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Disease Control and Pest Management

In Vitro Variability Among Isolates of Six Phytophthora Species in Response to Metalaxyl. M. D. Coffey, Associate professor, Department of Plant Pathology, University of California, Riverside 92521; L. A. Bower, staff research associate, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 74:502-506. Accepted for publication 10 January 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-502.

A comparison of isolates of Phytophthora parasitica and P. citrophthora, mainly from citrus, revealed a range of inhibitions of mycelial growth from 69.1 to 83.6% for the former, and only from 0 to 14.6% for the latter, at a metalaxyl concentration of 0.1 μg/ml. Similarly, P. palmivora was much more sensitive than either P. citrophthora or P. capsici (all from cacao) to the fungicide at 0.1 μg/ml. Among isolates of P. megasperma, those of P. megasperma f. sp. medicaginis from alfalfa were extremely sensitive to metalaxyl, followed by P. megasperma f. sp. glycinea from soybean. Isolates of P. megasperma from Douglas-fir were much less sensitive to metalaxyl. In contrast, isolates of P. infestans from potato did not display characteristic in vitro responses to metalaxyl, but variation in inhibition of mycelial growth ranged from a low of 11.8 to a high of 92.8% with metalaxyl at 1.0 μg/ml. For various Phytophthora spp., ED₅₀ values for inhibition of mycelial growth in vitro ranged from 0.013 μg/ml for an isolate of P. megasperma f. sp. medicaginis to 0.95 μg/ml for an isolate of P. megasperma from Douglas-fir.