ARC-Roodeplaat P/Bag X293, Pretoria 0001, South Africa
T. A. S.
Department of Botany, University of Pretoria, Pretoria 0002, South Africa
A destructive foliar disease was observed during field surveys of cowpea (Vigna unguiculata (L.) Walp.) in the Mpumalanga and Gauteng provinces in South Africa. Foliar symptoms begin as semicircular water-soaked lesions at the leaf margins. Lesions enlarge toward the center of the leaf, becoming necrotic. Sporulation is visible with the naked eye on the leaf surface as a black velvet mass. Occasionally, circular lesions are observed in the center of the leaf. Lesions begin as small brown spots, surrounded by a yellow halo. The lesions enlarge and become water-soaked, and black masses of conidia are visible on the brown, necrotic tissue surface. Alternaria cassiae (A. M. M. Juriar & A. Khan) was consistently isolated from the diseased leaf material. An isolate of the causal pathogen was identified by and deposited with the National Collection of Fungi, Plant Protection Research Institute, Pretoria, South Africa, and designated with the number PPRI 6393. Koch's postulates were proven by inoculating the leaves of cowpea seedlings with a 106 conidia ml-1suspension. Inoculated plants were maintained in a humidity chamber for 48 h and then returned to the greenhouse. After approximately 7 days, symptoms resembling those observed in the field were apparent. A. cassiae was reisolated from diseased tissue. The same foliar disease was observed on cowpea in Botswana. A. cassiae was isolated and identified from the characteristic lesions.