Effects of Nighttime Applications of Germicidal Ultraviolet Light upon Powdery Mildew (Erysiphe necator), Downy Mildew (Plasmopara viticola), and Sour Rot of Grapevine

Authors: GADOURY, DAVID - Cornell University; SAPKOTA, SURYA - Cornell University; Cadle-Davidson, Lance; Underhill, Anna; MCCANN, TYLER - Cornell University; GOLD, KAITLIN - Cornell University; GAMBHIR, NIKITA - Cornell University; COMBS, DAVID - Cornell University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2022
Publication Date: 5/18/2023
Citation: Gadoury, D.M., Sapkota, S., Cadle Davidson, L.E., Underhill, A.N., McCann, T., Gold, K., Gambhir, N., Combs, D. 2023. Effects of nighttime spplications of germicidal ultraviolet light upon Powdery Mildew (Erysiphe necator), Downy Mildew (Plasmopara viticola), and sour rot of grapevine. Plant Disease. https://doi.org/10.1094/PDIS-04-22-0984-RE.
DOI: https://doi.org/10.1094/PDIS-04-22-0984-RE

Interpretive Summary: We investigated nighttime Ultraviolet-C (UV-C) light for suppression of four of the most destructive grapevine diseases. The treatments were applied at night because pathogens are much more sensitive to UV at night, which allowed us to use a low UV dose that was harmless to plants. In the lab, UV-C applied during darkness strongly inhibited the spores of Grapevine Powdery Mildew (GPM). The same UV-C treatments protected grapevine leaves from Grapevine Downy Mildew (GDM) for up to 7 days, reducing GDM by more than 70% compared to untreated leaves. This indicated an increase of plant resistance to the diseases caused by the UV-C application. When testing moved from the lab to the vineyard, we used a mobile array of UV-C lamps configured for a vineyard that was either moved using a tractor or a fully autonomous robot. In 2019 and 2020, in a research vineyard planted to GPM- and GDM-susceptible variety ‘Chardonnay’ with a history of high disease pressure, weekly nighttime applications of UV-C strongly suppressed GPM on leaves and fruit. The highest intensity treatments were as effective for GPM as a standard fungicide program. However, unlike in our lab studies, none of the UV-C treatments worked against GDM in ‘Chardonnay’ vineyards. In commercial ‘Chardonnay’ vineyards that had a history of low disease pressure, UV-C provided even better control of GPM in 2020 and 2021. When we switched to the more GDM-resistant variety ‘Vignoles’ in 2021, twice-weekly UV-C treatments reduced the severity of GDM by over 50%, showing that UV-C has the potential to manage GDM on more disease-resistant grape varieties. We also discovered that UV-C provides excellent suppression of a third disease, a problematic complex referred to as sour rot. Sour rot is caused by an interaction between fungi, bacteria, and fruit-feeding insects. The efficacy of UV-C is especially significant against the combined attack by such a diverse group of pests. Thus far, we have not detected a significant effect of UV-C upon a fourth disease, Botrytis bunch rot. A comprehensive evaluation of UV-C for non-target effects showed no harmful impact of UV-C upon vine growth, fruit yields or quality, or upon non-target and presumably beneficial microbes living on the grapevine leaves or fruit. When used in accordance with the doses and frequencies detailed in this paper, nighttime UV-C can be an innovative and highly effective tool for maintaining the health of plants that are attacked by a diverse and destructive complex of pathogens and pests.

Technical Abstract: In laboratory studies, UV-C light (peak 254 nm, FWHM 5 nm) applied during darkness strongly inhibited the germination of conidia of Erysphe necator, and at a dose of 200 J/m2 germination was nil. Reciprocity of irradiance and duration of exposure with respect to conidial germination was confirmed for UV-C doses between 0 and 200 J/m2 applied at 4 or 400 seconds. When detached grapevine leaves were exposed during darkness to UV-C at 100 J/m2 up to 7 days before they were inoculated with zoospores of Plasmopara viticola, 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 E. necator on leaves and fruit at doses of 100 and 200 J/m2. In the same vineyard in 2020, UV-C was applied once or twice weekly at doses of 70, 100, or 200 J/m2, and severity of E. necator on both leaves and fruit was significantly reduced compared to untreated controls, and twice-weekly applications at 200 J/m2 provided suppression equivalent to a standard fungicide program. None of the foregoing UV-C treatments significantly reduced the severity of P. viticola on Chardonnay vines compared to the untreated control in 2020. However, twice-weekly applications of UV-C at 200 J/m2 to the more downy mildew-resistant Vitis interspecific hybrid cultivar Vignoles in 2021 significantly suppressed foliar disease severity. In commercial Chardonnay vineyards with histories of excellent disease control in Dresden, NY, E. necator remained at trace levels on foliage and was nil on fruit following weekly nighttime applications of UV-C at 200 J/m2 in 2020, and after weekly or twice-weekly application of UV-C at 100 or 200 J/m2 in 2021. In 2019, weekly nighttime applications of UV-C at 200 J/m2 also significantly reduced the severity of sour rot, a decay syndrome of complex etiology, on fruit of the Vitis interspecific hybrid cultivar ‘Vignoles’, but not the severity of bunch rot caused by Botrytis cinerea. A similar level of suppression of sour rot was observed on Vignoles vines treated twice-weekly with UV-C at 200 J/m2 in 2021. Nighttime UV-C applications did not produce detectable indications of phytotoxicity, growth reduction, or reductions of fruit yield or quality parameters, even at the highest doses and most frequent intervals employed.