Damon Smith, Oklahoma State University
Ripe fruit with sour rot will become soft and watery. Fruit of white cultivars will turn tan or brown. Fruit of purple cultivars will turn pale purple or pink. When the affected berries are ruptured, pungent odors like that of vinegar will be prominent.
The best method of managing this problem is to be diligent about preventing damage to fruit and controlling insect pests as part of a sound integrated pest management program. Fruit damage due to growth-related causes (e.g., excessive shading) can be prevented by canopy management, fruit thinning, and irrigation management. Tight clustered varieties may be more susceptible to berry split in wet conditions, especially if growing on heavy clay soils. If rain is forecast very near harvest, and the risk of fruit cracking is likely, harvest prior to the rain event to help reduce damage by sour rot.
Fungicides are not effective for managing this disease.
Sour rot is a significant bunch rot problem. The exact cause of sour rot is often difficult to determine because there are so many organisms involved. This disorder is a complex of fungi, bacteria, and insects that gain entry via wounds on fruit. Therefore, fruit that has been wounded by hail, birds, and other fungal infections are more likely to develop sour rot.
Grape Sour Rot, British Columbia Ministry of Agriculture
Summer Bunch Rot, University of California
Sour Bunch Rot, Michigan State University
Summer Bunch Rot, Texas A&M University
Field Guide for Integrated Pest Management in Pacific Northwest Vineyards, Washington State University
Reviewed by Fritz Westover, Texas AgriLife Extension and Stephen Jordan, University of Wisconsin-Madison