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Variability in Sensitivity to Metalaxyl of Isolates of Phytophthora cinnamomi and Phytophthora citricola. M. D. Coffey, Associate professor, Department of Plant Pathology, University of California, Riverside 92521; L. J. Klure(2), and L. A. Bower(3). (2)(3)Staff research associates, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 74:417-422. Accepted for publication 26 October 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-417.

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