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First Report of Colletotrichum coccodes on Soybean in the United States

Anthracnose symptoms were observed on soybean (Glycine max (L.) Merr.) cv. Yale in Illinois in September 1996. Lower stems were girdled by lesions that contained black fungal stroma. Colletotrichum coccodes (Wallr.) S. J. Hughes was isolated from surface-sterilized portions of diseased stems incubated on potato dextrose agar (PDA). Cultures produced abundant black sclerotia and acervuli with setae. Acervuli produced straight and fusiform conidia (15 to 23 × 3 to 4 μm) in honey-colored masses. Ovate or long clavate appressoria were formed on slide microcultures. Pathogenicity tests were carried out in the greenhouse on soybean plants, cvs. Panda and Pony at the V4 growth stage, by (i) spraying a conidial suspension (2 ×10⁶ conidia per ml) on seedlings and (ii) placing a mycelial PDA plug above the first two nodes of the stem, previously wounded with a sterile needle. Plain sterile water and plugs without mycelia were used as controls. Six plants per cultivar and per treatment were used. Plants were covered with polyethylene bags for 3 days. Anthracnose symptoms gradually appeared near maturity and plants senesced prematurely with both inoculation methods. Averaged over both cultivars, 100 and 50% of the plants showed symptoms when inoculated with a conidial suspension and mycelial plugs, respectively. Control plants did not have any symptoms. C. coccodes was consistently reisolated from stems, leaves, petioles, and pod peduncles with symptoms, and was not reisolated from noninoculated plants. Seeds collected from plants inoculated with either method showed infection rates up to 14 and 8% on cvs. Panda and Pony, respectively, while seeds collected from control plants showed 0% infection rate. The most common pathogen associated with soybean anthracnose is C. truncatum (Schwein.) Andrus and W. D. Moore, but other species have been reported to cause anthracnose. C. coccodes was reported on soybean in Italy (1). This pathogen has a wide host range and causes serious damage, mostly on solanaceous crops. This is the first report of the presence of the pathogen on soybean in the United States.

Reference: (1) G. Conca et al. Petria 4:193, 1994.