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A Semiselective and Diagnostic Medium for Gaeumannomyces graminis var. tritici. B. K. Duffy,graduate research assistant, Department of Plant Pathology, Washington State University, Current address: graduate research assistant, Institute of Plant Sciences/Phytomedicine, Swiss Federal Institute of Technology, Zurich, Switzerland; D. M. Weller, research plant pathologist, Agricultural Research Service, U. S. Department of Agriculture, Pullman, WA 99164-6430. Phytopathology 84:1407-1415. Accepted for publication 15 September 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1994. DOI: 10.1094/Phyto-84-1407.

Isolation of Gaeumannomyces graminis var. tritici, causal agent of take-all, from wheat tissues often is complicated by the presence of Pythium spp., Rhizoctonia spp., Fusarium spp. or secondary fungal colonists. Because G. g. tritici does not typically sporulate in culture, identification of putative isolates is based on cultural characteristics, pathogenicity tests, and production of perithecia on infected tissue. A semiselective medium designated R-PDA was developed that aids in the isolation and identification of G. g. tritici. The medium consists of dilute potato-dextrose agar amended with 100 ?g ml^–1 rifampicin and 10 μg ml^–1 tolclofosmethyl. Identification of G. g. tritici is aided based on its ability to alter the color of rifampicin in R-PDA from orange to purple. This reaction occurs in as little as 24 h. R-PDA was more effective in isolating putative isolates of G. g. tritici from wheat with symptoms of take-all than was SM-GGT3, another semiselective medium for G. graminis var. tritici.