VIEW ARTICLE

Disease Control and Pest Management

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.