Cercospora leaf spot is caused by Cercospora kukuchii. The disease is easy to identify by a mottled purple-to-orange discoloration of the uppermost leaves. The leaves also have a leathery appearance. In September, when soybean plants are approaching maturity, infected leaves turn orange or bronze (pictured on the left).
The causal fungus for this disease is a very close relative of the one that causes frogeye leaf spot (Cercospora sojina) and like frogeye leaf spot, it is seedborne. The infected seed have a purple discoloration, resulting in a disease call purple seed stain that is easily recognized (pictured below).
Management of Cercospora
Seed infection by either Cercospora fungus may cause poor seed vigor and reduced germination. Beans with substantial amounts of discoloration should not be saved for seed. However, there is currently no quantitative measurement to use as a threshold.
Tillage and crop rotation are effective ways to reduce the survival of these fungi from season to season on infested soybean residues. Rapid decay of infested residue prevents a build-up of the pathogen and potentially greater infection in the next soybean crop.