Disease Note.

First Report of Fusarium Wilt Race 3, Caused by Fusarium oxysporum f. sp. lycopersici, of Tomato in Mexico. J. G. Valenzuela-Ureta, Asgrow Seed Company, San Juan Bautista. CA CAADES. Sinaloa, Mexico. D. A. Lawn. R. F. Heisey, and V. Zamudio-Guzman, Asgrow Seed Company, San Juan Bautista. CA 95045 and Sinaloa, Mexico. Plant Dis. 80:105. Accepted for publication 16 November 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0105A.

Chlorotic flagging and wilting of randomly scattered tomato plants was observed in production fields near Culiacan, Mexico, in February 1995. In all cases, dark brown vascular discoloration was observed in infected plants. Diseased stem tissue was thoroughly washed, surface sterilized in 0.5% sodium hypochlorite for 2 min, rinsed in sterile distilled water, and then incubated on potato-dextrose agar (PDA), pentachloronitrobenzene (PCNB) agar, and water agar for 4 days at 25°C. Hyphal plugs from fungal colonies growing out of stem tissue were transferred to PDA and incubated at 25°C with alternating 12-h light cycles. Single-spore isolates from fungal colonies identified as Fusarium oxysporum were evaluated for pathogenicity and race determination. Differential host lines included Early Pac 7 (EP7), susceptible to all races Pakmor, resislant to race 1 Walter, resistant to races 1 and 2 and I3RI (J. Scott, University of Florida), resistant to races 1, 2, and 3. Roots of 7-day-old seedlings were washed and clipped to approximately 2 cm, soaked in a suspension of 1 x 107 spores/ml for 1 min, transplanted into a sandy loam soil mix, and incubated at 250C for 28 days. Susceptible reactions included dead and stunted seedlings and the presence of vascular discoloration. Most isolates were pathogenic on EP7 and Pakmor, suggesting race 2, and one isolate from the hybrid BR84 was pathogenic on EP7, Pakmor, and Walter, suggesting race 3. The fungus was successfully reisolated, completing Koch’s postulates. At the present time, no commercial tomato hybrids with resistance to Fusarium Wilt race 3 are available.