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Alternate Name(s): Cercospora leaf spot
Overview: Frog eye leaf spot is a fungus that primarily attacks soybean foliage. However, the fungus can also attack seeds, pods, and stems. Frog eye spots reduce photosynthetic area on infected leaves and significantly reduce soybean productivity when numerous. Ethylene produced in the leaf spots promotes premature defoliation, further reducing productivity. Development of the disease is favored by warm, humid conditions. The disease has historically been most important in the warm, humid Southeastern states of the USA but its distribution has increasingly widened over the last few years to include the southern Midwest.
Symptoms: The most common symptoms encountered on soybean are circular to angular spots on leaves that resemble the eyes of frogs, giving the disease its name. Lesions begin as dark, water-soaked spots, with or without lighter centers and develop into brown spots surrounded by narrow, dark reddish brown margins <1-5mm in diameter. Older spots are light to dark brown, translucent, and have white centers containing minute dark stromata. Several spots may coalesce to form larger, irregular spots. When spots cover about 30% of the leaf area, a blighting phase often occurs, and leaves wither and fall prematurely.
Causal Organisms: Cercospora sojina Hara; conidiophores (4-6 x 52-120 mm) arise in fascicles of two to 25 from a thin stroma, light to dark brown in color with one to several septa, covered with prominent bands (geniculations) and scars left by detached conidia; conidia or sympodulospores are indeterminately borne successively on new growing tips of the conidiophores, hyaline or dark colored, filiform (scolecospores), multiseptate. Multiple pathotypes exist.
Disease Cycle: Spots normally appear 8-12 days following inoculation with conidia, however, under continuous moist, warm conditions, spots may appear as early as 48 hrs post-inoculation. Conidia carried short distances by air currents and splashing rain cause secondary infections under favorable conditions throughout the season.
Management: Deployment of cultivars possessing the Rcs3 gene that gives resistance to all known pathotypes is the primary means to manage the disease. Registered fungicides applied at growth stages R2-R5 can effectively control the disease on susceptible soybean cultivars. Fungicides applied to the seed will control seed-borne infection.
Researchers: At the Laboratory for Soybean Disease Research, Curt Hill is conducting research on this disease.