Phenotype Patterns of Benomyl-Resistant Isolates of Venturia inaequalis in Eight Orchards in British Columbia, Canada. P. L. Sholberg, Agriculture Canada Research Station, Summerland, British Columbia V0H 1Z0, Canada. J. M. Yorston, British Columbia Ministry of Agriculture and Fisheries, Kelowna, British Columbia V1Y 4R2, Canada. Plant Dis. 75:616-619. Accepted for publication 4 December 1990. Copyright 1991 Department of Agriculture, Government of Canada. DOI: 10.1094/PD-75-0616.

Two hundred and fifty-nine orchard blocks from the interior of British Columbia were screened for benomyl resistance. In 22% of the samples, 85% or more of the conidia tested resistant to 1 µg/ml of benomyl. Eight orchards were selected from this group for further study. Single ascospore isolates were obtained from each orchard, totaling 991 from the eight orchards. Each isolate was tested for benomyl resistance over a range of concentrations from 1 to 500 µg/ml and for sensitivity to 0.5 µg/ml of diethofencarb. The majority of the isolates (74.5%) were resistant to benomyl and came from seven of the eight orchards. Three benomyl-resistant phenotypes were found—low benomyl resistance (LR), high benomyl resistance (HR), and very high benomyl resistance (VHR). Only one LR isolate was found. The HR phenotype was present in 41 isolates from four orchards, although 37 of the 41 isolates came from one orchard. The VHR isolate was found in all seven orchards with benomyl resistance and was present in 671 of the 712 benomyl-resistant isolates that were studied. All of the VHR isolates were sensitive to 0.5 µg/ml of diethofencarb and were negatively correlated cross resistant to N-phenylcarbamate. Four orchards had one or more isolates of the HR phenotype in addition to the VHR phenotype. In one orchard, 28.2% of the isolates were the HR phenotype and 67.2% were the VHR phenotype.