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Inhibition of Mortierella and Pythium in a Phytophthora-isolation Medium Containing Hymexazol. Peter H. Tsao, Professor, Department of Plant Pathology, University of California, Riverside, CA 92502; Stephen O. Guy, Staff Research Associate, Department of Plant Pathology, University of California, Riverside, CA 92502. Phytopathology 67:796-801. Accepted for publication 22 November 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-796.

Hymexazol (3-hydroxy-5-methylisoxazole, or HMI) at 50 μg/ml, which is noninhibitory to most Phytophthora spp., not only inhibited the development of Pythium as reported previously by Masago et al., but also Mortierella spp. which often are present in high populations in soil and can interfere with the detection and enumeration of Phytophthora in soil dilution plates. The pimaricin-vancomycin-pentachloro-nitrobenzene medium supplemented with hymexazol at 50 μg/ml (PVPH medium) was effective for selective isolation and quantitative estimation of Phytophthora cinnamomi and P. parasitica from roots and soils. Results of in vitro tests showed that linear growth of all 11 Mortierella isolates, but not all 10 Pythium isolates, was greatly or completely inhibited on PVPH medium. Some isolates of Phytophthora cactorum and P. palmivora were more sensitive to hymexazol than P. cinnamomi, P. citrophthora, P. parasitica, and the atypical black pepper strain of ‘P. palmivora’. Hymexazol reduced colony density of most test fungi, including some Phytophthora spp.

Additional keywords: selective medium, differential medium, selective inhibition.