A SEMISELECTIVE AND DIAGNOSTIC MEDIUM FOR GAEUMANNOMYCES-GRAMINIS VAR TRITICI

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 mu g ml(-1) rifampicin and 10 mu g 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.