Sour rot, a disease affecting grapes near harvest, has been observed in viticultural regions worldwide, yet much about basic disease biology and effective management remains unknown. It is characterized by rotting of grapes in the weeks preceding harvest, accompanied by the sour smell of acetic acid and the presence of Drosophila (fruit fly) species. We successfully reproduced the visual symptoms of sour rot and the characteristic accumulation of acetic acid following an initial production of ethanol by wounding healthy berries in the lab, inoculating them with Saccharomyces cerivisiae and Acetobacter aceti, and exposing them to D. melanogaster adults. Inoculation had a significant effect on ethanol production but not on generation of acetic acid. In contrast, exposure to Drosophila resulted in substantial acetic acid production accompanied by declining levels of its ethanol precursor, indicating that acetic acid production is dependent upon a factor or factors introduced by the flies, one of which is likely microbiological. The role of Drosophila in disease development was clarified using field trials conducted on the interspecific hybrid cv. Vignoles in 2013 to 2015, in which applications of both insecticide and antimicrobial treatments significantly reduced sour rot incidence and severity. In the 2015 trial, untreated control vines averaged 20.5% sour rot severity, while those receiving a combination of both the insecticide zeta-cypermethrin (Mustang MAX) and the antimicrobial hydrogen dioxide (OxiDate 2.0) weekly starting prior to the appearance of sour rot symptoms had 4.0% severity, an 81% reduction. Those receiving only insecticide sprays saw a 49% reduction in severity. These trials further support the hypothesis that a yeast-bacteria complex, in conjunction with the presence of Drosophila, is responsible for the development of sour rot and indicate that targeting these organisms can provide significant control of the disease.

Funding Support: New York Grape and Wine Foundation, New York Specialty Crop Block Grant