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Sensitivity of the apple scab pathogen, Venturia inaequalis, to SDHI fungicides

Katrin Ayer: Cornell University

<div>Apple scab, caused by the ascomycete <em>Venturia inaequalis</em>, is one of the most economically devastating diseases of apple requiring multiple applications of fungicides throughout the growing season to achieve control. Newer chemistries in the class of succinate dehydrogenase inhibitors (SDHIs) have been registered for use in apple and will become increasingly important as resistance to other fungicide classes becomes more prevalent. To establish protocols for monitoring field resistance, we determined the baseline sensitivity of <em>V. inaequalis</em> to all SDHIs registered for use on apple. Furthermore, we analyzed cross-sensitivities of conidial and mycelial growth between six SDHI fungicides for isolates representing both baseline and exposed orchard populations. All SDHI fungicides greatly inhibited conidial and mycelial growth of baseline <em>V. inaequalis</em> isolates, with mean EC<sub>50</sub> values of conidial growth for all SDHI fungicides ranging from 0.0016 to 0.176 μg ml<sup>-1</sup>. Preliminary trends indicate some correlation in sensitivity between all SDHI fungicides. Isolates with previous exposure to fungicides show a decreased level of sensitivity to SDHI fungicides when compared to baseline isolates, suggesting quantitative resistance mechanism may be playing a role in isolate response. Thus far, no mutations in the target <em>sdh</em> target genes have been identified. Phenotypic and genotypic assessment is ongoing to continually monitor resistance development.</div>

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