Assessment of Fungicide Resistance via Molecular Assay in Populations of Podosphaera leucotricha, Causal Agent of Apple Powdery Mildew, in New York.

Abstract

Podosphaera leucotricha, causal agent of apple powdery mildew, is a pathogen endemic worldwide where apples are produced. In the absence of durable host resistance, the disease is most effectively managed in conventional orchards with single-site fungicides. In New York State, increasingly erratic precipitation patterns and warmer temperatures due to climate change may create a regional environment more conducive to apple powdery mildew development and spread. In this scenario, outbreaks of apple powdery mildew may supplant the apple diseases of current management concern: apple scab and fire blight. Presently, there have been no reports from producers of fungicide control failures for apple powdery mildew, though increased disease incidence has been reported to and observed by the authors. As such, action was needed to assess the fungicide resistance status of populations of P. leucotricha to ensure key classes of single-site fungicides (FRAC 3, demethylation inhibitors, DMI; FRAC 11, quinone outside inhibitors, QoI; and FRAC 7, succinate dehydrogenase inhibitors, SDHI) remain effective. In a 2-year survey (2021 to 2022), we collected 160 samples of P. leucotricha from 43 orchards, representing conventional, organic, low-input, and unmanaged orchards from New York's primary production regions. Samples were screened for mutations in the target genes (CYP51, cytb, and sdhB) historically known to confer fungicide resistance in other fungal pathogens to the DMI, QoI, and SDHI fungicide classes, respectively. Across all samples, no nucleotide sequence mutations that translated into problematic amino acid substitutions were found in the target genes, suggesting that New York populations of P. leucotricha remain sensitive to the DMI, QoI, and SDHI fungicide classes, provided no other fungicide resistance mechanism is at play in the population.