Grape Powdery Mildew Fungicide Programs Affect Non-Target Fungi Populations Differentially in California Vineyards in a Single Growing Season

Grape powdery mildew (GPM) caused by Erysiphe necator is responsible for severe loss in grape production worldwide. Fungicide programs are implemented to control GPM each year. These frequent applications, while impacting the causal organism, may also influence populations of non-target fungi. This study aimed to investigate the effects of GPM fungicide programs on non-target fungal populations in grape vineyards using two fungal genera, Botrytis and Alternaria, collected from three grape vineyards in California. Vineyards included one field with six replicated GPM fungicide programs (treatments) and two fields with one GPM fungicide program for each. Monoconidial cultures of Botrytis and Alternaria species were obtained from leaf or rachis samples collected from each treatment. Isolates were evaluated and analyzed for: (1) resistance to three commonly used fungicides: pyraclostrobin (FRAC 11), boscalid, and fluopyram (both FRAC 7), (2) colony morphotype, (3) diversity of species (phylogeny), and (4) genetic diversity and population structure. Botrytis and Alternaria populations displayed significantly different fungicide resistance frequencies across the different treatments even within the same field, suggesting that GPM fungicide programs may have variable effects on non-target fungal populations. However, colony morphotype, genetic diversity and population structure did not show significant variability associated with treatment in either genus based on the small set of simple sequence repeat (SSR) markers evaluated. Species diversity among isolates of Alternaria was greater than that found among Botrytis isolates, but varied by field. Simple sequence repeat (SSR) markers identified significant genetic diversity associated with field and colony morphology for both genera.