Characterization of Verticillium dahliae isolates and wilt epidemics of pepper.
Epidemics of Verticillium wilt in pepper (Capsicum annuum) fields of the central coast of California, USA, and isolates of V. dahliae associated with these epidemics were characterized. The mean incidence of wilted plants per field ranged from 6.3 to 97.8% in fields with Anaheim, jalapeno, paprika, or bell peppers. Generally, wilt incidence in jalapeno and bell pepper crops was lower than in crops of other types of pepper. Inoculum density of V. dahliae in the surveyed fields ranged from 2.7 to 66.6 microsclerotia g-1 dry soil, and the correlation between disease incidence and density of microsclerotia was high (r=0.81, P<0.01). Distribution of Verticillium wilt was aggregated in a majority of the fields surveyed, but the degree of aggregation varied. Vegetative compatibility group (VCG) characterization of 67 isolates of V. dahliae indicated that 67% belonged to VCG 2, 22% to VCG 4, and 11% to a new group, designated VCG 6. The pathogenicity of isolates of V. dahliae from bell pepper and tomato plants was tested by inoculating 1-month-old bell pepper (cv. Cal Wonder) and tomato (cv. EP 7) seedlings and incubating the inoculated plants in the greenhouse. Seedlings of bell pepper were susceptible only to the isolates of V. dahliae from pepper, while seedlings of tomato were susceptible to both pepper and tomato isolates. Pepper isolates belonging to VCG 2, VCG 4, and VCG 6 were highly pathogenic to bell pepper and chilli pepper. Temperatures between 15 and 25°C were optimal for mycelial growth of a majority of isolates of V. dahliae. Molecular characterization of pepper isolates of V. dahliae using a polymerase chain reaction-based random amplified polymorphic DNA technique revealed minor variation among these isolates, but unique polymorphic banding patterns were observed for isolates belonging to VCG 6. Verticillium wilt of pepper is a major production constraint in the central coast of California. More aggressive isolates of V. dahliae may have been selected in this region as a result of intensive cropping practices.