Frogeye leaf spot is a common problem in the southern and central U.S. and can occur in some areas of the upper Midwest. This disease can cause severe defoliation during warm, humid weather. Frogeye leaf spot can be distinguished from other soybean disease by the reddish-brown or purple ring surrounding the round leaf spots.
Risk of frogeye leaf spot is highest when soybeans are grown continuously in the same field, especially in reduced tillage situations. The pathogen overwinters on soybean residue and seeds. Warm, humid weather promotes spore production, infection, and disease development. Irrigated fields (especially with overhead irrigation) will be at greater risk for disease development.
Frogeye leaf spot effect on yield potential can vary greatly depending on disease timing, soybean product susceptibility, and weather conditions during reproductive stages. If the disease begins late in reproductive stages (after R5 growth stage) or disease severity is low, the impact on yield will be minimal. However, if conditions are favorable and there are severe disease outbreaks early or just after flowering, yield losses can be up to 35 percent.
The risk for frogeye leaf spot is greater when:
Although symptoms may begin at any stage of development, frogeye leaf spot most often occurs after flowering. The best time to scout for this disease is beginning at the R3 growth stage after frequent rains, continuing if warm and humid conditions persist. Symptoms of frogeye leaf spot are most visible and typically seen on young leaves but can also occur on stems, leaves, pods, and seeds. This disease is fairly easy to recognize once you know what to look for. Initial symptoms appear as small dark spots on the leaves. The spots are angular with light gray centers and distinct purple to red-brown margins (Figure 1). Spots may coalesce to form larger, irregularly shaped spots. The lesions may appear to have small, dark hairs on the underside of the leaf which contain the conidia (infecting spores of the fungus). Spores will be produced from leaf lesions and secondary infections will continue as long as weather conditions remain conducive. Lesions on pods and stems appear reddish brown and darken as they mature, but lack the characteristic gray colored center. Lesions on pods may appear sunken and the seeds inside the pods may turn brown with cracked seed coats.
Early season infections from infected seeds result in stunted seedlings. Early season infection contribute to infection of foliage and pods later in the season. In severe cases, disease can cause premature leaf drop and spread to pods and stems.
To manage frogeye leaf spot, use resistant soybean products and pathogen-free seed. Rotating away from soybean for 2 years allows more time for inoculum in the field to degrade before soybean is planted again. When disease is severe, tillage can help bury infested residue. On susceptible soybean products, a foliar fungicide applications can help protect yield potential. If a fungicide treatment is needed, treating at the R3 growth stage is generally recommended to effectively control the disease and preserve yield potential. Thresholds at which a fungicide application is justified have not been established for FLS and control recommendations vary by region. In addition, new strobilurin-resistant strains of the FLS fungus have been reported in several states, rendering this type of fungicide ineffective at controlling these strains. Therefore, it is important to check with your local Extension agent for application timing and fungicide recommendations in your region.
Dorrance, A. and Mills, D. 2011. Frogeye leaf spot of soybean. The Ohio State University Extension publication AC-53-10. http://ohionline.osu.edu. Westphal, A., Abney, T., and Shaner, G. 2006. Frogeye leaf spot. Purdue University Extension publication BP-131-W. www.extension.purdue.edu. Bradley, C. et.al. 2016. Frogeye leaf spot. Crop Protection Network Soybean Disease Management publication CPN-1017. http://soybeanresearchinfo.com. Giesler, L. 2018. Frogeye leaf spot resistance increasing in soybean. University of Nebraska-Lincoln CropWatch. http://cropwatch.unl.edu. Faske, T. 2016. Soybean disease update and management: Frogeye leaf spot. University of Arkansas. http://www.arkansas-crops.com. Web sites verified 5/18/18.