Managing Grapevine Powdery Mildew with Ultraviolet-C Light in Washington State

Abstract

Context and purpose of the study – Germicidal ultraviolet-C (UV-C) light has shown promising results for suppression of several plant-pathogenic microorganims, including Erysiphe necator, which attacks grapevine. In Washington State the majority of winegrape production is in a semi-arid steppe environment, with historically low powdery mildew disease pressure, making it a promising area to deploy UV-C as a disease management tool. Trials focusing on UVC application timing and frequency will assist in developing regionally-appropriate application recommendations for eastern Washington State.

Material and methods – Trials were repeated for three years in a Vitis vinifera ‘Chardonnay’ vineyard at Washington State University (2020, 2021, and 2022) in Prosser, WA, USA. We evaluated the use of UV-C (200 J/m²) in a vineyard setting under different application regimes focused on timing (season-long, or early-season only), and frequency (intervals of application). In 2020, UV-C was applied weekly; in 2021 and 2022, UV-C was applied weekly or twice-weekly. Early-season applications were made between 15 cm shoot growth to prebloom. After prebloom, early-season vines switched to a standard fungicide spray program. In season-long trials, treatments were applied through 4-weeks postbloom. Control treatments included season-long unsprayed and a season-long fungicide program. In the early-season trials, a third control was included: unsprayed until prebloom, then sprayed after prebloom. Powdery mildew disease severity was visually rated on leaves and clusters bi-weekly from bloom until harvest.

Results – For early-season treatments, weekly UV-C in 2020 reduced foliar disease by 85% relative to the season-long unsprayed (p=0.003). In 2021, there was virtually no disease (extended “heat dome” events from bloom through véraison), and treatment separation.  In 2022, compared to the season-long unsprayed, weekly or twice-weekly early-season UV-C reduced foliar disease by 77% (p=0.0001) and 74% (p=0.0001); cluster disease was reduced by 78% (p=0.0001) and 74% (p=0.0001). For season-long treatments, UV-C in 2020 and 2021 did not significantly reduce foliar disease relative to the season-long unsprayed, though there was a trend for increasing control with more frequent UV-C intervals. For example, foliar disease was 25% and 50% less in 2020 and 2021 (respectively) with weekly UV-C; foliar disease was 79% less in 2021 for twice-weekly UV-C. In 2022, season-long weekly UV-C reduced foliar disease severity by 37% (p=0.03) and twice-weekly UV-C reduced foliar disease by 61% (p=0.0003) relative to the season-long unsprayed control. Cluster disease severity was reduced in 2022 by 40% (p=0.0001) with season-long twice-weekly UV-C and 16% (p=0.03) with season-long weekly UV-C compared to the season-long unsprayed control. While not always statistically significant, the consistent trend in reduction of powdery mildew disease severity by using UV-C weekly or twice-weekly, suggests that UV-C could be an additional tool for alternative powdery mildew management in eastern Washington winegrape vineyards.