Severe Outbreak of Chocolate Spot of Fava Bean, Caused by Botrytis fabae, in California

A damaging disease developed on commercial fava bean (Vicia faba L.) in coastal California following heavy rains in January through March, 1998. Two symptom types were observed. The most common symptoms on leaves and stems were small (1 to 5 mm), circular to oblong, dark red spots. As disease progressed, spots coalesced and caused large red areas on leaves and stems. Secondly, large (8 to 12 mm), oblong to irregular, dark brown necrotic spots with concentric circles formed on leaves, resulting in blighting and death of foliage. Severely affected plants lost over 50% of foliage and pod development was poor. Botrytis fabae was consistently isolated from both types of spots (1,2). Ellipsoidal to ovoid conidia measured 17 to 22 × 11 to 17 μm, and all isolates formed profuse, small (1 to 3 mm), spherical to oblong, black sclerotia. Pathogenicity was confirmed by inoculating isolates onto fava bean plants, incubating them in a humidity chamber for 48 h, then maintaining plants in a greenhouse. Inoculated plants developed both types of symptoms, and B. fabae was reisolated. Uninoculated controls developed no symptoms. The experiment was repeated and results were the same. While chocolate spot of fava bean has been observed previously in various parts of the state, this is the first documentation of this disease in California. The disease was also confirmed on fava bean grown as a winter cover crop in the same coastal area of California. References: (1) J. G. Harrison. Trans. Br. Mycol. Soc. 83:631, 1984. (2) J. G. Harrison. Plant Pathol. 37:168, 1988.