Sour Rot Disorder of Grape

Sour rot of grape is a disorder causing the breakdown of ripening berries rendering them unsuitable for wine production. Sour rot is caused by insect-associated microbes that expedite berry decay following entry through a wound. Many disorders, including sour rot, result in disease-like symptoms that can cause yield losses or reduce crop quality. 

Figure 1. Sour rot symptoms on white Traminette grapes (Lianna Wodzicki, The Ohio State University)

Figure 2. Sour rot symptoms on red Pinot Noir grapes (Yvonne Woodworth, The Ohio State University)

Figure 3. Sour rot and black rot symptoms on white Traminette grapes (Lianna Wodzicki, The Ohio State University)

Sour Rot Symptoms and Development 

Sour rot occurs on grape berries and causes discoloration, followed by berry shrivel and decomposition. White grape varieties afflicted with sour rot appear tan to light brown in color (Figure 1), while red grape varieties appear brownish-red or brick-colored (Figure 2). Symptoms of sour rot can look similar to fungal bunch rot diseases, such as black rot (Ivey, 2016) shown in Figure 3, or Botrytis bunch rot (Ellis, 2016). A notable difference, however, is that berries with sour rot have a strong vinegar-like smell and often attract high numbers of insects, including yellowjackets and flies. 

Sour rot requires the presence of wounded fruit and the interaction of fruit flies, yeast, and bacteria for symptoms to develop. For example, when berries are injured by birds, hail, insects, or pruning, the subsequent open wounds attract fruit flies that transmit sour rot-associated microbes to the fruit.

Two species of fruit flies are associated with sour rot development—the common fruit fly, Drosophila melanogaster, and the spotted-wing drosophila, D. suzukii. Common fruit flies can access fruits only after a wound has been inflicted, but spotted-wing drosophila has a specialized egg-laying organ that can cut the berry skin and create a wound that contributes to sour rot development (Sepesy, 2019). These flies introduce specialized bacteria (Acetobacter spp. and Gluconobacter spp.) into the wounds that interact with wild yeasts (Saccharomyces spp.) to produce acetic acid. Acetic acid is responsible for the distinctive vinegar-like odor associated with sour rot. The production of acetic acid further attracts  fruit flies and other insects, contributing to the spread of sour rot within and between clusters. 

Sour rot risk is greatest when the amount of sugar in the berries reach 15 °Brix, the temperature is warm (68 °F to 77 °F), and there is rain. As the temperature decreases, so does the risk of sour rot. At temperatures below 50 °F the risk of sour rot is negligible. For this reason, late developing cultivars such as ‘Catawba’ are less likely to develop sour rot.

Sour Rot Management

Variety Selection

All varieties of wine grapes can develop sour rot. However, early-maturing and tight cluster varieties are more likely to develop sour rot than late-maturing or loose cluster varieties (Table 1).

Sanitation and Cultural Control

Practices that minimize berry injury are crucial to preventing sour rot. Care should be taken to avoid mechanical injury of the berries during canopy maintenance and spraying. Bird management, especially from véraison (ripening) through to harvest, is recommended. Netting grapevines (Figure 4) is the most consistently effective method for deterring birds, although scare tactics such as cannons, distress calls, mylar tape, and air dancers can deter birds in the short term.

Figure 4. Placing netting over grapevines from véraison (ripening) through to harvest will deter birds from damaging grapes and reduce the risk of sour rot. (Lianna Wodzicki, The Ohio State University)

Cultural practices, such as leaf and cluster removal, can reduce sour rot incidence, severity, and spread. Leaf removal at trace bloom can reduce cluster tightness and increase air movement through the canopy, making the berries less susceptible to cracking and colonization by fruit flies that introduce Acetobacter spp. and Gluconobacter spp.  

Routine scouting, removal, and disposal of symptomatic clusters at véraison will help slow the progression of sour rot through the vineyard. Symptomatic clusters should be removed from the vineyard rather than left on the ground below grapevines. This practice will also reduce problems with wasps, like yellowjackets and hornets, which are attracted by sugars in fruit juice and fruit debris. Cluster removal may not be feasible for large scale commercial producers, and thus pesticides are recommended.

Chemical Control

Chemical control of sour rot focuses on reducing fruit fly populations in the vineyard, as well as the bacteria that contribute to acetic acid production. To manage fruit flies, insecticides should be sprayed when berries reach 15 °Brix or when symptoms are first observed, whichever comes first. Spotted-wing drosophila populations should be monitored using baited traps to determine date of first detection so that insecticides can be timed to protect ripening berries (Sepesy, 2019). 

Figure 5. Yellow jackets feeding on grape berries with sour rot. (Yvonne Woodworth, The Ohio State University)

Although wasps can be problematic in the vineyard at harvest, management of these insects with insecticides is not recommended. Wasps are pests more because they sting humans than because they damage clusters. Wasps are attracted to the sugary juices from fruit and often appear after berries have already been damaged by birds or other environmental factors (Figure 5). Importantly, insecticides that are effective against wasps have pre-harvest intervals that prohibit their use several days before harvest, and given that these insects live in colonies with thousands of individuals, new wasps readily take the place of those that are killed. Commercial or homemade wasp traps (Shetlar, 2012) can be placed in the vineyard to reduce the number of foragers in the vicinity; however, these traps will not eliminate wasp colonies and are most useful if used throughout the season.

To control the acetic acid-producing bacteria, a simultaneous application of an antimicrobial product containing hydrogen peroxide, peroxyacetic acid, or Banda de Lupinus albus doce (BLAD) is recommended. It is important to note that fungicides used to manage fungal diseases like Botrytis bunch rot,  powdery mildew, and the fungal-like disease downy mildew are not effective against sour rot. Therefore, chemical control of sour rot associated bacteria must be carried out in addition to any fungicide spray program being used. For current microbial agent and pesticide recommendations please consult the Midwest Fruit Pest Management Guide.  

References

Ellis, Michael A. “Botrytis Bunch Rot or Gray Mold of Grape.” Ohioline. The Ohio State University, April 13, 2016.
ohioline.osu.edu/factsheet/plpath-fru-03

Ivey, Melanie L. “Grape Black Rot.” Ohioline. The Ohio State University, April 15, 2016.
ohioline.osu.edu/factsheet/plpath-fru-24

Sepesy, Rosalie. “Monitoring and Managing Spotted Wing Drosophila in Fruit Crops.” Ohioline. The Ohio State University, March 6, 2019.
ohioline.osu.edu/factsheet/ent-86

Shetlar, David J. “Yellowjackets.” Ohioline. The Ohio State University, January 5, 2012.
ohioline.osu.edu/factsheet/HYG-2075-11

Useful Resources

Bordelon, Bruce. “Grapes: The Sour Rot Situation.” Facts for Fancy Fruit, issue 16-11. Purdue University, August 25, 2016.
fff.hort.purdue.edu/article/grapes-the-sour-rot-situation/

“Midwest Fruit Pest Management Guide 2021-2022.” Midwest Fruit Workers Group. (n.d.) Accessed August 3, 2023.
extension.illinois.edu/sites/default/files/2022_midwest_fruit_pest_mgmt_guide.pdf

This fact sheet was peer reviewed by Celeste Welty, Associate Professor, Department of Entomology, The Ohio State University; and by Anna Testen, Research Plant Pathologist, United States Department of Agriculture—Agricultural Research Center, Wooster, Ohio.