Frogeye leaf spot, caused by Cercospora sojina, is an economically important foliar disease of soybean (Glycine max (L.) Merr.) in areas where growing conditions are warm and humid. During a survey conducted in 2000 and 2001 in soybean fields in Wisconsin, reddish brown, circular to angular spots varying in diameter from 1 to 5 mm were observed on soybean leaves in four fields in Dane and Iowa counties, and in five and six fields in Lafayette and Green counties, respectively. Soybean plants were in growth stages between R3 and R5 during sampling. Disease incidence ranged from 30 to 100% with 5 to 10% of leaf area covered with leaf spot in 2000. In 2001, trace levels of the disease were detected in Dane County, but no symptomatic plants were present in the other counties. Symptomatic leaves were collected from all locations in 2000 and Dane county in 2001. Ten leaves were randomly picked from all samples for each year, placed in a 100 × 15 mm petri dish dampened with Whatman No.1 filter paper, and incubated overnight at 24°C. Fungal sporulation developed after 24 h. Fifteen spores were removed from the 10 leaves, placed on acidified potato dextrose agar (APDA), and incubated in the dark at 24°C. Cultures with dark pigmentation and associated conidia and conidiophores were observed after 3 weeks. The conidiophore, spore type, and leaf symptoms correspond to the description of C. sojina (1). Conidiophores were light-to-dark brown, one to four septate, and fasciculate. The conidiophores were also geniculate and measured 52 to 120 x 4 to 6 μm. Conidia were 0 to 10 septate, hyaline, elongate to fusiform, and measured 40 to 60 x 6 to 8 μm. Cultures were maintained on APDA, and spores for inoculations were produced on this medium. Spores from the 2000 cultures were harvested, bulked together, and used for pathogenicity tests. Pathogenicity tests were conducted in a growth chamber using a known susceptible soybean cultivar, Blackhawk. Ten-cm-diameter pots each containing 4 plants was used. Twenty plants were inoculated and 20 served as noninoculated controls. Ten-day-old plants were inoculated with a spore suspension of 3 × 105 spores/ml by spraying inoculum over the entire leaf surfaces with a spray atomizer. Control plants were sprayed similarly with sterile distilled water. Plants were incubated in an enclosed, transparent fiberglass box with a humidifier that provided 95 to 100% humidity. Lighting in the growth chamber was adjusted to 18-h light and 6-h dark during the inoculation period. Plants were removed from the box after 48 h and placed in a growth chamber with a 12-h photoperiod. The light output in the growth chamber was 300 μmol·m-2·s-1 and the temperature was maintained at 24 ± 3°C. The experiment was repeated once. Typical field symptoms appeared on each of the inoculated plant 8 days after inoculation, while the controls expressed no leaf symptoms. C. sojina was reisolated from all symptomatic plants. To our knowledge, this is the first report of C. sojina from soybean in Wisconsin.
Reference: (1) D. V. Phillips. Frogeye leaf spot. Page 20 in: Compendium of Soybean Diseases. 4th ed. G. L. Hartman, J. B. Sinclair, and J. C. Rupe, eds. American Phytopathological Society, St. Paul, MN, 1999.