By M. Moyer
Cause Sour rot can be caused by any number of primary or secondary pathogens, but is most commonly associated with various fungi, acetic acid bacteria, and fruit fly maggots. The "sour" or vinegar smell is typically associated with the invasion of the bacteria Acetobacter and Gluconobacter. The vinegar smell is due to both gluconic acid as well as acetic acid. If fruit is initially damaged as a result of excessive rain (as in the fall of 2013), excessive wind and/or other diseases or insect pests, many secondary invasions by fungal and bacterial microorganisms can occur. Sour rot typically does not spread from cluster to cluster, but can result in whole cluster loss.
Sour rots can be a significant problem regardless of harvest weather conditions. They are associated with varieties with compact clusters and varieties that are aromatic (and thus, attract insects). Conditions that favor development of grapevine powdery mildew or Botrytis bunch rot can also favor sour rot development.
Symptoms Sour rot is best described as a "wet rot" in ripening grape clusters. Typical symptoms include a brown discoloration of the fruit and wet or oozing berries, coupled with the smell of vinegar and the presence of fruit flies. Sour rot is often confused with (and in many cases associated with) the early stages of Botrytis bunch rot, but can be distinguished by the associated "sour" smell. As a general distinction, Botrytis bunch rot is a dry (fuzzy) rot of ripening fruit, whereas sour rot is considered a wet rot.
Cultural control Once sour rot begins in a vineyard, it cannot be controlled. Therefore, the primary tactic in managing sour rot is prevention of fruit damage as a result of other diseases, insect pests, and birds.
- Dropping affected clusters preharvest may help prevent inadvertent picking of diseased fruit.
- If sour rot symptoms are severe, pick as early as possible to reduce the amount of spoiled fruit.
Chemical control While there are no real effective chemical measures specifically designed for the management of sour rot, in-season programs that provide effective control of powdery mildew and Botrytis bunch rot may reduce the risk of developing sour rot in a vineyard. The following specifically mention sour rot.
- Luna Experience at 8 to 8.6 fl oz/A. Do not use if already used for powdery mildew control. Do not use on 'Concord' or with-in 14 days of harvest. Group 3 + 7 fungicide. 10-day reentry.
- Switch 62.5 WG at 11 to 14 oz/A. Do not use an adjuvant or within 7 days of harvest. Good control. Group 9 + 12 fungicide. 12-hr reentry.
- DoubleNickel 55 (Bacillus amyloliquefaciens strain D747) at 0.25 to 3 lb/A. Unknown efficacy. 4-hr reentry. O
- Serenade ASO (Bacillus subtilis strain QST 713) at 2 to 6 quarts/A plus an adjuvant. Active ingredient is a small protein. May be applied up to and including day of harvest. 4-hr reentry. O
Note Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Fracture.
Reference Moyer, M.M, and O'Neal, S. (eds). 2013. Field Guide for Integrated Pest Management in Pacific Northwest Vineyards. Pacific Northwest Extension Publication #PNW644.