| dc.description | The sensitivity of wheat powdery mildew (Erysiphe graminis DC. f.sp. tritici E. Marchal) populations to triadimefon for single spore-derived isolates collected from triadimefon-treated and -untreated wheat (cvs. Absolvent and Borden) fields in the Annapolis Valley (AV) and Colchester County (CC) of Nova Scotia, Canada, was studied in vitro. Results indicated that the populations of the fungus from the AV, where the fungicide has been used intensively, had higher mean EC$\sb{50}$ values and higher frequencies of isolates resistant to triadimefon than those from CC. The wheat cv. Absolvent was more favourable for the development of resistant isolates of the fungus than the cv. Borden. For both cultivars, the mean EC$\sb{50}$ values and frequencies of resistant isolates were higher for populations collected from triadimefon-treated fields compared to those from untreated fields. In the study of build-up of resistance, sensitivity tests were performed on powdery mildew isolates collected in 1992 from the AV and CC fields of the wheat cv. Absolvent before, and two and six weeks after triadimefon application. Results revealed that exposing powdery mildew populations to the selection pressure of triadimefon resulted in an increase in the EC$\sb{50}$ values of populations and in the frequency of resistant isolates within each population. In 1991, the competitive abilities of randomly selected triadimefon-sensitive (S) and -resistant (R) isolates of powdery mildew from treated and untreated fields of the AV in mixed isolate inoculations (75:25, 50:50, 25:75) were tested in the absence of triadimefon on the wheat cv. Absolvent. In 1992, the 50:50 ratio was used in studying the competition between R and S isolates that were collected from treated fields in the AV. Results indicated that, in the absence of triadimefon, the mean EC$\sb{50}$ values of mixtures of various R:S ratios decreased significantly after five generation cycles. Resistant isolates were less fit (less competitive) than sensitive ones in the absence of triadimefon. | en_US |
