A comparison of techniques for assessing benzimidazole resistance in Venturia inaequalis

This work examined the incidence of benomyl resistance in Venturia inaequalis obtained from trees which were treated the previous season with water, benomyl (MBC-BIC), or a methyl isocyanate homolog Azindoyle (MBC-MIC). Frequencies of sensitive, medium-resistant, high resistant or very-high-resistant phenotypes were determined with the germ tube index assay and polymerase chain reaction (PCR). According to germ tube index measurements, the majority of the isolates from the Azindoyle-treated trees were placed in the benomyl-sensitive category. In the benomyl group, isolates were equally distributed in the sensitive, medium and very-high resistance categories. PCR analysis indicated approximately equal numbers of isolates carrying the ben(VHR) allele, responsible for very high resistance to benomyl, among all three treatments. The ben(HR) and ben(MR) alleles were not observed. PCR results were confirmed by Tha I digestion and DNA sequence analysis of the amplification product and radial growth assay on benomyl-amended and non-amended media. This research demonstrates that PCR-based detection of benomyl resistance is fast, accurate and more reliable than that based on germination and/or germ tube length criteria. Any estimation of the distribution of benomyl-resistant strains in a population of V. inaequalis may be greatly affected by the method of detection.