DISTRIBUTION OF MYCLOBUTANIL FUNGICIDE SENSITIVITIES AMONG POPULATIONS OF VENTURIA INAEQUALIS, THE CAUSAL AGENT OF APPLE SCAB, IN ONTARIO

Sensitivity of five populations of V. inaequalis, collected from four geographical locations in Ontario, to myclobutanil, a demethylation inhibitor fungicide, was investigated. Sensitivities of isolates to fungicide were determined by ED50 values and a sensitivity test based on the relative growth (mean colony diameter on media amended with discriminatory myclobutanil doses per mean colony diameter on unameded media x 100) of mycelial colonies of a baseline population at a single discriminatory dose (0.1 µg/ml). Both assays were highly correlated. The wild-type population had a baseline sensitivity (mean ED₅₀) of 0.074 µg/ml of myclobutanil and showed lognormal distributions to myclobutanil. The mean ED₅₀ values were 0.114, 0.074, 0.076, 0.071 and 0.581 µg/ml of myclobutanil for orchards 1, 2, 3, 4, and 5, respectively. With the exception of the orchard 5, all populations showed lognormal distributions to myclobutanil. Mean isolate sensitivities between baseline population and populations sampled from orchards that have never been or rarely treated with myclobutanil were not significantly different (P = 0.20). Isolates from one experimental orchard, orchard 5, that was treated with myclobutanil showed tolerance to the fungicide and had significantly different mean isolate sensitivities than the baseline population ( P = 0.03). The resistance factor for orchard 5 was 7.8 (mean ED₅₀ of resistant population/ mean ED₅₀ of the baseline population), while resistance factors for four other orchards ranged from 0.9 to 1.5 for myclobutanil. Although fungicide resistance to myclobutanil in V. inaequalis was not observed in the commercial orchards, a shift in fungicide sensitivities was observed in an an experimental orchard.