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Fighting sour rot: How damaged grapes, fruit flies can impact wine quality

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  • Staff Report 

Damaged grape berries combined with vinegar flies are a recipe for promoting sour rot, a disease that lowers vineyard yields and wine quality, according to a Cornell University study reporting on field experiments in New York state.

The study revealed that wasps, birds and wet weather can damage the skin on grapes and create openings for vinegar or fruit flies to lay eggs in the grape’s exposed flesh. When they do so, the flies may also inadvertently spread yeast and bacteria that interact to cause sour rot, creating favorable conditions for infections.

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“Sour rot creates a set of symptoms that lower the quality of wine made from infected grapes,” said Greg Loeb, professor of entomology at Cornell and senior author of the study.

From 2021 through 2022, the researchers tested the effects of different types of damage in the presence or absence of vinegar flies on a sour rot-susceptible Vignole cultivar in fields located at Cornell AgriTech in Geneva, New York.

In both years, they found, sour rot was most severe on mechanically injured clusters – to mimic bird pecks –compared with other treatments. When fruit flies were present, it was worse. They discovered a similar pattern when berries were damaged by yellowjackets, though the presence of fruit flies increased infections only in the second year.

“We’ll use this knowledge to help make recommendations to growers about how to try to control this type of cluster rot,” Loeb said.

One such recommendation is to use bird netting and selectively apply insecticides to deter insects and limit berry damage in the latter part of the growing season, Loeb said. Though insecticides are known to work well for fruit flies, they come with costs. 

“We now know there is a considerable amount of insecticide resistance developing in fruit flies in New York and around the country,” Loeb said. “So that’s another part of this. We now understand what the risk factors are and when it makes sense for growers to apply insecticides and other kinds of microbicides.”

Growers may apply treatments when extended wet weather creates cracks in berries, for example. Experiments are also underway to develop nets with small mesh to protect the berries from yellowjackets.

“It’d be nice to get to a place where we can develop risk models that growers can use to help them make [treatment] decisions,” Loeb said.

The study was funded by the New York Wine and Grape Foundation and the United States Department of Agriculture.