Abstract
Background and goals Sour rot, a variable and difficult to manage bunch rot disease of grapevines, is environmentally triggered by warm, humid weather during ripening. The recommended treatment of weekly insecticide and fungicide application starting at 12 to 15 Brix is costly for growers, especially during low-risk years, and can increase the risk of insecticide resistance developing in vinegar fly populations. This study investigated the efficacy of alternative, non-insecticide management strategies to mitigate sour rot disease.
Methods and key findings Fruit-zone leaf removal, ultraviolet-C (UV-C) light, and the combination of both were applied to Vignoles grapes over three growing seasons and compared to untreated controls. Metrics of sour rot occurrence and damage were taken at harvest. The environmental conditions necessary for sour rot varied by year, with 2022 having the most severe outbreak. Leaf removal and UV light significantly decreased sour rot damage in all 3 yr. Combining these treatments did not enhance their effects and was not different from untreated controls in the most severe sour rot year (2022).
Conclusions and significance Alternative, non-insecticidal management strategies are successful at reducing the impact of sour rot disease. Our results provide options for growers to reduce insecticide sprays while still protecting their vineyards from sour rot disease.
- canopy management
- fruit zone leaf removal
- late season bunch rot
- non-insecticidal management
- ultraviolet light
- vineyard disease management
Introduction
Sour rot is a grapevine bunch rot disease of particular economic concern that is caused by an environmentally triggered complex of vinegar flies, yeast, and bacteria in the warm, humid weather of the Mid-Atlantic and northeast United States (U.S.) production regions (Huber 2016, Rombaut et al. 2017, Hall et al. 2018a, Ioriatti et al. 2018, Zhu et al. 2023, Bhandari et al. 2024a, Brischetto et al. 2024a). Thin-skinned, tight clustered grape varieties such as Riesling, Pinot noir, and Vignoles are highly susceptible to this disease (Barata et al. 2012, Bhandari et al. 2024a, Brischetto et al. 2024b). Sour rot chemically alters grape berry composition, reducing the quantity and quality of grape must and wine produced (Zoecklein et al. 2000, Meneguzzo et al. 2008, Barata et al. 2008, 2011a, 2011b, Steel et al. 2013, Lleixà et al. 2018). Disease is variable across years but can cause large yield losses in years with warm, humid weather during fruit ripening. A survey of New York growers in 2019 indicated ~36.5% of vineyard acreage was affected by sour rot in 2018 (ranging from 5 to 90%), with estimated losses averaging at 24% crop yield (ranging from 5 to >50%).
Vinegar flies like Drosophila melanogaster and Drosophila suzukii are a primary vector of disease etiology, and as such, insecticides play a crucial role in disease management (Rombaut et al. 2017, Hall et al. 2018b, Ioriatti et al. 2018, Entling and Hoffmann 2020, Bhandari et al. 2024b, Neugebauer et al. 2024, Jumbam et al. 2025). The commercial standard for disease management is to apply insecticides and fungicides weekly once grapes reach 12 to 15 Brix—an expensive but effective management regime (Hall et al. 2018b). Growing insecticide resistance in vinegar flies further challenges effective disease management, and resistance to several commercial standard products has been documented in recent studies (Sun et al. 2019, Kenney 2020, Hubhachen et al. 2022, Mertz et al. 2022, 2023, Bhandari et al. 2024b, Breeden et al. 2025).
Non-insecticide based management strategies are needed for grapegrowers to combat the risk of sour rot. Non-chemical treatments such as ultraviolet (UV) light and fruit-zone leaf removal at fruit set have been successful at reducing infection from other bunch rots such as Botrytis cinerea (Mosetti et al. 2016, Steel et al. 2016, Hickey and Wolf 2018, VanderWeide et al. 2020, Mundy et al. 2022, Wegher et al. 2022, Gadoury et al. 2023, Hed and Centinari 2024). Canopy density, reduced through leaf removal, has been identified as a key risk factor contributing to sour rot development (Hall et al. 2018b). Removing leaves early in the season (before or at bloom) alters cluster architecture, decreases berry density, and lowers bunch rot impact (VanderWeide et al. 2020, 2021, Wegher et al. 2022). Management of sour rot by leaf removal has seen promising results in several studies, indicating that it is a viable alternative to insecticides (Zoecklein et al. 1992, Ferrari et al. 2017, VanderWeide et al. 2020), but the U.S. Environmental Protection Agency (EPA) has raised concerns over pesticide exposure during leaf removal (Carter 2021, Chandgoyal et al. 2024). The practice is time and labor intensive, and most effective when conducted early in the season prior to symptom development. Mechanized leaf removal studies have shown success in reducing the effects of bunch rots, but the outcomes are variable depending on training system, variety, and disease (Hed and Centinari 2021, 2024). A key motivation for the present study was to provide growers with data on the costs and benefits of leaf removal so they can use these results to inform their disease management efforts early in the season before the preharvest weather is known.
UV light damages microbial cells through DNA and RNA damage and has been shown to increase plant defense pathways against microbial pathogens (Bonomelli et al. 2004, Urban et al. 2018, Vanhaelewyn et al. 2020, Gadoury et al. 2023, Ramalingam et al. 2024). UV-B (280 to 315 nm) and UV-C light (200 to 280 nm) have been explored as pathogen preventatives in several crop species, including grapevine (Kakani et al. 2003, Raviv and Antignus 2004, Paul et al. 2012, Janisiewicz et al. 2016, Urban et al. 2018, Vanhaelewyn et al. 2020, Meyer et al. 2021, Yemmireddy et al. 2022, Gadoury et al. 2023, Lopes et al. 2023, Ramalingam et al. 2024, Wong et al. 2024); for example, it is now well established that nighttime UV-C light application is an effective approach for grapevine powdery mildew control (Gadoury et al. 2023, Wong et al. 2024). UV treatments have been explored for the management of bunch rots such as sour rot and B. cinerea, with mixed success (Gadoury et al. 2023, Ramalingam et al. 2024, Wong et al. 2024). Previous work in Vignoles grapes indicates that weekly applications of UV-C light at 200 J/m2 were as equally effective as fungicides in reducing sour rot severity, and these metrics were used to inform our experimental design (Gadoury et al. 2023). Understanding the crossapplicability of UV-C technology for sour rot control would help growers make informed decisions on whether to invest in UV-C technology for vineyard disease management, while improving sustainability through reduced insecticide use. The goal of this study was to evaluate the efficacy of UV light, fruit-zone leaf removal, and a combination of both, against sour rot across 3 yr of replicated study in Vignoles grapevine in northern New York.
Materials and Methods
The Cornell Pathology Vineyards in Geneva, NY were used to examine the efficacy of non-insecticide management strategies to mitigate sour rot disease. The experiments were conducted on Vignoles grapes in a block of top-wire cordon-trained vines (planted in 2000) with 7.32-m panels. The majority of vines were own-rooted, with some replants on rootstock 3309. Rows were oriented north-to-south, with 2.74-m row spacing and 1.83-m vine spacing. Vineyard floor management comprised of pre-emergent herbicide and two cultivations per year. Vines were spur pruned, pruning three to five buds per spur, and mechanically hedged two times per season. Fungicide and insecticide applications are outlined (Supplemental Table 1). Vines were fertilized in the fall with 1681.28 cg/m2 of sulfate of potash magnesia (Sul-Po- Mag; 22% sulfur, 22% potassium [K], 11% magnesium) with borate, and in the spring with 1120.85 cg/m2 each of 22-0-0 (22% nitrogen, 0% phosphorus, 0% K) and potassium chloride. No irrigation system was used.
Treatments were investigated across three growing seasons (2022 to 2024) and included UV light, fruit-zone leaf removal, a combination of UV light and leaf removal, and an untreated control group. Treatments were replicated four times each year in a randomized complete block design across plots with nine to 20 vines. Additionally, the commercial standard for managing sour rot (insecticide sprays) was tested in 2022. In addition to the variable treatments, multiple materials were applied for control of black rot, phomopsis, downy mildew, powdery mildew, grape phylloxera, and Japanese beetle (Supplemental Table 1). Weather data for each year were collected from a Network for Environment and Weather Applications (NEWA) weather station located at the agricultural field station where the research vineyard is located (Supplemental Table 2 and Supplemental Figure 1).
Manual leaf removal from the fruit zone occurred during the early bloom stage (mid-to-late June). Basal leaves were removed up to one to two leaves past the distal cluster (e.g., if the distal cluster was at node 4, leaves were pulled on nodes 1 to 5 or 1 to 6, depending on the length of the shoot). This was done the same way in each season but the arrangement of treatments was not identical across years. UV treatments were applied weekly after grapes had reached 12 Brix (mid- August to mid-September). This timing was variable so that the number of weekly UV exposures differed across years (Supplemental Table 3). UV light was applied with a custom over-the-row apparatus designed to deliver 200 J/m2 (see Gadoury et al. [2023] for specifications). Daytime applications of UV light were used in 2022 to 2023; day and night applications of UV treatments were compared in 2024 but did not show differences in any measured metrics so they were combined in subsequent analyses. In 2022, the commercial standard treatment of insecticides (Mustang Max, 2.8 cg/m2; Induce, 2.8 cg/m2) and microbial pesticides (Oxidate, 44.83 cg/m2) was administered weekly after 12 Brix (D. Combs, unpublished data). Insecticides and antimicrobial pesticides were applied with a custom hooded boom sprayer attached to a 67 kW (90 hp) tractor operating at 690 kPa (100 psi). Disease assessments were made on 20 clusters randomly selected from the center two vines of each fourvine panel. Disease incidence (proportion of assessed clusters exhibiting signs of disease) and severity (proportion of berries per infected cluster exhibiting disease symptoms) were visually estimated following a modified Horsfall-Barrat rating scale, and proportion of total damage was calculated from these data using the equation (incidence × severity)/ 100 (Horsfall and Barratt 2025). Assessments were made at the end of the experiment, which was concluded at typical Vignoles harvest time in mid-to-late September (Supplemental Table 2).
Statistical analyses
All data manipulations and analyses were conducted in the statistical program R (ver. 4.5.1) using the user interface RStudio (2025.09.0+387) (R Core Team 2025). Three metrics of disease impact were examined: incidence, severity, and total damage (Supplemental Tables 4 to 6). Incidence was calculated as the number of clusters that showed any sign of sour rot damage, severity as the proportion of infected berries on affected clusters, and total damage as (incidence × severity)/100. Incidence was analyzed using a generalized linear mixed effects model (GLMM) with a binary distribution, using the glmer function from the ‘lme4’ package (Bates et al. 2015). Treatment and year were included in the model as factorial fixed effects, with block included as a random intercept. The interaction term between year and treatment was dropped as non-significant. Severity and total damage were analyzed with linear mixed-effects models (LMM), using the lme function from the ‘nlme’ package (Pinheiro et al. 2026). Treatment, year, and their interaction were included as factorial fixed effects; block was included as a random intercept. Post-hoc analyses were conducted by calculating the estimated marginal means of pairwise treatments, using the emmeans function from the ‘emmeans’ package (Lenth and Piaskowski 2026). Separate analyses with the same parameters were conducted on the data from 2022 to compare the alternative disease management treatments to the commercial standard mix of insecticides and microbial pesticides.
Results
Sour rot disease was successfully managed by UV treatment, leaf removal, and the combination of both across the three trial years. The trial years differed in the amount of sour rot present in the vineyard, with 2022 having much higher disease occurrence than 2023 to 2024 (LMM, p < 0.001; Supplemental Figure 2B, Supplemental Tables 4 to 6). Sour rot incidence was reduced by the alternative disease management treatments compared to untreated controls, but remained high, especially in the high sour rot year of 2022 (GLMM: p < 0.001; Figure 1B and 2B, Supplemental Figure 2B, Supplemental Table 4). All treatments reduced sour rot severity and total damage across years (LMM: severity: p < 0.001; total damage: p < 0.001; Figure 1A and 1C), but the combination of UV and leaf removal was less effective in 2022—the most severe sour rot year during the trial (Supplemental Figure 2A and 2C, Supplemental Table 4).
The effects of ultraviolet-C (UV-C) light, leaf removal, and UV-C + leaf removal compared to untreated controls (UTC) on the (A) total damage, (B) incidence (number of infected clusters), and (C) severity of sour rot across 3 yr (2022 to 2024) in Vignoles grapes in Geneva, NY. Boxes plot the median value, the first and third quartiles, while whiskers indicate the range of values within 1.5 * interquartile range; data outside this range are plotted individually. Letters designate statistical differences in pairwise comparisons (p < 0.05), calculated post-hoc with estimated marginal means. Sample size for each treatment is n = 240.
Effects of ultraviolet-C (UV-C) light, leaf removal, UV-C + leaf removal, and commercial standard insecticide sprays compared to untreated controls (UTC) on the (A) total damage, (B) incidence (number of infected clusters), and (C) severity of sour rot in 2022 for Vignoles grapes in Geneva, NY. Boxes plot the median value, the first and third quartiles, while whiskers indicate the range of values within 1.5 * interquartile range; data outside this range are plotted individually. Letters designate statistical differences in pairwise comparisons (p < 0.05), calculated post-hoc with estimated marginal means. Sample size for each treatment is n = 80.
The commercial standard treatment for sour rot was compared to UV light, leaf removal, and their combination in 2022. Sour rot incidence was not reduced by either the commercial standard or the combination of UV and leaf removal (GLMM: commercial standard: p = 0.225; UV + LP: p = 0.106; Figure 2B and Supplemental Table 4). While UV and leaf removal individually decreased sour rot incidence, it remained high at ~70% of clusters showing infection (GLMM: UV: p = 0.003; leaf removal: p = 0.002; Figure 2B and Supplemental Table 4). In contrast, UV and leaf removal reduced severity (LMM: UV: p = 0.004; leaf removal: p = 0.002) and total damage (LMM: UV: p = 0.003; leaf removal: p = 0.001) by an average of 40 to 45% compared to untreated controls (Figure 2A and 2C, Supplemental Table 4). The combination of leaf removal and UV light reduced sour rot severity and total damage (LMM: severity: p = 0.048; total damage: p = 0.038), but the effect was smaller than either treatment alone, with an average decrease of 25% compared to untreated controls (Figure 2 and Supplemental Table 4). The commercial standard treatment did not reduce sour rot severity or total damage compared to the untreated controls (LMM: severity: p = 0.086; total damage: p = 0.063; Supplemental Table 4).
Discussion
The overall goal of this study was to investigate the efficacy of non-insecticide management strategies for sour rot disease in grapevines. This form of bunch rot is variable across years due to environmental factors of disease etiology, which makes management decisions difficult, especially because the greatest impacts are seen in late season just before harvest. This presents a tradeoff for growers: invest in preventative, high expense, weekly pesticide treatments without knowing if weather conditions will trigger sour rot, or risk high-yield losses if the preharvest season is warm and humid. While recent research indicates that minimizing insecticide sprays to only symptom onset after veraison and just before harvest can provide comparable protection to weekly treatments, this has yet to be tested at a commercial scale (Hall et al. 2018b, Bhandari et al. 2024). Developing alternative management strategies for growers is crucial in combating this variable but damaging disease, to decrease both grower expenses as well as pressure for insecticide resistance in vinegar flies. UV light treatment and leaf removal have been used to prevent plant pathogen damage and reduce bunch rots in vineyards, and both present viable disease management approaches for controlling sour rot.
In this study, UV light and leaf removal showed significant suppression of sour rot in Vignoles grapes in both high- and low-risk years. The combination of these treatments was not additive and actually decreased the efficacy of sour rot management in the most severe disease year. It is unclear why the leaf pulling and UV combination did not have additive results; cultural control practices may have an upper limit to their efficacy, above which they fail to have synergistic effects. Additionally, while leaf removal is generally effective at reducing bunch rots, it can be variable in its effects across years (Hed et al. 2015, VanderWeide et al. 2021). These findings indicate that either leaf removal or UV treatment are viable strategies for growers to control sour rot damage, but there are minimal benefits from using both. Leaf removal was an effective disease management strategy across all trial years, reducing the total damage caused by sour rot by up to 46.23% in 2022. However, leaf removal is a labor intensive practice and tends to be more effective at disease control when applied earlier in the season: at bloom or fruit set, rather than at veraison (Zoecklein et al. 1992b, Komm and Moyer 2015, Ferrari et al. 2017, VanderWeide et al. 2020, Hed and Centinari 2024). There have been investigations into the efficacy of manual versus mechanical leaf removal, which indicate that less labor-intensive mechanical trimming can be effective at reducing Botrytis bunch rot, but more exploration of timing, methods, and the effect of vine training system are warranted (Hed and Centinari 2021, 2024). Additionally, leaf removal can alter the sugar and chemical composition of berries and reduce berry weight and total yield, which may not be desired by growers (Guidoni et al. 2008, Tardaguila et al. 2010, Kemp et al. 2011, Pisciotta et al. 2013, Komm and Moyer 2015, Mosetti et al. 2016, Ferrari et al. 2017, Hed and Centinari 2018, 2024, VanderWeide et al. 2020). In contrast, UV treatment was effective in our study when applied during veraison (starting at 12 to 15 Brix), reducing total sour rot damage by up to 40.62%. This later starting date gives growers more flexibility in deciding if and when to start treatment for sour rot, rather than automatically starting early in the season when preharvest weather conditions are difficult to predict. However, the equipment for UV light application is not currently commercially available (but can be custom fabricated) and is most effective when applied at night (Gadoury et al. 2023).
Conclusion
The results of our study provide grapegrowers with effective, non-insecticide alternatives for managing sour rot disease in susceptible varieties. Our results show that leaf removal and UV light treatment individually are effective disease management strategies, but they do not have additive effects when used in combination. While leaf removal reduces sour rot damage, it is labor intensive and applied early in the season, before the risk of sour rot is known. Alternatively, UV light treatment can be deployed during veraison when environmental factors for sour rot development are present.
CRediT Authorship Contributions
MM, SE: Formal Analysis, Visualization, Writing – Original Draft; MM, SE, KG: Writing – Review & Editing; SE, HWP, KG: Conceptualization; SE, DBC, HWP, KG: Project Administration; DC, DBC, KG: Data Curation; DC, DBC, HWP: Investigation; DC, DBC, HWP, KG: Methodology; HWP, KG: Funding Acquisition, Resources, Supervision
Conflict of Interest
The authors declare no conflicts of interest.
Supplemental Data
The following supplemental materials are available for this article in the Supplemental tab above:
Supplemental Table 1 Materials applied to Vignoles grapevines to control for non-target pests and pathogens in each trial year. Research vineyards were located in Geneva, NY and trials were conducted in 2022 to 2024.
Supplemental Table 2 Monthly weather data during the summer of each trial year (2022 to 2024), collected from a Network for Environment and Weather Applications (NEWA) weather station located in Geneva, NY at the agricultural research station that houses the research Vignoles vineyard. Errors = standard deviation.
Supplemental Table 3 Plot trial details for each trial year (2022 to 2024) for alternative management strategies of sour rot disease in Vignoles vineyards in Geneva, NY. UV-C, ultraviolet-C.
Supplemental Table 4 Metrics of sour rot in Vignoles grapes in a research vineyard in Geneva, NY in response to alternative disease management strategies in the 2022 trial year. SD, standard deviation; UTC, untreated controls; UV-C, ultraviolet-C.
Supplemental Table 5 Metrics of sour rot in Vignoles grapes in a research vineyard in Geneva, NY in response to alternative disease management strategies in the 2023 trial year. SD, standard deviation; UTC, untreated controls; UV-C, ultraviolet-C.
Supplemental Table 6 Metrics of sour rot in Vignoles grapes in a research vineyard in Geneva, NY in response to alternative disease management strategies in the 2024 trial year. SD, standard deviation; UTC, untreated controls; UV-C, ultraviolet-C.
Supplemental Figure 1 Monthly weather data for each trial year (2022 to 2024), collected from a Network for Environment and Weather Applications (NEWA) weather station located in Geneva, NY. (A) Mean monthly temperature and (B) relative humidity are shown with error bars = standard error. The total precipitation accumulated each month is also shown (C).
Supplemental Figure 2 Effect of year on the efficacy of management treatments on the (A) total damage, (B) incidence (number of infected clusters), and (C) severity of sour rot in Vignoles grapes in a research vineyard in Geneva, NY for the trial years 2022 to 2024. UTC, untreated control; UV-C, ultraviolet-C.
Data Availability
The data underlying this study were deposited into Zenodo at: https://doi.org/10.5281/zenodo.18792300
Footnotes
This work was supported by the New York Wine and Grape Foundation and the Finger Lakes Grape Program (FLGP). The authors thank Ellen Coyne from the FLGP for field assistance during this project.
Moore ME, Emery SE, Caldwell D, Combs DB, Walter-Peterson H and Gold KM. 2026. UV-C and leaf removal provide equivalent and effective sour rot control in Vignoles grapevine. Am J Enol Vitic 77:0770023. DOI: 10.5344/ajev.2026.26010
By downloading and/or receiving this article, you agree to the Disclaimer of Warranties and Liability. If you do not agree to the Disclaimers, do not download and/or accept this article.
- Received March 2026.
- Accepted June 2026.
- Published online September 2026
This is an open access article distributed under the CC BY 4.0 license.
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