Stem cankers were observed during 1998 on bolting stalks of cabbage (Brassica oleracea var. capitata L.) in seed production fields in western Washington. In 1999, approximately 4 ha of cabbage hybrid 'Wk 121, was severely affected. Lesions occurred at the base of seed stalks after they emerged from heads of plants overwintered in the field, or on flower branches and seed-bearing stalks that developed during the growing season. Lesions girdled a branch or stalk, and killed or weakened it so that it broke during pod fill. Isolates of Botrytis cinerea Pers.:Fr. were obtained by plating spores from lesions onto potato dextrose agar. To confirm pathogenicity, stems of 12-day-old seedlings of ‘Wk 121’ were scraped with a razor blade or left intact, atomized with sterile 0.01 % Tween 80 or a suspension of Botrytis cinerea at 1.0 × 106 conidia/ml, and kept at 20°C in a dew chamber in plastic bags. The fungus was reisolated from small lesions on wounded stems inoculated with B. cinerea after 3 days. No lesions developed on non-wounded or wounded control plants. B. cinerea is reported to cause storage rot of cabbage (2) and gray mold on Brassica oleracea L. (cabbage, kale, kohlrabi, wild cabbage) in Washington (1) but not stem canker. This new seed crop disease may be the result of predisposition to infection by freezing injury or mechanical damage on a highly susceptible cultivar grown under cool, wet weather.
References: (1) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. American Phytopathological Society. St. Paul, MN. (2) O. C. Yoder and M. L. Whalen. Can. J. Bot. 53:691, 1975.