Alexa L. McDaniel

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 season-long UV-C exposure on basic fruit chemistry. Methods and key findings UV-C doses of 100 and 200 J/m² 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/m² dose was equivalent to that of the control (horticultural oil) treatment. UV-C was also applied in Vitis vinifera Chardonnay vineyards at 200 J/m² 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 season-long 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 disease-pressure 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.

Background and goals Ozone (O₃) is a powerful oxidizer. When dissolved in water, O₃ offers a direct, residue-free application to potentially control unwanted pests. Ozonated water spray (OWS) has been shown to suppress powdery mildew growth in greenhouse settings and cause insect mortality under in vitro conditions. Our goal was to evaluate the ability of OWS in lab and field settings to manage two main pests in eastern Washington vineyards: grapevine powdery mildew (Erysiphe necator) and grape mealybug (Pseudococcus maritimus). Methods and key findings The potential curative effects of ozonated water on nascent E. necator colonies and its lethal effects on P. maritimus nymphs were evaluated using O₃ concentrations of 1 or 7 mg/L, delivered through a laboratory OWS system. These studies were coupled with field evaluations using a commercial OWS retrofit system to manage E. necator prebloom and P. maritimus as a delayed-dormant treatment. Our lab studies against E. necator confirmed that OWS was not effective at reducing colony formation. Due to low disease pressure years during the study, we cannot confirm the field efficacy of OWS applied prebloom. Laboratory-scale OWS was inconsistent at increasing P. maritimus nymph mortality. OWS field evaluations for P. maritimus management did not reduce the nymph population. Conclusions and significance Current OWS systems are not an effective tool for E. necator and P. maritimus management when applied at the O3 concentrations that current commercial units can generate.