Role of Berry Injuries and Drosophila Vinegar Flies in Sour Rot Disease Etiology

Background and goals Sour rot is caused by the interaction of yeast, acetic acid bacteria, vinegar flies, and suitable abiotic conditions. Mechanical- and insect-induced berry injuries may provide entry for microbes and encourage egg laying by vinegar flies such as Drosophila melanogaster, but the impact of these types of injuries on sour rot development is unknown. The goals of our study were to assess the role of injuries caused by several common vineyard agents on sour rot development in the presence and absence of D. melanogaster, to assess injury impact on D. melanogaster reproductive output, and to survey the most prevalent yellowjacket species—important agents of berry injury—in vineyards in 2021 and 2022.

Methods and key findings A manipulative field experiment using inclusion bags on clusters was conducted over two years using a 4 × 2 factorial design. We found that sour rot severity (% infected berries in a cluster) on mechanically injured clusters was greater in both years than other treatments, and especially so in the presence of D. melanogaster. We observed a similar pattern with feeding damage from yellowjackets, though the enhanced effect of D. melanogaster was significant only in 2022. Feeding injury by grape berry moth larvae had minimal effect on sour rot severity, except in the presence of D. melanogaster in 2022. Vespula maculifrons and Vespula germanica were the most dominant yellowjacket species observed in vineyards.

Conclusions and significance Both mechanical and insect berry injuries encourage sour rot development in the presence of vinegar flies. Understanding the role of injuries in grape berries on sour rot development and severity will help improve sustainable integrated management.