A comparison of techniques for assessing benzimidazole resistance in Venturia inaequalis

This work examined the incidence of benomyl resistance in Venturia inaequal is obtained from trees which were treated the previous season with water, b enomyl (MBC-BIC), or a methyl isocyanate homolog Azindoyle (MBC-MIC). Frequ encies of sensitive, medium-resistant, high resistant or very-high-resistan t phenotypes were determined with the germ tube index assay and polymerase chain reaction (PCR). According to germ tube index measurements, the majori ty of the isolates from the Azindoyle-treated trees were placed in the beno myl-sensitive category. In the benomyl group, isolates were equally distrib uted in the sensitive, medium and very-high resistance categories. PCR anal ysis 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 amplific ation product and radial growth assay on benomyl-amended and non-amended me dia. This research demonstrates that PCR-based detection of benomyl resista nce 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.