Tomato is one of the important crops produced in Boushehr Province especially in fall and winter. Tomato plants were diseased and injured by some fungi in this survey. Samples were collected and fungi isolated. According to the morphology and host range of pathogenicity, fungi were recognized as Alternaria alternata f.sp. lycopersici. The symptoms of disease were characterized by the formation of dark-brown to black canker on the stem. Foliar symptoms included epinasty of the petiole with inward rolling and angular interval necrotic areas on the midrib. The culture filtrate of fungi incited symptoms of disease in Peto early cultivar. This experimet indicated that the external metabolites of fungi act as a causal agent of this disease. For determination of resistance and susceptibility in six tomato cultivars, an experiment was carried out in a completely randomized design in pots under greenhouse conditions. The results showed that there were significant differences among the cultivars in resistance and susceptibility to tomato stem canker. Based on MSTAT- C analysis and Duncan’s Multiple Range Test, cultivars were placed in 3 groups. The cultivar ACE was immune to disease, while early Pak and Peto early are found to be susceptible.
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