How sour rot can impact wine quality in New York State

GENEVA, N.Y. (WSYR-TV) — Nobody wants to drink bad wine.

Cornell University conducted field experiments out of New York State on how damaged grape berries and vinegar flies can create sour rot.

But what is sour rot?

According to Cornell, 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.

This is what deteriorates the quality of the grapes, and then the wine.

“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.

Researchers tested the effects of different types of damage in the presence or absence of vinegar flies on sour-rot-susceptible Vignole grapes from 2021 to 2022. This took place in fields located at Cornell AgriTech in Gevena, New York.

Cornell explained that 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.

However, there is a cost that comes with using insecticides.

“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.”

They say growers can use treatments when large amounts of wet weather create cracks within the grapes. According to Cornell, 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.  

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