Effects of Ultraviolet-C Light on Grapevine Powdery Mildew and Fruit Quality in Vitis vinifera Chardonnay

Background and goals Germicidal ultraviolet light (UV-C) has suppressed grapevine powdery mildew in multiple cropping systems. Our goals were to further characterize the curative effects of UV-C light against grapevine powdery mildew (Erysiphe necator) in laboratory experiments, to understand how UV-C can be integrated into vineyard disease management programs, and to determine the effects of seasonlong UV-C exposure on basic fruit chemistry. Methods and key findings UV-C doses of 100 and 200 J/m 2 were applied to developing E. necator colonies at 24 to 144 hrs after inoculation in the lab. Both doses reduced colony development relative to the untreated control, and the effect of the 200 J/m 2 dose was equivalent to that of the control (horticultural oil) treatment. UV-C was also applied in Vitis vinifera Chardonnay vineyards at 200 J/m 2 once or twice weekly, under different application regimens (early-season only or season-long). In the most disease-favorable years, both regimens reduced foliar and cluster disease severity relative to the untreated control. Basic fruit chemistry was not affected by UV-C treatments in either the seasonlong UV-C trials or in an additional UV-C trial in Dresden, New York. Conclusions and significance The ability of UV-C to reduce powdery mildew on leaves and fruit in both low and high diseasepressure years, with no adverse effects on yield or fruit quality, suggests that it could be an additional, non-chemical tool to manage grapevine powdery mildew.