Causal Role of Drosophila spp. in Sour Rot Development
Megan Hall,* Greg Loeb, and Wayne Wilcox
*Cornell University, 630 W. North St., Barton Laboratory, Geneva, NY 14456 (firstname.lastname@example.org)
Sour rot is a disease complex characterized by rotting of the grape berry partnered with internal development of acetic acid and is typically associated with the abundant presence of Drosophila fruit flies in the vineyard. Acetic acid production within rotted berries is a two-step process, requiring yeasts first to produce ethanol and bacteria subsequently to oxidize this to acetic acid, and we have characterized the microorganism species involved. To investigate potential microbial and nonmicrobial contributions of Drosophila spp. within the sour rot complex, we produced axenic D. melanogaster eggs using standard protocols and reared larvae under axenic conditions, yielding adults devoid of gut or surface microbiota, and compared their effects with wild-type flies. Wounded berries exposed to wild-type D. melanogaster for eight days developed sour rot symptoms, but berries in the presence of axenic flies required a co-inoculation with Saccharomyces cerevisiae and Gluconobacter oxydans, by dipping the wounded berries into an aqueous suspension of the two organisms, for sour rot to develop. In contrast, wounded berries dipped in the aqueous suspension but unexposed to flies did not develop sour rot symptoms, nor did wounded berries exposed to axenic flies but not inoculated with microorganisms. In related experiments to compare individual Drosophila species, we saw simi-lar contributions from D. suzukii (Spotted Wing Drosophila) and D. melanogaster. Thus, sour rot development appears to require not only the presence of a wounded berry, yeast and acetic acid bacteria, but also Drosophila spp. Although wild-type flies in the vineyard clearly can vector microbes that cause the disease, our work indicates that the flies also play a crucial role via some nonmicrobial mechanism. In multiple replicated field trials, antimicrobial plus insecticide sprays consistently provided significant sour rot control, while antimicrobial sprays alone did not.
Funding Support: Specialty Crop Block Grant Initiative, New York State Department of Agriculture & Markets, New York Wine and Grape Foundation