Sour rot of grapes is caused by various undesirable yeasts and acetic acid bacteria rather than a single pathogen.  Sour rot impacts both grape yield and wine quality by spreading throughout grape clusters.  Infected grapes give an unpleasant, vinegary finish to wines and increases volatile acids.

Symptoms of infected grapes appear similar to botrytis rot.  Grapes leak juice that smells like vinegar.  White varieties will appear brick colored and red varieties will appear purple or brown.  Large numbers of fruit flies are common and will spread the infection to other clusters.

Sour rot infection is favored by warm, humid, wet weather and tight clustered varieties with little room for berry expansion.  The most efficient way to control sour rot is to plant loose-clustered varieties like Frontenac and La Crescent, and utilize pruning and training systems to improve air circulation which promotes rapid fruit drying.  Leaf removal between fruit set and veraison may also be beneficial. 

References

Grape Sour Rot Pest Management, 2006, Ministry of Agriculture and Lands, The Government of British Columbia, http://www.agf.gov.bc.ca/cropprot/grapeipm/sourrot.htm.

Schilder, A. 2007, Sour Bunch Rot – Bacteria, yeasts and fungi, MSU Integrated Pest Management Program, http://www.grapes.msu.edu/sourrot.htm.