Sour rot is a disease that affects grape berries once they reach ripeness and involves pre-harvest cluster decay accompanied by a smell of vinegar. It is caused by yeasts and acetic acid bacteria who are frequently vectored by vinegar flies and are all regulated by the actual weather conditions. For getting new insights in the etiology of sour rot we studied and manipulated concurrently the microbial community, transmission patterns as well as meteorological conditions. A co-inoculation experiment indicated that the two yeast genera Candida and Hanseniaspora contribute to the development of sour rot symptoms and acetic acid production and that they might well be able to provoke sour rot on their own. When studying the vectoring capacity of vinegar flies, females of Drosophila suzukii induced more severe sour rot damage than their males or D. melanogaster. This might indicate that egg laying and/or larval development favour microbial colonisation and/or sour rot development. Yet, we were also able to show that direct fruit to fruit transmission is common when there is physical contact between diseased grapes and healthy but injured berries. This mechanism is probably responsible for the propagation of sour rot within grape clusters as frequently observed in the vineyards by the direct aggregation of several diseased berries. Moreover, we were able to demonstrate in the laboratory that the optimal temperature for the development of sour rot lays around 30 °C. Above and below this temperature, acetic acid production was reduced with being almost absent at 20 °C and below. Finally, a greenhouse experiment revealed that temperature and precipitation affected the number of eggs laid by D. suzukii females in the berries of caged grapevines, which thereafter determined the severity of sour rot acetic and consequently acid concentration within grapes. Overall, our study provides additional insights in the etiology of sour rot and contribute to a better understanding of the complex interaction between microbial communities, sour rot transmission, assumed vectors and favourable weather conditions in vineyards.
