Issue link: http://winesandvines.uberflip.com/i/918844
January 2018 WINES&VINES 123 PRACTICAL WINERY & VINEYARD GRAPEGROWING Determining the cause of sour rot Our study began with the collection of sour- rotted clusters from 16 vineyards in the Finger Lakes region of New York. High levels of both ethanol and acetic acid were found (averag- ing 1.14 g/L and 1.58 g/L, respectively, in two years of sampling). More than 1,300 indi- vidual microbial isolates were recovered, more than 90% of which fell into four catego- ries: two Metschnikowia spp. yeast strains, Gluconobacter cerinus (an acetic acid bacte- rium, or AAB), and a ubiquitous environmen- tal bacterium (Rahnella sp.) neither associated with acetic acid production nor considered a plant pathogen. A series of experiments was begun in the lab to determine whether the recovered micro organisms were, in fact, causing the disease. Berries of V. vinifera cultivars Caber- net Franc, Chardonnay and Red Globe were wounded and inoculated with 21 different individual yeasts, mold fungi, bacteria or combinations thereof. One set of inoculated berries was exposed to fruit flies (Drosophila melanogaster), while a companion set was not. After eight days of incubation, berries were rated for rot and browning on a scale of 0-4, then macerated and the juice measured for acetic acid content. Berries were deter- mined to have sour rot if they had a visual KEY POINTS Although sour rot is sometimes used as a catch-all term to refer to a complex of late-sea- son bunch rots that develop in tight-cluster or thin-skin varieties caused by numerous fungi, the authors have defined it more narrowly as a condition involving decayed berries with brown (oxidized) skins often free of fungal contaminants and with pulp that smells of vinegar (acetic acid). Sour rot-affected berries collected from numerous vineyards contained significant amounts of ethanol (produced by various yeast species) and acetic acid (produced by specific bacte- ria) and were always associated with large populations of Drosophila fruit flies. Experimental inoculation in the laboratory with microbes recovered from diseased clus- ters was not sufficient to reproduce sour rot symptoms unless inoculated berries also were exposed to Drosophila fruit flies. Sour rot results from the interaction of yeasts that ferment grape pulp to ethanol, specific bacteria that oxidize ethanol to acetic acid and Drosophila fruit flies, which appear to be responsible for the breakdown of infected berries and subsequent spread of the disease. Three years of vineyard spray trials with antimicrobial treatments targeting the causal yeast and bacteria alone achieved modest reductions in sour rot severity. Insecticides targeting Drosophila fruit flies significantly reduced sour rot severity, and combining antimicrobial sprays with the insecticide improved it even further. Mid-wire cordon-trained Vignoles grapevines had less sour rot when trained with vertical shoot positioning than when trained with a high-wire cordon system where drooping shoots enclosed the fruit within an umbrella-like canopy. Management of sour rot involves controlling both the microbes and the Drosophila fruit flies that cause the disease and is aided by viticultural practices that enhance ventilation and exposure of the fruit.