Developing management solutions and understanding environmental associations of sour rot in grapes
Hall, Megan and Evans, KJ and Loeb, G and Wilcox, W, Developing management solutions and understanding environmental associations of sour rot in grapes, 16th Australian Wine Industry Technical Conference 2016, 24-28 July 2016, Adelaide, SA (2016) [Conference Extract]
Sour rot is a poorly-understood disease of grapes affecting growers worldwide where preharvest rains occur, characterized by oxidation of the berry skin and decay of the pulp accompanied by the smell of acetic acid and presence of Drosophila vinegar flies. Visual and olfactory symptoms were first reproduced in the lab in 2014, and were associated with ethanol production and its subsequent conversion to acetic acid. Healthy fruit were wounded,
inoculated with Saccharomyces cerevisiae and Acetobacter aceti, and exposed to D. melanogaster adults. Inoculation without exposure to flies significantly promoted ethanol production but not acetic acid generation, whereas exposure to flies resulted in both. In field trials conducted on Vitis interspecific hybrid 'Vignoles' in 2013-15 in the Finger Lakes region of New York, both insecticide and antimicrobial treatments significantly reduced sour rot development. In 2015, untreated vines averaged 20.5% sour rot severity, which was reduced by 73-81% on vines treated prophylactically post-veraison with weekly sprays containing the insecticide zeta-cypermethrin plus the antimicrobial potassium metabisulfite or hydrogen dioxide. These results support the hypothesis that sour rot is caused by a complex of yeast, bacteria, and Drosophila, and that targeting these organisms can reduce disease development. In a 2016 study in a commercial vineyard of Vitis vinifera ‘Riesling’ in Tasmania, disease severity and incidence increased by 25.5% and 58% respectively over a two-week period before harvest. A mixed effects model showed a highly significant relationship between incidence and severity values and the minimum and maximum daily dew points during this time.