Frogeye Leaf Spot (Soybean)

Black Rot

Cercospora sojina

Soybean leaf infected by Frogeye Leaf Spot
Soybean leaf infected by Frogeye Leaf Spot. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Frogeye leaf spot is a prevalent soybean disease in the southern U.S. but has been known to move throughout the northern states as well. It tends to affect younger leaves and results in lower quality yields for both seed and food-grade varieties.
Frogeye leaf spot is caused by a fungus that survives in warm, humid weather. It lives in infected seed and crop residue. Spores that infect the plants are usually windblown and can build up substantially in continuous soybean areas.
Infection can occur throughout the growing season, most often preceded by flowering. Initially, the symptoms are manifested as small yellow spots on the leaves. These spots begin to enlarge to about 1/4 inch, producing a gray or brown center and reddish or purple margins. Sometimes they grow together in large, irregular-shaped lesions. Heavily diseased leaves usually wilt, eventually dropping off. In some cases, the lesions develop on the seed pod or stem. Stem lesions are generally more red and darken with age. Pod lesions are slightly sunken in, sometimes infecting the seed itself. Lesions throughout the plant will grow as long as weather allows, greatly reducing the green leaf area and hindering growth.
Soybean leaf with frogeye leaf spot
Soybean leaf with frogeye leaf spot. Photo courtesy of Clemson University. Licensed under CC BY 3.0.
Overall yield losses in severely infected varieties have gone up to 30 percent. In cases where varieties are resistant and infection is minor, there is no economic damage. If the infection begins after flowering, there is a greater chance of it resulting in yield loss.
Tillage and crop rotation can be very beneficial in controlling frogeye leaf spot. In tillage, it's important that residue is completely buried. If infection does occur, it is best to rotate out of soybeans for at least one year, and then follow the nonhost crop with a resistant variety of soybeans.
Foliar fungicides are most effective in controlling frogeye leaf spot outbreaks. Applications are most effective when applied from R1 to R3. Fungicide application becomes very economical if applied before spots number greater than 2 every 25 feet. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
Soybean varieties have been developed that are somewhat resistant to frogeye leaf spot. However, it is important to select varieties that are more resistant to diseases with greater impact than frogeye leaf spot.