PATHOGENICITY OF CORYNESPORA CASSIICOLA ON SOYBEAN

Corynespora cassiicola was pathogenic on the roots and hypocotyls of soybean seedlings grown in infested soil at Ottawa. Extensive superficial necrotic lesions developed on seedlings from inoculated seeds planted in the field in mid-May, but few symptoms appeared on seedlings planted in late May and in June. Infected plants were initially stunted but recovered with little effect on size or yield. In controlled temperature studies, severe root and hypocotyl rot were produced on inoculated seedlings at 15–20 °C. Symptoms were less severe at temperatures alternating from 15 or 20 °C to 25–35 °C and were negligible on seedlings kept constantly at 25–35 °C. Optimum growth of the fungus on agar media occurred at 20 °C. At 5 °C and 35 °C conidia germinated, but no appreciable mycelial growth occurred; at 30 °C growth was greatly restricted. Typical symptoms of target spot were produced on the inoculated foliage of soybeans in the greenhouse and in the field, but secondary infection was not observed in the field. Foliage infection occurred on cowpea (Vigna sinensis), sesame (Sesamum indicum), and Hartsville cotton (Gossypium hirsutum) in the greenhouse. C. cassiicola was isolated from overwintered soybean root debris, from the roots of mature field bean (Phaseolus vulgaris), and from the roots of soybean seedlings grown in soil not previously cropped with soybeans.