Location: Grape Genetics Research Unit (GGRU)Title: Grape sour rot: a four-way interaction involving the host, yeast, acetic acid bacteria, and insects
|HALL, MEGAN - Cornell University - New York|
|LOEB, GREGORY - Cornell University - New York|
|EVANS, KATHERINE - University Of Tasmania|
|WILCOX, WAYNE - Cornell University - New York|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2018
Publication Date: 11/5/2018
Citation: Hall, M.E., Loeb, G.M., Cadle Davidson, L.E., Evans, K.J., Wilcox, W.F. 2018. Grape sour rot: a four-way interaction involving the host, yeast, acetic acid bacteria, and insects. Phytopathology. https://doi.org/10.1094/PHYTO-03-18-0098-R.
Interpretive Summary: Sour rot is a grape disease that has not been clearly defined. Reported symptoms of the disease include browning of the berry skin, oozing of berry pulp and the smell of vinegar (acetic acid), all in the presence of fruit flies. Based on samples of diseased grapes collected from commercial and research vineyards, we defined sour rot symptoms as decaying berries containing a minimum of 0.83 grams per liter acetic acid. In laboratory inoculations, sour rot disease symptoms only developed when fruit flies, yeast, and acetic acid bacteria were all present, and not when any of these components were absent. The required presence of fruit flies indicates the insects play an important role in the disease. We conclude that sour rot results from yeasts that convert grape sugars to ethanol, and bacteria that oxidize the ethanol to acetic acid, and that this process is mediated by fruit flies.
Technical Abstract: Sour rot, a disease affecting berries of cultivated Vitis spp. worldwide, has not been clearly defined. Reported symptoms of the disease include browning of the berry skin, oozing of disintegrated berry pulp and the smell of acetic acid, all in the presence of fruit flies (Drosophila spp.). We determined acetic acid concentrations in multiple collections of symptomatic berries; isolated and identified microbes from them; and inoculated commonly-isolated organisms into healthy berries with and without concurrent exposure to wild-type or axenic Drosophila melanogaster. Co-inoculations combining one of several yeasts (Metschnikowia spp., Pichia spp., Saccharomyces sp.) plus an acetic acid bacterium (Acetobacter sp., Gluconobacter spp.) reproduced sour rot symptoms, defined here as decaying berries with a loss of turgor and containing a minimum of 0.83 g/l acetic acid, based on observed field levels. Symptoms developed only in the presence of D. melanogaster, either wild type or axenic, indicating a non-microbial contribution of these insects in addition to a previously-suggested microbial role. We conclude that sour rot is the culmination of co-infection by various yeasts, which convert grape sugars to ethanol, and bacteria that oxidize the ethanol to acetic acid, and that this process is mediated by Drosophila spp.