Sour rot is a disease of grapes caused by various yeasts and bacteria rather than a single pathogen. It spreads through grape clusters and causes infected grapes to leak vinegar-smelling juice, giving an unpleasant finish to wine. Symptoms include brick-colored white grapes and purple or brown red grapes, as well as an abundance of fruit flies. Sour rot favors warm, humid conditions and tight clusters, and can be controlled through good cultural practices like pruning, improved airflow, and leaf removal to reduce berry skin lesions.

1. Sour Rot
Sour rot of grapes has not yet been observed on Minnesota, grapes, but is still an
important disease to understand. It 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 cultivars will appear brick colored and red cultivars 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 cultivars.
The most efficient way to control sour rot is the use of good cultural practices. Utilize
pruning and training systems to improve air circulation which promotes rapid leaf drying.
Remove leaves between fruit set and veraison to increase berry skin thickness to reduce
the risk of lesions.
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.