Sour Rot on Grapes

Sour Rot on Grapes

By Luke Freeman, NCAT Horticulture Specialist

The grape harvest has begun here in Arkansas and we have all been enjoying delicious locally-grown table grapes. If you are growing grapes yourself you may be noticing some diseases that have caused your grapes to rot before you can harvest them.

One disease you may see, which is prevalent here in the humid Southeast, is a disease complex called sour rot. Sour rot causes grape berries to become soft, watery and brown. It moves from one grape to another within a bunch and eventually causes the whole bunch to turn brown and ooze with a distinct vinegar smell. Given enough time berries will start falling off the cluster, called “shatter.”

Not Just Sour Grapes

A Faith grape cluster that has been severely infected by sour rot
A Faith grape cluster that has been severely infected by sour rot. Photo: Luke Freeman, NCAT

Horticulture Professor Elena Garcia at the University of Arkansas has been researching table grapes in high tunnels on several farms across the state. She has said that sour rot has been the most serious disease problem they have encountered on table grapes in high tunnels, even with regular fungicide sprays. Other grape diseases like black rot and powdery mildew can be more easily controlled with timely fungicide applications. However, sour rot is much more difficult to control, likely because it is a complex of fungi, yeast, and bacteria. You can watch a video of Dr. Garcia’s comments on sour rot here.

High tunnels create an ideal environment for grapes which leads to rapid vine growth and higher yields. However, high tunnels can be problematic for disease management. Because there is so much vine growth inside the high tunnel, air flow can be restricted. This leads to higher humidity and lack of air circulation to dry off the grape clusters. Diseases such as sour rot thrive in warm humid environments, which makes the high tunnel an ideal environment for the disease.

Pecking by birds and other damage caused by animal pests also increases the severity of sour rot in a vineyard. When birds peck at ripening berries they leave wounds in the fruit—an infection point for the sour rot disease. From one infection point in a cluster sour rot can spread to infect the entire cluster, rendering it unmarketable.

Managing Sour Rot

The grape cultivar Gratitude (on the left) is less susceptible to sour rot than Faith (on the right). Photo by Luke Freeman, NCAT

Organic and conventional grape growers can use management practices to reduce the incidence of sour rot. Growers can protect clusters from bird damage by using netting. In addition, good pruning practices will encourage air flow through the canopy. Cluster thinning reduces the number of clusters on a vine; both cluster thinning and leaf thinning around the clusters will promote air flow and rapid drying. Another consideration is that tight grape clusters are more susceptible to sour rot. Selecting grape cultivars that have less dense clusters and berries with thicker skins can help reduce the incidence of the disease.

In addition, harvesting the grape clusters in a timely manner can reduce the incidence of sour rot. The longer the grapes hang on the vine, the greater the chance that sour rot will infect some of the clusters.

Additional Resources

For more information on growing table grapes in high tunnels see the publications and videos linked below. You can also contact Luke and our other horticulture specialist if you have questions! Call 1-800-346-9140 or email askanag@ncat.org.

High Tunnel Grapes Playlist on YouTube

This playlist from NCAT’s ATTRA Sustainable Agriculture program contains a series of instructional videos on high tunnel grape management. It includes information on planting, trellising and training, winter pruning, cluster thinning, and more.

“High Tunnel Grapes: Establishment” publication

This ATTRA Sustainable Agriculture publication covers the topics of selecting and building a high tunnel, preparing the soil, selecting table grape cultivars, planting grapevines, and managing young vines.

Grape Production in Arkansas website

This website from the University of Arkansas Division of Agriculture provides links to resources on grape production in Arkansas. It includes information about the Arkansas grape cultivars used in the high tunnel research cited in this publication.

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