Venturia inaequalis, the causal agent of apple scab, is controlled primarily by fungicides. Long-term, extensive fungicide use has led to the development of resistance to multiple fungicides. To assess fungicide resistance, isolates of V. inaequalis were collected from Indiana and Michigan orchards. Single-spore derived isolates were evaluated by mycelium growth assays with previously determined discriminatory doses on media containing dodine, kresoxim-methyl, myclobutanil, or thiophanate-methyl. Of 195 isolates tested, 5.2, 0.7, 57.0, and 92.6% of isolates were found to be resistant to dodine, kresoxim-methyl, myclobutanil, and thiophanate-methyl, respectively. This is the first report of kresoxim-methyl field resistance in these states. Isolates resistant or shifted to a single fungicide were often found to have multiple fungicide resistance. Of all isolates tested, 38% were identified as resistant or shifted to two fungicides, and 12% were resistant or shifted to all four fungicides tested. No fitness penalty was found for isolates resistant to multiple fungicides based on a statistical analysis of mycelial growth and conidial production.

Venturia inaequalis, the causal agent of apple scab, infects both commercial apples and ornamental crabapples. We found four classes of benzimidazole fungicide sensitivity in the Indiana population: sensitive (S) isolates unable to grow on 0.5 μg active ingredient (a.i.)/ml; low resistant (LR) isolates that grew at 0.5 μg a.i./ml, but not at 5 μg a.i./ml; moderately resistant (MR) isolates that grew at 5 μg a.i./ml, but not at 50 μg a.i./ml; and very highly resistant (VHR) isolates that grew rapidly at 50 μg a.i./ml. Polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) of the β-tubulin gene with two restriction enzymes, BstUI and Cac8I, enabled us to rapidly identify benzimidazole resistance among all tested isolates. Sixty-nine percent of the resistant isolates tested possessed the BstUI RFLP at codon 198 that corresponds to VHR, and the remaining LR and MR isolates possessed the Cac8I RFLP corresponding to a newly identified resistance allele at codon L240F. Combined, PCR-RFLP correctly identified the resistance status of all isolates tested to date. The preponderance of benzimidazole-resistant isolates from commercial apple orchards and their absence in the landscape on ornamental crabapple suggests that two distinct populations of V. inaequalis coexist in Indiana.