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Characterization of the VisdhC and VisdhD Genes in Venturia inaequalis, and Sensitivity to Fluxapyroxad, Pydiflumetofen, Inpyrfluxam, and Benzovindiflupyr.

Succinate dehydrogenase inhibitors (SDHI) are an important class of fungicides for management of apple scab, especially as resistance to other classes of fungicides has become prevalent in the northeastern United States. Considering their single-site mode of action, there is high risk of resistance development to SDHI fungicides. Such risk mandates the need for proper monitoring of shifts in population sensitivity. This study aims to provide a means for phenotypic and genotypic characterization of SDHI fungicide resistance for Venturia inaequalis, the causal agent of apple scab. To complement the published sequence of VisdhB, target genes VisdhC and VisdhD were identified using sequences of homologous genes in other fungal organisms and a draft genome of V. inaequalis. Using mycelial growth and conidial germination assays, baseline sensitivities and cross sensitivities of V. inaequalis were determined for several SDHI fungicides. Mean baseline EC50 values for conidial germination of benzovindiflupyr, fluxapyroxad, pydiflumetofen, and inpyrfluxam were found to be 0.0021, 0.0284, 0.014, and 0.0137 μg ml-1, respectively. Mean baseline EC50 values for mycelial growth of benzovindiflupyr, fluxapyroxad, pydiflumetofen, and inpyrfluxam were found to be 0.0575, 0.228, 0.062, and 0.0291 μg ml-1, respectively. A significant and positive correlation in sensitivity was found between benzovindiflupyr, fluxapyroxad, pydiflumetofen, and inpyrfluxam as well as penthiopyrad and fluopyram, with the highest correlation between benzovindiflupyr and penthiopyrad for mycelial inhibition of V. inaequalis (r = 0.950, P < 0.001). For inhibition of conidial germination, the highest correlation was observed between penthiopyrad and fluopyram (r = 0.775, P < 0.001). Furthermore, the sequences of the VisdhC and VisdhD genes were identified and characterized for baseline isolates of V. inaequalis. Residues of similar position to mutations found in other systems that confer resistance to SDHI fungicides were identified in baseline isolates, but no mutations were identified in baseline isolates or those previously exposed to SDHI fungicides. This study will serve as a reference for future monitoring of resistance to SDHI fungicides in V. inaequalis at both a phenotypic and genotypic level.