Tackle Summer Bunch Rot and Sour Rot Now
By Dr. Justin Tanner, UCCE San Joaquin County Viticulture Farm Advisor
As California vineyards are coming out of dormancy now, the battle against summer bunch rot and sour rot is far from over. The aftermath of the previous season's afflictions, which were fueled by weather conditions that promoted vine vigor and dense canopy growth as well as increasing fungal disease pressure, has growers affected by rot in 2023 focused on preventing a repeat this year. Key to this is the standard practice of removing infected fruit during winter pruning to reset the stage for a clean start this coming season. This critical step involves carefully pruning away dormant canes along with diseased clusters, placing them in the row middles, and incorporating them into the soil. Infected material, such as berries, canes, and leaves, can harbor Botrytis cinerea, a primary pathogen in bunch rot (Jaspers et al., 2015) along with many other fungal pathogens. Rigorous sanitation during winter pruning is essential to minimize the potential for disease in the upcoming season, especially in vineyards that experienced high levels of rot last season.
Image 1 Unharvested grapes affected by summer bunch rot. These can be a source of inoculum in the next season if allowed to remain in the vineyard. Diseased fruit should be cut off with final pruning, placed in the row middles, and incorporated into the soil before the start of bud burst in spring to reduce disease pressure next season.
Early Season Vigilance: Canopy Management
Beginning shortly after bud burst, effective and timely canopy management is foundational for disease prevention for the season. The objective is to create a canopy that allows for optimal air circulation and sunlight penetration. This involves strategic thinning and spacing of vine shoots, tailored to each vineyard's site and vine vigor, and production goals. Regular leaf pulling and shoot positioning are essential in maintaining an open canopy, which significantly reduces the humid conditions that favor rot development. Canopy management techniques such as shoot thinning, leaf removal, and light pruning can modify canopy architecture, influencing reproductive performance, and berry ripening. Shoot thinning and leaf removal are particularly effective in decreasing leaf area index and increasing canopy porosity and light interception, positively affecting berry ripening, and reducing disease pressure (Wang, De Bei, Fuentes, & Collins, 2019). Additionally, early leaf removal has been shown to affect the source-sink balance in grapevines, leading to a reduction in fruit set, which could result in looser clusters and improved grape composition (Frioni et al., 2019).
Image 2 Shoot thinning early in the season promotes air circulation, light infiltration and increases spray penetration to help decrease fungal diseases throughout the of the season.
Understanding and Managing Summer Bunch Rot and Sour Rot
Prevention is the most effective approach against summer bunch rot and sour rot. Summer Bunch Rot infects fruit by one of two means. The first is during bloom, where flowers can become infected through the stigma or scar tissue leftover from where the calyptra detached. These infections will then lay dormant until sugar starts to accumulate in the berry post veraison. The second means of infection happens later in the season. Fungal spores can exploit any wounds on the berry skins, such as those caused by mechanical damage, birds pecking, powdery mildew infection scars, feeding and/or oviposition damage from insects including certain moths, wasps, mealybugs, and thrips, or even sunburn. Therefore, minimizing these injuries is crucial, which includes implementing bird control measures, using gentle handling during mechanical operations, managing insects that damage fruit, and employing sunburn prevention tactics like canopy and irrigation management and berry thinning to reduce cluster compaction. Summer bunch rot is a disease complex caused by one or more of multiple organisms such as Botrytis cinerea, Aspergillus tubingensis, A. carbonarius, A. niger, Alternaria sp. Cladosporium sp., Rhizopus sp., and Penicillium sp. Sour rot is a polymicrobial disease involving yeasts and acetic acid bacteria, particularly in the presence of Drosophila fruit flies. Sour rot is primarily caused by native yeasts and acetic acid-forming bacteria. Research has shown that the disease is a result of a complex interaction involving these microorganisms, which leads to the decaying of berries with high amounts of undesirable volatile acidity (Hall et al., 2019).
A Preventative Stance
A proactive approach to disease management involves more than just properly timed fungicide applications. It's about creating a canopy environment less conducive to disease. This means balancing vine vigor through careful water and nutrient management, particularly in drought-prone areas like California. Excessive vigor, often resulting from over-irrigation or over-fertilization, can lead to dense canopies that favor disease development. In California’s Central Valley, for example, monitoring evapotranspiration using remote sensing data has proven to be an effective tool for optimizing irrigation management, ensuring vines receive adequate but not excessive water, thus reducing disease pressure (Semmens et al., 2016). When it comes to fungicides, the key is timing and rotation. Applications should be strategically timed based on disease forecasts (https://ipm.ucanr.edu/weather/grape-powdery-mildew-risk-assessment-index/) and vineyard conditions, and products should be rotated to prevent resistance build-up.
Impact of Insects on Disease Development
Insects, particularly fruit flies, play a significant role in the facilitation and development of sour rot. The presence of these insects significantly increases the severity and incidence of berry rot diseases (Madden et al., 2017). In grapevines, fruit flies prefer the shelter of a dense canopy which provides a more humid environment sheltered from wind. When sour rot occurs several weeks before harvest and fruit flies are present, simply dropping the fruit below the vine is not enough to prevent it from spreading. New adults emerge from infested fruit after only 7-8 days and will simply migrate back up into the canopy to repeat the cycle of infection and rot if not removed from the vineyard. In mechanically harvested wine grapes, if sour rot occurs close to harvest and rot levels are low, dropping fruit may be done right before harvest to exclude it from compromising the quality of the crop. The emergence of resistance to fungicides and insecticides among pathogens and insect vectors respectively is a growing concern. Studies have shown that Drosophila melanogaster populations in vineyards have developed resistance to commonly used insecticides due to their numerous short reproductive cycles within a season, leading to control failures of sour rot (Sun et al., 2019). This highlights the importance of monitoring for resistance and the use of integrated pest management strategies accordingly.
For grape growers, the challenge of managing summer bunch rot and sour rot requires a blend of careful planning, vigilant monitoring, and adaptive management practices. Each season presents an opportunity to learn and refine these strategies, aiming for a balance between environmental stewardship and effective disease control. With diligence and a commitment to these practices, growers can look forward to a season with reduced disease pressure and healthier vineyards. Additional management considerations can be found at https://ipm.ucanr.edu/agriculture/grape/summer-bunch-rot-sour-rot/.
A) Botrytis and B) Sour rot on clusters of Riesling. While similar in appearance due to oxidative browning of affected berries, sour rot can be easily distinguished by the leakage of fluid and vinegar odor emitted from rotting fruit.
Bibliography:
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Hall, M., O’Bryon, I., Wilcox, W., Osier, M., & Cadle-Davidson, L. (2019). The epiphytic microbiota of sour rot-affected grapes differs minimally from that of healthy grapes, indicating causal organisms are already present on healthy berries. PLoS ONE, 14. https://doi.org/10.1371/journal.pone.0211378.
Jaspers, M., Seyb, A., Trought, M., & Balasubramaniam, R. (2015). Necrotic grapevine material from the current season is a source of Botrytis cinerea inoculum. European Journal of Plant Pathology, 144, 811-820. doi:10.1007/s10658-015-0726-4.
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Vogel, A. R., White, R., MacAllister, C., & Hickey, C. (2020). Fruit Zone Leaf Removal Timing and Extent Alters Bunch Rot, Primary Fruit Composition, and Crop Yield in Georgia-grown ‘Chardonnay’ (Vitis vinifera L.). HortScience. doi:10.21273/hortsci15090-20.
Wang, X., De Bei, R., Fuentes, S., & Collins, C. (2019). Influence of Canopy Management Practices on Canopy Architecture and Reproductive Performance of Semillon and Shiraz Grapevines in a Hot Climate. American Journal of Enology and Viticulture, 70, 360-372.