected. Infected seeds have light to dark gray discolored blotches that vary in size and cover the entire seed in severe cases. The seed coat often cracks.
The fungus survives in infested crop residue and infected seed. Early season infections contribute to infection of foliage and pods later in the season. Warm, humid weather promotes spore production, infection and disease development. Young leaves are more susceptible to infection than older leaves, but visible lesions take two weeks to develop after infection. It is common for disease to be layered with the the canopy. This is a result of little to no infection during dry periods and higher levels of infection during wet or humid weather.
The best time to scout this disease is R3 through R6; after frequent rains. Check plants in areas where moisture collects from extended dew periods. Found more frequently in continuous soybean fields.
Variety selection: Resistant varieties are available and should be used where disease is a potential problem. Several races of the pathogen have been identified, and varieties with resistance to all known races are available.
Crop rotation and tillage: Rotation and tillage will reduce survival of Cercospora sojina. Crops no susceptible to this pathogen are alfalfa, corn and small grians. If tillage is considered to promote decay of crop residue, great care should be taken to minimize soil erosion and maintain soil quality.
Fungicides: Foliar fungicides applied during late flowering and early pod set to pod-filling stages can reduce the incidence of frogeye leaf spot and improve seed quality and yield.
Photo by Daren Mueller