Sour Rot in Grapes: alternative management strategies update

Cornell Integrated Pest Management
Plant Pathology and Plant-Microbe Biology Section
Viticulture and Enology
Crops

Sour rot is a variable and difficult to manage bunch rot that affects thin-skinned, tight clustered grape varieties. It affects vineyards during the later part of the season, after grapes have reached 12-15 Brix, when weather conditions are warm and humid. Since sour rot is dependent on weather conditions, some years are heavily impacted and have up to 50% reduced yields, while other years have minimal damage. Current recommended management practices involve weekly pesticide sprays of insecticides and antimicrobials during ripening. However, these treatments are not cost effective in years when sour rot risk is low, and can be minimally effective during severe outbreaks. Additionally, insecticide resistance is increasingly reported in vinegar fly populations, emphasizing the need for non-insecticidal management strategies for sour rot. Researchers at Cornell Agritech investigated the efficacy of leaf pulling, UV-light, and peroxide in reducing sour rot damage, alone and in combination, across three years. Their results show that both leaf pulling and UV applications, but not peroxide, significantly limited sour rot damage, providing alternative management options for growers in combatting this disease.

Sour rot is a potentially devastating, late-season bunch rot of particular concern on tight clustered grapevine varieties in warm, wet years. This causes the disease to have sporadic effects on grape harvests, with some years showing only small yield losses, and others with >50% of fruit unable to process. Vinegar flies, Drosophila melanogaster and D. suzukii, are important components of the sour rot disease complex, so antimicrobial pesticides are ineffective for control when applied alone. The recommended treatment schedule for sour rot involves spraying insecticides and antimicrobial pesticides weekly once grapes reach 12-15 Brix. This is not only costly for growers, especially in low risk years where the benefits are minimal, it also increases insecticide resistance in Drosophila. Populations of D. melanogaster already show high resistance levels to three out of the five insecticide classes labeled in New York state for controlling Drosophila in grapes: pyrethroids (Mustang Maxx,), organophosphates (malathion) and neonicotinoids (Assail) (Sun et al. 2019). Several populations of D. melanogaster in New York also showed moderate levels of resistance to Delegate, raising concerns that efficacy for this insecticide class may also start to decline if not used judiciously (Scott et al. 2023, 2024).

The combined threat of yield loss, weekly pesticide applications, and increased insecticide resistance, emphasize the need for alternative management strategies. Researchers at Cornell AgriTech investigated several non-pesticide management strategies for sour rot control in a Vignoles vineyard block across three seasons from 2022-2024. Treatments tested across all three years and compared to an untreated control included leaf pulling, UV light treatment, and combination of leaf pulling and UV light. Leaf pulling is performed at fruit set, decreasing humidity and increasing air flow around bunches and through the canopy. UV light damages disease-causing microbes and is applied later in the season, after 12 Brix. A commercial grower standard and combination treatments including peroxide (Oxidate) which damages disease-causing microbes, were also tested in a subset of years. The 2022 season had very high incidence and severity of sour rot while in the 2023 and 2024 seasons sour rot caused minimal damage. The incidence—the number of bunches that had sour rot—and the severity—the proportion of diseased berries within clusters—and the total damage of sour rot in the experimental vineyards were measured across all treatments.

The results of these experiments in Vignoles indicate that these alternative management strategies can be successful at controlling sour rot but their impact is highly season dependent. Across all three years UV was as effective as leaf pulling for reducing total damage and both sour rot incidence and severity. There was no added benefit to combining leaf-pulling and UV. In the warm, wet year in which trials were conducted with high sour rot damage (2022), the commercial standard did not appear to reduce sour rot more than the untreated control, however it also did not perform significantly worse than the UV and leaf-pulling treatments. In mild sour rot years, with cooler, dryer weather, UV and leaf pulling, alone or in combination, reduced total damage significantly compared to untreated controls. Peroxide treatment in isolation or with UV was not successful at reducing total damage, and did not seem to increase the efficacy of leaf pulling treatments. However, severity was very low even in the untreated control groups during these years (2023-2024), affecting less than 25% of berries in infected clusters.

box plots comparing control vs. treatments

Figure 1. Total damage caused by sour rot in response to alternative management treatments shared across the three trial years (A: UV, leaf pulling, and combination, 2022-2024), and the treatments applied only in 2023-2024 (B: peroxide and its combinations with UV and leaf pulling treatments). Letters indicate significant differences between groups (p<0.05) in post hoc analyses. UTC = untreated controls.

From these results, UV and leaf pulling are both effective management options to reduce sour rot incidence and severity in vineyards. Peroxide was not successful in reducing sour rot impacts on its own, and did not increase the efficacy of the other management strategies, though it should be noted this was only tested in low sour rot years. UV appears somewhat less effective than leaf pulling in mild years, but is far less labor intensive. The efficacy of UV treatments both in high sour rot years and across all three years of trials is particularly noteworthy, since it remains a treatment option much later in the season if conditions appear to favor sour rot.

About the authors: Dr. Elizabeth Moore is a postdoctoral research in the Emery Lab at Cornell Agritech in Geneva, NY. Their research currently focuses on sour rot disease in grapes, including developing action threshold models to reduce insecticide sprays, the ecology of the sour rot microbial and vinegar fly complex, and how biotic and abiotic factors influence disease etiology. They are broadly interested in how the ecology of insects and microbes can inform applied agricultural research.They can be contacted via email: mem558 [at] cornell.edu (mem558[at]cornell[dot]edu).

Dr. Sara E. Emery is an Assistant Professor in the Entomology Department of Cornell University at Cornell AgriTech. Her lab quantifies the effects of extreme weather events, landscapes and local management on complex pest and pathogen patterns in agriculture. She uses historic pest monitoring data to deepen our understanding of pest populations and to understand the potential for ecological intensification, in which agroecosystems can be managed to enhance ecosystem service benefits. She integrates novel data streams, like satellite-based remote sensing, with pest, pathogen and predator community data. As an applied community ecologist, she identifies the functional attributes of pest life history traits, agricultural management and extreme weather events that contribute to pest and pathogen outbreaks. Her extension program creates tools that growers can use for site-specific predictions that support thriving sustainable production systems and resilient agroecosystems. She can be contacted via email: see68 [at] cornell.edu (see68[at]cornell[dot]edu) .

Don Caldwell is the Wine Extension Specialist at New Mexico State University and a former Senior Extension Associate in Viticulture & Enology with Cornell Cooperative Extension. Dave Combs is a Research & Extension Support Specialist II in Dr. Katie Gold’s Grape Pathology Laboratory at Cornell AgriTech in Geneva, New York. He runs Cornell Grape Pathology’s annual fungicide efficacy evaluation program. Hans Walter-Peterson is a Senior Extension Associate and Viticulture Extension Specialist in Cornell Cooperative Extension. He is also the team leader for the Finger Lakes Grape Program. Dr. Katie Gold is an Assistant Professor of Grape Pathology, and Susan Eckert Lynch Faculty Fellow, in the Plant Pathology and Plant-Microbe Biology Section of the School of Integrative Plant Science at Cornell University. Her Grape Sensing, Pathology, and Extension Lab at Cornell (GrapeSPEC) studies grape disease detection and management.

Sour Rot in Grapes: alternative management strategies update

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