Frogeye Leaf Spot of Soybean

Frogeye leaf spot of soybean is a foliar disease found in soybean growing regions throughout the United States and first identified in North Dakota in 2020. To date, yield loss has not yet been reported in North Dakota, but southern U.S regions that frequently experience favorable conditions for disease development have reported yield losses greater than 30%. Consequently, scouting is important to monitor occurrence of the disease and to make informed management decisions if needed.

Frogeye leaf spot of soybean is caused by the fungal pathogen Cercospora sojina. The fungus can survive for at least two years as filamentous fungal growth, overwintering in infested soybean residue. It can also survive in infected seed, although seed transmission is rarely observed in the field. It is also believed that the fungus can overwinter in North Dakota, although the level of survival is unknown. Warm temperatures between 70 and 80 F and humid conditions promote spore production from the infested residue, leading to spread, infection and disease development. The infection process begins when fungal spores are disseminated to healthy plant tissue through wind and rain splash. Once spores infect tissue, small lesions will develop at the point of infection. Lesions are often not visible until two weeks after infection. As lesions expand, the pathogen will begin producing new spores on the infected tissue. Spore production can occur within 48 hours of visible symptoms under favorable conditions, and when infection occurs early in the growing seasons, an explosive epidemic may occur. Cercospora sojina can infect soybean at any growth stage; however, younger leaves are more susceptible than older leaves. In soybean producing areas where the disease is more common and more severe, symptoms are most frequently observed after flowering (R1). At the time of this printing, frogeye leaf spot has only been observed in North Dakota late in the growing season.

Photo Credit:

Sam Markell, NDSU

Figure 1. Frogeye leaf spot lesions on the upper side of the leaf. Lesions at varying stages of infection. Younger lesions appear as small dark, water-soaked spots. Mature lesions develop into irregular or circular lesions surrounded by a narrow red or purple border.

Frogeye leaf spot lesions are often first observed on the upper side of the leaves. Initially, lesions appear as small dark, water-soaked spots that develop into irregular or circular lesions approximately ¼ inch in diameter (Figure 1). As the lesions mature, the center of the lesions gradually progress in color from gray to brown to tan surrounded by a narrow red or purple border (Figure 2A). Depending on soybean variety, a light-green or yellow (chlorotic) halo surrounding the lesion may also develop. Under favorable conditions, the center of the lesions may sporulate resulting in a fuzzy gray appearance on the underside of the leaf (Figure 2B). As lesions continue to enlarge, they may coalesce on the leaf (Figure 3). In severe cases, this can result in premature defoliation. Lesions can also appear on the stems, pods and seeds in highly conducive conditions. However, stem and pod symptoms differ from foliar symptoms and can be difficult to identify. Symptoms on the stem often appear as long, discolored lesions. Pod lesions can often resemble foliar lesions and appear to have an oblong shape. Pods that are severely infected may also lead to infected seed. Infected seed may be symptomless or have a purple or gray discoloration.

Photo Credit:

Sam Markell, NDSU

Figure 2. Comparison of lesions at varying stages of infection on (A) the upper side of the leaf and on (B) the underside of the leaf. In larger lesion, note the circular to irregular gray center surrounded by dark purple halo on the top side of the leaf and fuzzy gray sporulation in the center of the lesion on the underside of the leaf.

Photo Credit:

Sam Markell, NDSU

Figure 2B

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