SENSITIVITY DISTRIBUTION OF VENTURIA-INAEQUALIS TO FENARIMOL IN QUEBEC APPLE ORCHARDS

This study was initiated to quantify the baseline sensitivity of apple scab (Venturia inaequalis) to fenarimol, an ergosterol synthesis-inhi biting fungicide. In 1988, 576 monoconidial isolates of Venturia inaeq ualis were collected from 26 commercial orchards throughout Quebec. Se nsitivity to fenarimol was assessed by radial growth inhibition assay. The ED50 values for the 26 orchards ranged from 0.024 to 5.212 mug mL -1 with a mean ED50 of 0.156 mug mL-1. Reduced sensitivity, expressed as ED50, was found in three orchards for an overall frequency of 4.51% of isolates. Sensitive isolates had a mean ED50 of 0.079 mug mL-1, wh ereas isolates with reduced sensitivity had a mean ED50 of 1.714 mug m L-1, yielding a resistance factor of about 22. Four populations were i dentified based on the frequency distribution of ED50 values.