Illuminating Ultraviolet-C Light for Grapevine Powdery Mildew Management
Alexa McDaniel,* Maria Mireles, and Michelle
Moyer
*Washington State University, 24106 N Bunn Rd, Prosser, WA, 99350
(alexa.mcdaniel@wsu.edu)
Germicidal Ultraviolet-C (UVC) is a promising management tool for grapevine powdery mildew (Erysiphe necator). We evaluated UVC disease management efficacy and impact on berry quality in 2020 and 2021 in a mature Vitis vinifera Chardonnay vineyard at WSU Prosser. UVC was applied at 200 J/m2 30 min post-sunset with an over-the-row light array. In 2020, we evaluated application intervals of once per week and every 14 days; in 2021, we evaluated application intervals of twice per week and once per week. Unsprayed and standard spray programs were included. Plot sizes included 30 vines replicated four times throughout the vineyard. Treatments began at 7 to 15 cm shoot growth and ended at approximately three weeks post-fruit set. These vintages were challenging for disease evaluation, as there was naturally low disease pressure. End-of-season disease severity in the untreated controls was 43% (2020) and 16% (2021) for foliar and 3% (2020) and <1% (2021) for clusters. Despite this low pressure, positive trends were seen with UVC treatment. Disease severity was converted to accumulated area under the disease progress curve (AUDPC) and the presented statistics represent this. In 2020, while UVC did not statistically reduce foliar disease severity, treatment once per week reduced foliar disease by 25% compared to the unsprayed control. In 2021, the twice per week application reduced disease by 79% and the once per week by 50% compared to the untreated control, though treatments were not statistically different (p = 0.09). Berry weight, Brix, pH, and titratable acidity were not affected by UVC treatment, but there was an increase in total skin phenolics when treated with UVC once per week in 2020 (p = 0.009); 2021 data is still being processed. The consistent disease reduction with UVC applications once or twice per week, even under very low disease pressure, and the lack of detrimental impact on berries, suggests that UVC could be a complementary powdery mildew disease management tool.
Funding Support: Washington State Grape and Wine Research Program
