THE EFFECTS OF SHOOT THINNING AND POST-BLOOM LEAF REMOVAL ON FRUIT ROTS AND DOWNY MILDEW FUNGICIDE RESISTANCE IN MID ATLANTIC VINEYARDS

Abstract

This thesis investigates integrated disease management strategies by evaluating essential canopy management practices and their effects on disease risk and development. Field trials were conducted across two vineyards in 2023 and 2024, involving three sites and five wine grape cultivars. Treatments included various combinations of high and low leaf removal and shoot density. Results demonstrated that low leaf removal significantly reduced ripe rot severity across trials and years, while low shoot density treatments lowered cluster powdery mildew and Botrytis severity. Disease severity varied by cultivar, with Merlot showing increased susceptibility to ripe rot and Vidal blanc to sour rot in 2024. Additionally, this research determined the fungicide resistance profiles of Plasmopara viticola, the causal agent of grapevine downy mildew. Between 2019 and 2023, 352 downy mildew samples were collected from 27 vineyards and 32 cultivars in Maryland and Pennsylvania. Resistance to common fungicides was assessed using whole-leaf bioassays, while amino acid mutations known for resistance development were confirmed via Sanger sequencing and PCR-RFLP. Overall, resistance was detected to azoxystrobin (69%), mandipropamid (39%), and phosphorous acids (33%) across all regions. Individual isolates, simultaneously displaying resistance to two or more chemical classes of fungicides, were frequently detected (20%). Notably, this study represents the first report of phosphorous acid resistance in North America, detected as early as 2020 and widespread across regions tested. These findings underscore the increasing threat posed by P. viticola to grape production in the Northeast, as rising resistance to multiple fungicide classes significantly limits control options for growers. Further, canopy management practices such as leaf removal and shoot thinning can be tailored to specific diseases by forming less conducive microclimates.