Disease Note.

A Watermelon Leaf Blight Caused by Thanatephorus cucumeris in Brazil. J. R.M. Santos, CNPH/EMBRAPA, CP. 0218, 70359-970 Brasilia-DF, Brazil. L. S. Boiteux, CNPH/EMBRAPA, CP. 0218, 70359-970 Brasilia-DF, Brazil. Plant Dis. 78:830. Accepted for publication 24 March 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0830A.

A previously undescribed foliar disease was observed within commercial plantings of watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) 'Charleston Grey' in Manicore County in the Amazon Basin of Brazil. Symptoms included a severe leaf blight characterized by large necrotic brownish lesions, followed by intensive defoliation. A mycelial net was found commonly covering the leaf surface and often held two or more blighted leaves together. Fruit infection was not found. Fields with 100% of plants affected were commonly observed. Yield losses ranged from 20 to 50%. The pathogen was identified as Thanatephorus cucumeris (AB. Frank) Donk from the morphology of sterigmata, metabasidia, and basidiospores (1). The incompatibility (anastomosis) group of this isolate was not determined. Koch's postulates were completed using selected disease-free plants in areas within one commercial field. Plants were inoculated by placing the mycelial net of infected leaves in contact with healthy leaves. Disease symptoms were reproduced in about 4 to 5 days in the inoculated leaves but not in the healthy controls. High temperatures (24-35 C), heavy rains, and the large host range of T. cucumeris provide optimal epidemiological conditions for severe outbreaks of this disease. This seems to be the first report of the teleomorph T. cucumeris as the causal agent of a severe leaf blight disease on watermelon in Brazil.