Competition Between Triadimefon-Sensitive and Triadimefon-Resistant Isolates ofErysiphe graminisf. sp.tritici

The competitive abilities of randomly selected triadimefon-sensitive (S) and triadimefon-resistant (R) single-pustule derived isolates of Erysiphe graminis f. sp. tritici obtained from triadimefon-treated and -untreated fields of the Annapolis Valley (AV), Nova Scotia, were tested. In 1991, mixed inoculations of S/R isolates in 75:25, 50:50, and 25:75 ratios were studied in vitro in the absence of triadimefon on wheat cultivar Absolvent. In 1992, a 50:50 (S/R) inoculation ratio was used to study the competition between R and S isolates collected from triadimefon-treated fields in the AV. Results indicated that in the absence of triadimefon the mean EC 50 values of mixtures of various S/R ratios tested in 1991 and of the 50:50 ratio tested in 1992 decreased significantly (P < 0.05) after five generation cycles. Resistant isolates were less fit (less competitive) than sensitive ones in the absence of triadimefon.