Frogeye leaf spot is caused by Cercospora sojina, and is a close relative of the fungus that causes Cercospora leaf spot and purple seed stain (Cercospora kukuchii) . Frogeye leaf spot is becoming more prevalent in Iowa and is common in soybean fields in late summer.
The disease can be recognized by small, gray spots with reddish-brown borders on the upper leaves in mid to late August. In severe cases, the disease can cause premature leaf drop and spread to stems and pods. The fungus infects seed, and consequently, the seed coat of infected seeds turns gray.
As with other Cercospora and Phomopsis leaf spots, this disease can be seedborne and it is important to not save seed from fields with severe infestation. A severe infestation would be considered one in which seed discoloration at harvest is extensive, or heavy defoliation occurred. Seed quality should not be affected in fields with light infection, which is more commonly the case.
Management of Frogeye Leaf Spot
The frogeye pathogen survives in infested soybean residue. Therefore, tillage and crop rotation are very effective means of reducing the population from season to season. The earlier a tillage operation is made, the more effective it is in reducing the pathogen population. It is best to till infested residues directly after the soybean harvest, rather than after a subsequent corn rotation.
Some soybean varieties are less susceptible to frogeye leaf spot than others. Consider a resistant variety if a field has a history of the disease.