Evidence for the predisposition of fungicide-resistant isolates of Venturia inaequalis to a preferential selection for resistance to other fungicides

In the United States, populations of the apple scab pathogen Venturia inaeq ualis have progressed through three consecutive rounds of fungicide resista nce development, first to dodine, then to the benzimidazoles, and most rece ntly to the sterol demethylation inhibitors (DMIs). Analysis of extensive m onitoring data have to date provided no indication of detectable cross-resi stance or partial cross-resistance of V. inaequalis populations to the thre e unrelated classes of fungicides prior to the selection of resistant subpo pulations. However, in this study, resistance to both benomyl and DMIs deve loped to significantly higher frequencies within the previously established dodine-resistant population than in the population sensitive to dodine. Ac celerated selection of phenotypes double resistant to dodine and the DMI fe narimol was apparent over the course of distinct seasons of apple scab mana gement with either dodine or fenarimol. The data provide evidence for an ac celerated speed of resistance development among phenotypes of V. inaequalis already resistant to an unrelated fungicide. This finding represents a dep arture from the previous model, which assumed entirely independent rounds o f resistance developments. The data indicate that phenotypes of V. inaequal is might not only be selected for the trait of fungicide resistance but als o for traits allowing a more flexible response to changes in the environmen t where they compete.