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Selective Inhibition of Pythium spp. on a Medium for Direct Isolation of Phytophthora spp. from Soils and Plants. H. Masago, Professor, Laboratory of Plant Pathology, Faculty of Agriculture, Kyoto Prefectural University, Shimogamo, Kyoto 606, Japan; M. Yoshikawa(2), M. Fukada(3), and N. Nakanishi(4). (2)(3)(4)Assistant Professor, and Undergraduate Students, respectively, Laboratory of Plant Pathology, Faculty of Agriculture, Kyoto Prefectural University, Shimogamo, Kyoto 606, Japan. Phytopathology 67:425-428. Accepted for publication 28 September 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-425.

The growth of Pythium spp., but not Phytophthora spp., was selectively inhibited by 3-hydroxy-5-methylisoxazole (HMI). Incorporation of HMI at 25 and 50 μg/ml into a potato-dextrose agar medium containing other antimicrobial agents (benomyl, nystatin, pentachloronitrobenzene, rifampicin, and ampicillin at 10, 25, 25, 10, and 500 μg/ml, respectively) almost completely prevented colony development of bacteria, nonpythiaceous fungi, and 12 species of Pythium, but did not significantly affect mycelial growth and spore germination of various Phytophthora spp. With this selective medium, the presence and propagule density of Phytophthora spp. in soil could be quantitatively determined. The medium also was useful for isolating Phytophthora spp. from plant tissues.

Additional keywords: selective medium, selective antimicrobial chemical, differential toxicity, phycomycetes, pathogen population.