Effects of Nighttime Applications of Germicidal Ultraviolet Light Upon Powdery Mildew (), Downy Mildew (), and Sour Rot of Grapevine.

Nighttime applications of germicidal ultraviolet were evaluated as a means to suppress three diseases of grapevine. In laboratory studies, UV-C light (peak 254 nm, FWHM 5 nm) applied during darkness strongly inhibited the germination of conidia of , and at a dose of 200 J/m, germination was zero. Reciprocity of irradiance and duration of exposure with respect to conidial germination was confirmed for UV-C doses between 0 and 200 J/m applied at 4 or 400 s. When detached grapevine leaves were exposed during darkness to UV-C at 100 J/m up to 7 days before they were inoculated with zoospores of , infection and subsequent sporulation was reduced by over 70% compared to untreated control leaves, indicating an indirect suppression of the pathogen exerted through the host. A hemicylindrical array of low-pressure discharge UV-C lamps configured for trellised grapevines was designed and fitted to both a tractor-drawn carriage and a fully autonomous robotic carriage for vineyard applications. In 2019, in a Chardonnay research vineyard with a history of high inoculum and severe disease, weekly nighttime applications of UV-C suppressed on leaves and fruit at doses of 100 and 200 J/m. In the same vineyard in 2020, UV-C was applied once or twice weekly at doses of 70, 100, or 200 J/m, and severity of on both leaves and fruit was significantly reduced compared to untreated controls; twice-weekly applications at 200 J/m provided suppression equivalent to a standard fungicide program. None of the foregoing UV-C treatments significantly reduced the severity of on Chardonnay vines compared to the untreated control in 2020. However, twice-weekly applications of UV-C at 200 J/m to the more downy mildew-resistant interspecific hybrid cultivar Vignoles in 2021 significantly suppressed foliar disease severity. In commercial Chardonnay vineyards with histories of excellent disease control in Dresden, NY, remained at trace levels on foliage and was zero on fruit following weekly nighttime applications of UV-C at 200 J/m in 2020 and after weekly or twice-weekly application of UV-C at 100 or 200 J/m in 2021. In 2019, weekly nighttime applications of UV-C at 200 J/m also significantly reduced the severity of sour rot, a decay syndrome of complex etiology, on fruit of 'Vignoles' but not the severity of bunch rot caused by . A similar level of suppression of sour rot was observed on 'Vignoles' vines treated twice-weekly with UV-C at 200 J/m in 2021. Nighttime UV-C applications did not produce detectable indications of metabolic abnormalities, phytotoxicity, growth reduction, or reductions of fruit yield or quality parameters, even at the highest doses and most frequent intervals employed.