BUILDUP OF RESISTANCE TO TRIADIMEFON FOR ISOLATES OF ERYSIPHE-GRAMINIS F SP TRITICI FROM NOVA-SCOTIA, CANADA

Sensitivity to triadimefon, a triazole demethylation inhibitor (DMI), of Erysiphe graminis f. sp, tritici populations collected in 1992 from fields of the wheat cultivar Absolvent in the Annapolis Valley and Co lchester County of Nova Scotia, Canada, was studied in vitro. E. g. f. sp. tritici isolates were collected before, as well as at two and six weeks after triadimefon application. Exposing powdery mildew fungal p opulations from the Annapolis Valley to the selection pressure of tria dimefon resulted in an increase in the log(10) transformed mean value of EC(50) from 0.30 mu g/mL for population of isolates collected befor e triadimefon application, to 0.85 mu g/mL for population of isolates collected six weeks after triadimefon application. In the case of Colc hester County, the log,, transformed mean value of EC(50) increased fr om 0.34 mu g/mL to 1.04 mu g/mL for isolates collected before, and six weeks after triadimefon application, respectively. The frequency dist ribution of EC(50) values within each population of fungal isolates co llected at each sampling time was lognormal. The findings suggest that isolates of E. g. f. sp. tritici with reduced sensitivity to triadime fon had existed in winter wheat fields before the fungicide was applie d. After exposing fungal populations to triadimefon more isolates with reduced sensitivity were built-up, and by the end of the season they dominated the populations.