Cercospora leaf blight has become more prevalent in Iowa. Yield losses due to this disease are common in the southern United States, but serious losses have not been reported despite widespread distribution of the disease in the North Central region. Diseased plants are usually widespread within a field. Foliar symptoms usually are seen at the beginning of seed set and occur in the uppermost canopy on leaves exposed to the sun. Affected leaves are discolored, with symptoms ranging from light purple, pinpoint spots to larger, irregularly shaped patches typically only on the upper leaf surface. As disease develops, affected leaves may become leathery and dark purple with bronze highlights. Symptoms may be confused with sunburn. Discoloration may extend to the upper stems, petioles and pods. Infection of petioles and severe symptoms may lead to defoliation of the uppermost leaves and give the appearance of a maturing crop. However, petioles of fallen leaves remain attatched to the stem, and lower leaves of the plant remain green. Symptoms of purple seed strain are distinct pink to dark purple discolorations of seed. Discolored areas vary in size from small spots to the entire surface of the seed coat; however, infected seeds may not show symptoms.
The fungus survives winter in infested crop residue and infected seed. Most early season infection of foliage and pods later in the season. Warm and wet weather is favorable for infection. Foliar symptoms are the result of an interaction between a toxin produced bny the fungus and sunlight. Weather conditions during flowering and plant maturity will affect the incidence of purple seed stain. Despite being caused by the same organism, there is no consistent relationship between the occurrence of Cercospora leaf blight and purple seed stain.
The best time to scout for this disease is R3 through R6. Check plants in areas of field where moisture collects from extended dew periods.
Variety selection: There are commercially available varieties resistant to Cercospora leaf blight. However, there are no known sources of resistance for purple seed stain.
Fungicides: Foliar fungicides are registered for Cercospora leaf blight. Application made during pod-filling stages can reduce the incidence of purple seed stain, but may not affect soybean yield.
Crop rotation and tillage: Rotation to non-host crops such as alfalfa, corn and small grains and tillage to bury infested crop residue will reduce pathogen levels. If considering tillage, use proven conservation practices to maintain soil quality.
Photo by Daren Mueller
A Summer 2009 Update on Diseases in Iowa Soybean