EVALUATION OF INTEGRATING UV-C AND CANOPY MANAGEMENT FOR ERYSIPHE NECATOR CONTROL, AND AN ANALYSIS OF ENVIRONMENTAL DRIVERS AND ADJUVANT USE ON SULFUR PHYTOTOXICITY IN VITIS VINIFERA

Jesse Stevens

Winegrapes (Vitis vinifera) are a major crop in Washington State but require effective management of Erysiphe necator, the causal agent of powdery mildew. Reliance on synthetic fungicides has raised sustainability concerns, prompting research into alternative strategies. Two experiments were conducted at the Washington State University Irrigated Agriculture Research and Extension Center in Prosser, WA. The first evaluated germicidal ultraviolet-C light (UV-C, 254 nm) integrated with canopy management as a modified disease management program. Treatments included standard fungicides at the start and end of the season with UV-C (200 J/m²) applied once or twice weekly during the critical window for powdery mildew management (bloom to four weeks post–full bloom), an unsprayed control, and a full-season fungicide program. Canopy management treatments included prebloom shoot thinning, shoot thinning plus fruit-zone leaf removal, and a no-management control. End-of-season cluster disease did not differ between fungicide-only and fungicide + UV-C programs. Canopy management improved control in both systems, reducing cluster severity by 25–91%. Overall, a modified disease management program that included UV-C during the critical window hybrid UV-C programs was as effective as conventional season-long fungicide programs, with canopy management enhancing—but not required for— effective control. The second study assessed sulfur phytotoxicity risk in eastern Washington. Although sulfur is widely used for powdery mildew control, its use is often limited by concerns over heat-induced phytotoxicity despite limited supporting evidence. Field, potted vine, and controlled environment studies were conducted to determine the drivers for potential sulfur phytotoxicity. In outdoor experiments, two sulfur formulations (applied a concentration of 19.15 g/L) and six adjuvants were sprayed on vines on days when the daytime high temperatures exceeded 32.2 °C. Resulting phytotoxicity, when it occurred (incidence was low) was less than 13% severity. In controlled conditions, sulfur was applied with three adjuvants and incubated under set combinations of temperature (26.7 or 37.1 °C) and relative humidity (43.1 or 80.1%). Temperature–humidity interactions significantly affected phytotoxicity, with the highest severity requiring both high temperature and high humidity. Additional factors, including adjuvants and leaf age, also influenced phytotoxicity risk. Overall, though the frequency and severity of sulfur-based phytotoxicity, regardless of temperature, humidity, and adjuvant use, remained low. The outdoor conditions required for high incidence and severity are not common in eastern Washington growing regions and therefore fears of phytotoxicity are likely overstated.

EVALUATION OF INTEGRATING UV-C AND CANOPY MANAGEMENT FOR ERYSIPHE NECATOR CONTROL, AND AN ANALYSIS OF ENVIRONMENTAL DRIVERS AND ADJUVANT USE ON SULFUR PHYTOTOXICITY IN VITIS VINIFERA

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