Alternariastem canker caused by Alternaria alternata f. sp. lycopersici was first reported in California, but is now found in many tomato production areas (Gro-gan et al. 1975). Dark brown to black cankers form on infected plants near the soil line or just above the ground. This disease can girdle and destroy the plant. Foliar and fruit symptoms may also occur. Alternaria stem canker resistance is controlled by a single dominant gene designated Asc, which is incompletely dominant to a host-specificpathotoxin (Groganet al. 1975; Clouse and Gilchrist 1987). Witsenboer etal. (1989) originally reported Asc to be located on chromosome 3 and linked to the solanifolium gene. RFLP markers TG134 and TG442 were reported to be tightly linked to Asc (van der Biezen et al. 1995; Mesbah et al. 1999).
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