Genetic diversity of Alternaria alternata isolated from tomato in California assessed using RAPDs

Black mould lesions were caused by Alternaria alternata in 76% of 228 tomat o fruit with characteristic sunken black lesions collected from fields of p rocessing tomatoes in California. Analysis of 29 RAPD primers revealed a hi gh level of genetic diversity among the 69 isolates tested. Two major phene tic groups (Group 1 with 55 isolates and Group 2 with 14) were identified i ndependently by PGA and by UPGMA of Jaccard similarity coefficients. Only 3 4 of 137 RAPD markers were monomorphic for all isolates and the genetic sim ilarity between the two groups was 50%. Co-infection of black mould lesions by genetically distinct strains of A. alternata occurred in two of 10 isol ates tested. There was no evidence for geographic clustering of isolates wi th high levels of genetic similarity, suggesting that isolates are widely d ispersed across California. Only one isolate was identified which also caus ed stem canker disease on a susceptible tomato cv., suggesting that these s trains play a minor role in causing black mould on processing tomatoes in C alifornia. This isolate and two other known stem canker isolates were clust ered together with 11 other isolates in Group 2. Group 2-specific bands wer e also identified in a survey of seven additional isolates known to produce host-specific toxins.