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Incidence of Myclobutanil- and Kresoxim-Methyl-Insensitive Isolates of Venturia inaequalis in Quebec Orchards

Sensitivity of baseline and exposed populations of Venturia inaequalis to myclobutanil and to kresoxim-methyl were evaluated in vitro. For myclobutanil, the population was constructed with 238 monoconidial isolates of V. inaequalis collected from 48 orchards. For kresoxim-methyl, the population was constructed with 251 monoconidial isolates collected from 49 orchards. Baseline populations were constructed with 34 and 29 monoconidial isolates collected from apple trees that had never been treated for myclobutanil and kresoxim-methyl, respectively. Sensitivity to fungicides was evaluated based on 50% effective dose (ED₅₀) values. The V. inaequalis population that was not exposed to myclobutanil had a baseline sensitivity (mean ED₅₀) of 0.064 μg/ml and showed a lognormal distribution. The V. inaequalis population constructed with isolates from commercial orchards had a mean ED₅₀ of 2.600 μg/ml, which was significantly higher than the baseline sensitivity. The distribution of ED₅₀ values did not follow a lognormal distribution. In response to declining levels of scab control with myclobutanil and other sterol demethylation inhibitor fungicides (DMIs), three orchards were more deeply investigated. The mean ED₅₀ values were 1.618 (n = 23), 3.079 (n = 29), and 1.500 μg/ml (n = 20) in orchards one, two, and three, respectively. Resistant isolates, according to criteria set by other studies, accounted for 39, 76, and 85% of the isolates tested. The V. inaequalis population that had never been exposed to kresoxim-methyl had a baseline sensitivity (mean ED₅₀) of 0.092 μg/ml and showed a lognormal distribution. The V. inaequalis population constructed with isolates from commercial orchards had a mean ED₅₀ of 6.093 μg/ml, which was significantly higher than the baseline sensitivity. The distribution of ED₅₀ values followed a lognormal distribution. However, when a subsample of isolates was retested for their sensitivity to kresoxim-methyl with the addition of salicylhydroxamic acid (an inhibitor of alternative oxidase) at 100 μg/ml to the growth medium, more than 98% inhibition was observed for all isolates. The results from in vitro tests showed a high level of resistance to myclobutanil and a low level of resistance to kresoxim-methyl, suggesting that the use of myclobutanil and DMIs should be discontinued or significantly reduced before practical resistance is reached.