sour rot, known as pourriture acide in French, is a breakdown of mature grapes caused by a mixture of fungi, bacteria, and yeast which invades damaged berries. The fruit takes on the smell of vinegar, and juice from rotting berries can spread the infection, as can fruit fly. Common entry points for the mixture of microbes are bird pecks as well as splits in berry skin caused by rain. Some organisms involved are the fungi Aspergillus, Botryosphaeria, Cladosporium, Monilia, Penicillium, and Sclerotinia and the yeast Saccharomyces. The rot is encouraged by rain and high humidity, and control relies on avoiding fruit damage as well as encouraging fruit aeration.
