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Isolation and Characterization of a Metalaxyl-Insensitive Isolate of Phytophthora megasperma f. sp. medicaginis. J. P. Stack, Former graduate research assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; R. L. Millar, professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 75:1014-1019. Accepted for publication 3 May 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1014.

Investigations on the survival and activity of Phytophthora megasperma f. sp. medicaginis in a naturally-infested, nonsterile field soil were hampered by lack of specificity of the alfalfa seedling bait bioassay, interference by fast-growing Pythium spp., and an inability to distinguish between added and indigenous propagules of P. m. f. sp. megasperma. An isolate (Pm20) of the pathogen with insensitivity to the fungicide metalaxyl was obtained without mutagenesis by screening encysted zoospores on an agar medium amended with metalaxyl. Except for insensitivity to metalaxyl, Pm20 did not differ from the wild type in total growth, growth rate, sporulation, propagule germination, or pathogenicity to alfalfa. Among 41 isolates of P. megasperma from nine hosts in six states, Pm20 was unique in its insensitivity to metalaxyl. A relationship was observed between colony morphology on cornmeal agar, sensitivity to metalaxyl, and pathogenicity to alfalfa. Pm20 was easily recovered after 6 wk in a nonsterile field soil on an antibiotic medium amended with metalaxyl. On this medium, there was no growth of P. m. f. sp. megasperma from soil to which Pm20 had not been added. By using Pm20 and incorporating metalaxyl into an antibiotic recovery medium selective for Phytophthora, the obstacles of assay nonspecifity, interference by Pythium spp., and lack of distinction between added and indigenous P. m. f. sp. megasperma were overcome.

Additional keywords: alfalfa, ecology, Phytophthora root rot, survival.