Jeneylyne F. Colcol, Monsanto Company, DeForest, WI 53532; and
Lynn Esther Rallos and
Anton B. Baudoin, Department of Plant Pathology, Physiology and Weed Science, Virginia Tech, Blacksburg 24061
Grape powdery mildew (Erysiphe necator) isolates were collected from 2005 to 2007 from vineyards mostly in Virginia but also some in Maryland, North Carolina, and Pennsylvania. Using a leaf disc assay, the isolates were tested against five demethylation inhibitor (DMI) fungicides. Most isolates exhibited reduced sensitivity to the five DMIs when compared with a sensitive group (n = 12) and compared with unexposed populations reported from other areas. The median resistance factor (RF) was highest for tebuconazole (RF = 399) and myclobutanil (RF = 378), followed by triflumizole (RF = 70), triadimefon (RF = 62), and fenarimol (RF = 44). The sensitive group used as the basis for comparison appears to have been more sensitive than unexposed isolates in New York and California. Our finding that the greatest resistance shift occurred with tebuconazole and myclobutanil contrasts with earlier reports from New York and California, where the greatest resistance shift was observed with triadimefon or triadimenol. Sensitivities to all five DMI fungicides were strongly correlated (pairwise r values of 0.70 to 0.87) but our data suggest that some may retain greater utility than others.