Variability in Sensitivity to Metalaxyl of Isolates ofPhytophthora cinnamomiandPhytophthora citricola

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Abstract
Among isolates of P. cinnamomi, both A1 and A2 mating types, there was a significant variation in the response of mycelial growth to metalaxyl with ED50 values ranging 0.07-0.29 .mu.g/ml. The ED50 values among A2 isolates from a single host, avocado (Persea americana), ranged 0.07-0.14 .mu.g/ml, and ED90 values 0.45 to 1.14 .mu.g/ml. Isolates of another species, P. citricola pathogenic on avocado, had ED50 values ranging 1.18-4.61 .mu.g/ml, and ED90 values from 31.5 to 192.4 .mu.g/ml of metalaxyl. Chlamydospore production in 3 isolates of P. cinnamomi was inhibited more than 60% with 0.1 .mu.g/ml, and was completely inhibited at 0.25 .mu.g/ml. Germination of chlamydospores was much less sensitive than their production, with about 70% inhibition at 10 .mu.g/ml. Sporangium production in P. cinnamomi was extremely sensitive to metalaxyl; 0.25 .mu.g/ml generally was completely inhibitory. Isolates of P. citricola were less sensitive, with 1.0 .mu.g/ml giving about 70% inhibition. Cyst germination in both P. cinnamomi and P. citricola was insensitive to metalaxyl, with even 100 .mu.g/ml having essentially no effect. Oospore formation in P. cinnamomi was sensitive compared to that of P. citricola. With P. cinnamomi 1.0 .mu.g/ml prevented oospore formation, while with P. citricola some formation still occurred at 10 .mu.g/ml of metalaxyl.