ABSTRACT
Sour rot in grapevines is thought to result from berry infection by yeasts, acetic acid bacteria and vinegar flies. Our results confirm previous studies having shown that sour rot supersedes grey mould when both diseases are present in a vineyard, but also indicate that the two diseases can cohabit at an advanced stage of grape decay. To better characterize the role of each of the actors involved in sour rot expression, we conducted experiments involving the isolation of 1593 fungi and bacteria to understand the composition and dynamics of the microbiomes associated with healthy berries, diseased berries and insect vectors. As some grape clusters became symptomatic for sour rot in the absence of acetic acid bacteria, the latter do not always appear necessary for disease expression. The yeast genus Geotrichum, is reported here for the first time to be able to initiate sour rot in grapes, similar to other yeast genera. By allowing or denying the access of insect vectors to intact or artificially wounded grapes, we demonstrate that vinegar flies accelerate disease expression but only when berries are previously injured. Furthermore, the yeast communities identified on native and introduced vinegar flies (Drosophila suzukii) are very similar and yeast-dominated. This confirms the key role of both introduced and endemic vinegar flies in the transmission of sour rot fungi. However, berry skin wounds, induced by heavy rainfall leading to high water uptake and rapid berry swelling, appear to be the main factor in disease expression.
IMPORTANCE This study sheds new light on the complex interactions between microbiomes, insect vectors and physical factors favoring the development of sour rot. While previous studies suggested that acetic acid bacteria were mandatory for sour rot expression and that the introduced Drosophila suzukii flies were the main insect vector, our results indicate that grape sour rot can also result solely from yeast infection and that endemic Drosophila flies are equally important in yeast transportation and transmission in the vineyard. The fruit flies accelerate the infection process leading to disease expression in the weeks prior to harvest, but we demonstrate here that they can do this only on the condition that the skin of grape berries shows already injuries, for example due to excessive water uptake after heavy rain. Consequently, sour rot of grapes is suggested to depend principally on climatic conditions during the maturation of grape berries.
