Frogeye leaf spot (FLS), caused by the pathogen Cercospora sojina, is a common soybean foliar disease of many soybean-producing regions worldwide. In the U.S., the disease is established in southern production regions and has recently become prevalent in the Midwest and Upper Midwest. It’s believed that the range expansion and increased disease severity are caused by widespread planting of susceptible varieties, warmer winter temperatures, and the increased adoption of conservation tillage practices, which, together, lead to increased inoculum levels. FLS does not always cause yield loss, but yield loss of up to 60% has been reported with severe infection rates.

FLS overwinters in infected residue, primarily residue left on the soil surface; infected seeds can also carry the disease. FLS infections occur in warm, moist conditions, mainly on new leaves, because older leaves are less susceptible to disease development. Rain splashing spreads spores from the ground to young leaves. The disease is polycyclic, meaning that multiple infection cycles can occur within the same year because the FLS disease lesions produce spores. If warm, moist, or rainy conditions persist, new infection of young leaves can occur as long as conducive weather continues. Storms and wind can spread spores throughout a field and to new fields. Hurricanes move spores great distances and result in northward movements of the disease.

Symptoms typically occur from growth stage R1 to R6 in soybeans, though symptoms can occur at any time. FLS causes small, up to ¼ in., lesions that are circular or slightly irregular in shape. The lesions begin as small, gray, water-soaked spots and develop into tan or gray necrotic lesions with red to brown borders. Sometimes, the lesions can be surrounded by a chlorotic halo. Lesions are thin and can leave holes in the leaves. In highly susceptible varieties or severe infections, lesions may join together to form larger lesions on the leaves. In rare and extreme situations, pod and stem infections have been observed. Severe infections may result in premature leaf drop. Seeds may be infected when lesions occur on the pods. Gray blotches may appear on infected seeds, or the disease may be asymptomatic on the seed. Since FLS infects new tissue growth, it is common to fi nd symptoms in one layer of the canopy (i.e., on the 6th trifoliate) but not on leaves above or below that point if favorable conditions for disease development did not continue. When favorable conditions persist, symptoms will continue to appear on new leaves and exist in multiple layers of the canopy.

Frogeye leaf spot overwinters in infested soybean residue. Crop rotation out of soybeans for at least one season will reduce inoculum. Tillage will bury residue and decrease inoculum; however, neither of these will completely mitigate the risk of infection. Since seed can be infected, it is import ant to plant certified seed that is free of infection and to use a comprehensive seed treatment.

Resistance genes have been identified for FLS. Planting resistant varieties is one of the most cost-effective means of controlling FLS. If FLS is a known problem, plant a variety that is rated high for resistance to FLS.

Applying fungicides that are labeled for control of FLS is an effective strategy to control this disease. FLS has shown widespread resistance to the Qol or strobilurin group of fungicides (FRAC code 11). It’s recommended to apply a fungicide with at least two effective modes of action to prevent resistance development. Beck’s Practical Farm Research (PFR)® has identified the R3 reproductive stage as the most economical time to apply a foliar fu ngicide. In a study conducted by The Ohio State University, fungicide applications were economical when one to two lesions were present in 25 ft. of the row at R2.

https://www.extension.purdue.edu/extmedia/bp/bp-131-w.pdf

https://www.apsnet.org/edcenter/disandpath/fungalasco/pdlessons/Pages/FrogeyeLeafSpot.aspx

A Farmer’s Guide to Soybean Diseases. 2016. The American Phytopathological Society and the Crop Protection Network.