Abstract
Background and goals Sour rot, a major challenge in grape production, is caused by the interaction of microorganisms and Drosophila flies. It is distinct from another common grape rot in vineyards, Botrytis bunch rot, which is caused by Botrytis cinerea. As fungicides used against Botrytis do not control sour rot, to effectively implement management practices targeted for sour rot, it is crucial to understand, identify, and differentiate the symptoms of sour rot from those of Botrytis.
Methods and key findings These videos educate audiences on recognizing the symptoms of sour rot and Botrytis bunch rot by emphasizing each disease’s key etiological and visual distinctions. Part 1 informs viewers on how to differentiate sour rot disease symptoms and causal agents. Part 2 provides an overview of integrated pest management (IPM) approaches for sour rot, including chemical controls for the causal agents as well as cultural practices that reduce disease risk.
Conclusions and significance Effective management of sour rot requires correct identification of symptoms and use of IPM strategies, as commonly-used fungicides for rots are not effective against sour rot. These videos help growers and researchers identify, adopt, and explore different IPM options for sour rot.
Part 1: Understanding factors influencing sour rot and its management strategies in vineyards
Part 2: Management strategies for grape sour rot
Part 1: Understanding Factors Influencing Sour Rot and its Management Strategies in Vineyards
Have you ever been walking through a vineyard and seen soft, brown grape berries that smell like vinegar and are surrounded by fruit flies in the weeks before harvest? These are the symptoms of sour rot, a major concern to winegrape growers around the world, and especially those in cool-climate regions where it rains during harvest. Sour rot negatively impacts fruit quality, causing substantial financial losses for vineyards and wineries alike.
Sour rot disease creates significant quality concerns and translates into reduced yields, as growers must decide whether to dispose the fruit before harvest or let the wineries sort out rotten fruit at the crush-pad. Sorting out fruit at the winery or in the field requires extra labor, resulting in additional processing costs and volatile acidity mitigation strategies.
In this video we will describe the signs and symptoms of sour rot in more detail, including the causal agents, and the factors and circumstances that contribute to sour rot development as we currently understand them. Finally, we will summarize some cultural and chemical approaches to sour rot management.
What are the causal agents and symptoms of sour rot?
Sour rot is a disease complex caused by acetic acid bacteria, yeast, and Drosophila vinegar flies. The infected berries are soft, red, or brown and emit a pungent vinegar smell. These symptoms may resemble those of Botrytis bunch rot; however, Botrytis bunch rot is caused by the fungal pathogen Botrytis cinerea, which produces fuzzy grey fungal growth on the berries’ surface with no characteristic vinegar smell like sour rot. Similarly, Botrytis infected berries are intact while sour rotted berries usually disintegrate under wet environmental conditions. Because sour rot is caused by a combination of yeast, acetic acid bacteria, and vinegar flies, fungicides that manage fungal diseases such as Botrytis bunch rot are not effective at controlling sour rot, as they do not target all the causal microorganisms.
What causes an outbreak of sour rot disease?
Yeast and acetic acid bacteria are naturally present on the surface and in the pulp of healthy grapes. In addition, they can be vectored to and between berries by insects such as Drosophila vinegar flies. As grapes have a thick skin, an opening in the skin, whether the result of pest damage or excessive water in the berry, is generally necessary for the disease cycle to begin. Yeast converts sugar in the berry pulp into ethanol, and with the wound site allowing for oxygen to enter, acetic acid bacteria then convert the ethanol to acetic acid, which accounts for the tell-tale aroma associated with the disease.
This all takes place in the presence of Drosophila larvae in the interior of the berry, liquifying and disrupting the berry pulp. While we do not yet fully understand why, the activity of fruit fly larvae significantly contributes to sour rot symptoms. Perhaps larvae liquify the berry pulp and speed up the oxidation reaction from ethanol to acetic acid.
There are several species of Drosophila vinegar flies (generally referred to as vinegar fruit flies) that can infest grape berries and contribute to the sour rot complex. The most common species found is Drosophila melanogaster.
Drosophila suzukii, a relatively new, invasive species of vinegar fly in the U.S., commonly known as spotted wing drosophila, can be especially abundant late in the growing season and contribute to sour rot development. D. suzukii gets its common name from the presence of black spots on male wings. Female spotted wing drosophila are characterized by having a large, serrated ovipositor that allows her to lay eggs directly into the skin of ripe and ripening soft-skinned fruits, such as raspberries and cherries.
Most grape cultivars have thicker skins than these other types of fruit, which makes it difficult for spotted wing females to lay eggs into healthy grape berries, although some thinner-skinned cultivars such as Pinot noir are somewhat susceptible. The more common way for spotted wing drosophila to gain entry into grape berries, however, is through wounds.
Spotted wing Drosophila is very abundant in NY and surrounding states at the time grapes are ripening. They can contribute to sour rot, just as D. melanogaster can, as they also possess causal bacteria and yeast associated with the disease, present on their body and in their gut. Similarly, the early infestation of grapes by spotted wing drosophila can increase the volatile acidity in infested berries that may further increase D. melanogaster populations, aggravating sour rot symptoms and spread within the vineyard.
What are the factors that contribute to sour rot disease development?
Several factors contribute to the risk of developing serious sour rot problems, including grape cultivar; berry ripeness; physical damage to berries by hail, birds, and insects; and importantly, weather conditions.
Grape cultivars with tight clusters and thin-skinned berries are more prone to cracking and leaking of juice as the berries are squeezed against each other. Some examples of cultivars with tight clusters that are prone to sour rot include Pinot noir, Vignoles, Riesling, Sauvignon blanc, and Zweigelt.
Berry ripeness appears to be an important factor, as we tend not to observe sour rot until berries reach 13 to 15 Brix, which is an estimate of total soluble solids (TSS). This can be easily measured using a handheld refractometer.
Physical damage to berries can create opportunities for vinegar flies and microorganisms that cause sour rot to gain access, as well as creating an aerobic environment inside the berry, allowing for the conversion of ethanol into acetic acid, which causes the characteristic vinegar smell of sour rot.
Sour rot is more of an issue in vineyards in warm regions compared to cool regions. The long dry periods in summer followed by heavy rainfall, inconsistent irrigation, and rapid change of soil water moisture level can lead to berry cracking. Similarly, mid-season hail can cause damage to berries, as can bird pecks. Insects such as grape berry moth larvae that feed inside ripening berries may also create wound sites that lead to sour rot, though this has not been carefully investigated. Several species of yellow jackets feed on ripe berries near harvest, causing obvious wound sites. It’s also likely that they spread microbes as they move from cluster to cluster.
Weather, especially later in the season, plays an important role in the development of sour rot symptoms. Sour rot was severe in both 2018 and 2021 in New York and many other areas in the eastern U.S. 2018 and 2021 were characterized by greater than normal amounts of rainfall during August, September, and October compared to the 50-year average rainfall in those months. Similarly, there were many days with night temperature above 60°F [15.6°C]. While more research is needed, it appears that warm and wet conditions near harvest promote sour rot, while dry conditions reduce the severity of sour rot.
As a recap, in this video we discussed different aspects of sour rot disease. First, we described the causal microbes involved. Then, we discussed how vinegar flies promote disease symptoms. Lastly, we discussed the risk factors associated with sour rot, including physical injuries by insects, birds, and hail; as well as cluster architecture, phenology, and weather conditions.
For information on managing sour rot disease and associated insects, don’t forget to watch our other video.
Thank you for watching!
Part 2: Management Strategies for Grape Sour Rot
Hello everyone, welcome to our second follow up video on sour rot disease and associated Drosophila vinegar flies in vineyards. In our previous video, we discussed the symptoms, causal agents, and risk factors associated with sour rot disease. In this video, we will be discussing management of sour rot disease and associated insects in vineyards.
What options are there for managing sour rot?
Adoption of Integrated Pest Management strategies that include prevention, monitoring, and control or mitigation of grape contamination from microbes, vector insects, and sour rot-conducive environmental conditions is important for timely management of sour rot in vineyards.
Shoot thinning, thinning of diseased clusters, leaf removal, and training systems can all play a major role in successful management of sour rot, as high canopy density may increase the environmental suitability around the clusters for sour rot development. Managing bird and insect damage during the season prior to harvest is critical.
Controlling Drosophila vinegar flies with approved insecticides in combination with antimicrobial pesticides such as Oxidate or Problad has been shown to reduce incidence and severity of sour rot if the sprays are initiated before symptoms develop and continue weekly.
If growers wait until symptoms are apparent and vinegar flies are present at high numbers, it is difficult to manage both flies and sour rot-associated microorganisms. In most of our studies, we have applied insecticides weekly up to near harvest, however, these weekly applications may not be required. Recent research in our research vineyards suggests two sprays, once after veraison when sour rot symptoms are just beginning, and a second treatment near harvest, provided similar efficacy as weekly sprays. However, more research at the vineyard scale is required.
When discussing the use of insecticides for managing Drosophila, grapegrowers also need to be aware of the potential for insecticide resistance in Drosophila populations in New York and surrounding states to several classes of insecticides including pyrethroids such as zeta-cypermethrin, organophosphates such as malathion, and neonicotinoids like acetamiprid.
This does not necessarily mean that these materials are failing to control flies in the field in most locations, but the potential for resistance exists, meaning that growers should try to eliminate unnecessary sprays. We strongly recommend rotating among different insecticide classes during the season since Drosophila vinegar flies could be selected for resistance even if they are present below the economic injury level.
More research on sour rot is necessary, but we know a significant amount now about how best to manage it. Here is a summary of some things we know can help.
Minimize berry injury
Be diligent about your disease and insect management strategies for the entire season, as many pests can cause wound sites, which increase the likelihood of sour rot development.
Bird netting or bird distractors can be used to reduce bird feeding damage to ripening fruit, thereby reducing the likelihood of sour rot development. Exclusion netting could possibly be used to prevent or reduce damage from yellowjackets and other insect pests, although this needs to be further examined. Overall, use of netting, traps, and chemical control to reduce injuries to berries, along with a comprehensive disease management strategy, are all critical for management of sour rot.
Watch out for Drosophila vinegar flies in the vineyard
Seeing these flies is an indication of a sour rot problem. It’s important to monitor their activity as another indicator that management of sour rot is needed. The visual observation of Drosophila vinegar flies could be the only reliable way to monitor these vinegar flies so far. However, often by the time Drosophila vinegar flies become sufficiently abundant to notice, sour rot probably has already become a problem. Application of insecticides and antimicrobial pesticides when the grape cultivars are sour rot-susceptible, weather conditions are favorable for sour rot, Brix [TSS] levels are high enough, and especially when vinegar flies are detected, may help in reducing their selection for insecticide resistance.
Canopy management
Some trellis systems can create a protected area near the fruiting zone in which clusters are protected from wind, which is an ideal location for fruit flies and microbes to live and reproduce. Opening up the canopy in a way that increases air circulation among the clusters and vines is helpful for discouraging those insect populations and increasing spray deposition on clusters.
Adjusting harvest time
If you know you are growing a susceptible cultivar, plan your harvest accordingly and be prepared to adjust your plan if sour rot develops. While early harvest may not be ideal, there are tools that help to make desired quality wine using less ripe grapes, such as by adding polysaccharides and increasing alcohol through chaptalization, whereas there are few methods for making good quality wine using sour rot-affected fruit, such as sorting out rotten berries and adding SO2 to reduce volatile acidity, that are cost and labor intensive.
Last but not the least,
Keep checking weather forecast
Be vigilant about weather condition in the vineyard, with special attention placed on temperature. If the night temperature is predicted to be above 60°F [15.6°C] for many days, accompanied or followed by prolonged rainfall events, there is high chance of sour rot outbreak. After veraison, it may rain many times leading up to the harvest. So it is important to apply chemicals in between the rainfall event or reapply chemicals after rainfall, for effectively managing vinegar flies and microbes.
Sour rot is a multi-faceted disease which cannot be dealt with using a single strategy.
Management plan can start from selection of resistant cultivars against sour rot. Ample sunlight exposure and air circulation in vine canopy determine cluster health, so plan for shoot thinning right after budbreak, and crop thinning in the late summer. There is also good evidence that reducing cluster compactness can reduce susceptibility to sour rot. Similarly, sugar level impacts sour rot susceptibility both by affecting the community of microbes present, and abundance of insects such as yellowjackets and vinegar flies that contribute to sour rot. So, keep sampling Brix [TSS] levels and stay alert and monitor sour rot symptoms, berry injuries, and berry damaging agents like yellowjackets and birds once the berries hit 10 to 12 Brix. Usually, rainfall events during this time may enhance the chances of sour rot outbreaks. Application of insecticides and antimicrobial pesticides two times, once when the berries hit 15 Brix and once one week before the harvest, can provide equal benefits by reducing sour rot as weekly sprays after 15 Brix.
Thank you for watching our video.
Footnotes
Bhandari R, Zaman F, Hall M, Gold K, Wise A, Walter-Peterson H et al. 2024. Understanding grape sour rot complex. Am J Enol Vitic 75:0750026. DOI: 10.5344/ajev.2024.22072
Some data underlying this study cannot be shared publicly. However, the remaining data are available on request from the corresponding author.
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Disclaimer: This transcript is not a standalone work and is intended to be a complementary resource to the video article. It is not intended to be cited directly, as it is not representative of the authors’ full work. The references and acknowledgments are included at the end of this video.
- Received November 2022.
- Accepted August 2024.
- Published online December 2024
This is an open access article distributed under the CC BY 4.0 license.