In the absence of commercially acceptable host resistance and a sustainable workforce for agriculture, apples require considerable fungicide applications to manage disease and ensure sustainable production. Of the fungicides available, single-site fungicides are effective at smaller quantities, have fewer off-target effects, and possess pre- and post-infection activity. Because their single-site mode of action, there is a high risk of fungicide resistance development. To ensure sustainable use of single-site fungicides, we conducted a series of multi-year studies to understand the development and occurrence of practical resistance at the molecular and field population level for the apple scab pathogen <i>Venturia inaequalis</i>. We found that the distribution of individuals with resistant genotypes or phenotypes was more important for determining commercial failure of fungicides than the detection of a few highly-resistant individuals. We also found that the environment and host genetics (cultivar) were equally important as pathogen genotype/phenotype for determining commercial fungicide failure. In extensive multi-year surveys, we found that individuals with resistance to an older fungicide dodine were being slowly selected out of regional populations. We hypothesize that curtailed use of dodine for more than 20 years combined with subsequent selection for resistance to newer chemistries was the likely explanation for the selection against dodine resistance from regional populations. This discovery lead to careful commercial re-introduction of an effective fungicide previously lost to resistance. Present field research efforts test the dose and fungicide mixture hypotheses for resistance management over several seasons. Prior to the establishment of resistance, we found that populations shift towards resistance with frequency of use irrespective of dose or fungicide mixture, and that overall inoculum level may be the primary consideration for resistance development. In summary, these investigations have provided new guidance on fungicide use recommendations to stakeholders as well as insights on determining practical resistance in managed plant systems. Given that the apples are treated for fungal diseases many times within a season, the information resulting from our studies may be suitable in other host plant or animal host systems where multiple applications may be made against resident populations in a nine to twelve month-period.

Management strategies for fungicide resistance in apple

2019 Spring National Meeting

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