Wines & Vines

January 2018 Unified Symposium Issue

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122 WINES&VINES January 2018 GRAPEGROWING PRACTICAL WINERY & VINEYARD S our rot is a term that is widely and somewhat inexactly used to refer to a group of late-season bunch rots that are particularly problem- atic on tight-cluster or thin-skin grape va- rieties. In California, it is another name given to a complex of fungal species and other microorganisms often referred to as summer bunch rot, which is reported to begin attacking berries when they reach a maturity stage corresponding to 8° Brix (see "Summer Bunch Rot" and accompanying photo on page 126). In many other parts of the world, and in most of the published literature, sour rot is a syndrome characterized by decay, oxida- tion (browning) of the berry skin and the smell of acetic acid (vinegar) emanating from diseased grapes that break down and begin to leak their contents, hence the "sour" designation. Although mold fungi such as those in the California complex are sometimes present as secondary decay organisms, they are not necessary components of this form of sour rot elsewhere (see photo above). Studies con- ducted on Riesling, Pinot Noir and other V. vinifera cultivars in Ontario, Canada, have shown that they are not susceptible to such disease development until they reach a ma- turity stage of 15° Brix. To successfully manage a disease, it is important to identify the specific organisms involved and how they interact with the plant host and environment to produce the condition. In the past four years, we have studied sour rot in the laboratory and a series of vineyard trials in New York to identify specific organisms and conditions that are needed for sour rot to develop there and, consequently, how to manage it. This has allowed us to develop a definition of the disease as seen in eastern North America, Europe, Oregon and elsewhere—as well as a pesticide program to manage it—and to identify significant differences in disease severity related to trellising systems. Wounds are important Although sour rot occurs sporadically, it has been reported worldwide where and when conditions for the disease are favorable. Such conditions are poorly defined but often in- clude pre-harvest rains and temperatures regularly exceeding 65° F during the final stage of ripening. Once sour rot begins, typi- cally following rain that occurs after berries reach 15° Brix, it often spreads rapidly. Wounds are necessary for entry of the yeast, bacteria and fruit flies that are causal components of the sour rot complex, and these microorganisms are distributed to them by rain or heavy dew. Although such wounds are sometimes due to birds and insects, sour rot more commonly begins through cracks in the grape skin caused by rain or by swelling of the berries as they take up water and pull away from pedicels (berry stems) in tightly compressed clusters shortly before harvest. In the summer bunch rot complex in Cali- fornia, rain is not a requirement. Skin-split- ting from irrigation, temperature flux and/ or rapid growth appear to provide wounds for entry of the causal organisms, then wind, insects and perhaps birds seem to facilitate disease spread. It also seems likely that the various mold fungi that enter these wounds as early as 8° Brix cause decay and further breakdown of the berries, allowing yeast and bacteria to enter later and produce the acetic acid responsible for the smell of vinegar, although this has not been proven. Wounds also let oxygen into the berry. This is necessary for the conversion of etha- nol, first produced by yeasts, to acetic acid by specific bacteria within the berry. In turn, volatilization of ethanol and acetic acid at- tract Drosophila fruit flies that feed on the yeast and bacteria producing these com- pounds, and the insects then spread the microorganisms to new berries. Fruit Flies Play Role in Sour Rot Complex Training system can impede development of disease By Megan Hall, Gregory Loeb and Wayne Wilcox MEGAN HALL This Riesling cluster affected with sour rot is shown shortly before harvest. Note the association of multiple Drosophila fruit flies (arrows) with diseased berries and the lack of fungal mold growth. EDITOR'S NOTE This text was adapted from the origi- nal version, which appeared in Appel- lation Cornell, a quarterly newsletter published by Cornell University.

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