How to Stop Sour Rot of Grapes

How to Stop Sour Rot of Grapes

Sour rot of grapes

While thin-skinned and tight-cluster grape varieties are particularly susceptible to sour rot, under the right conditions it can strike vulnerable grapes at any time.
Photo by Megan Hall

Vineyards in the U.S. and throughout the rest of the world have a common enemy: sour rot. Also known as summer bunch rot, the infected grapes crack, ooze, and break down. While thin-skinned and tight-cluster grape varieties are particularly susceptible to this disorder, under the right conditions it can strike vulnerable grapes at any time.

The name sour rot is accurate on many levels: The problem results in bitter grapes and unhappy growers. Sour rot is the product of a perfect storm. Thanks to Dr. Megan Hall, whose Ph.D. work on the etiology of sour rot helped pinpoint the exact cause, we now know that four components work together to sour the grapes.

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“If you don’t have all of these things, you won’t get sour rot,” Hall says.

Four Components
First, Hall says, there must be a wound site. Because these sites are much less common in loosely clustered and thick-skinned grapes, those varieties are less prone to sour rot. Wound sites are also caused by bird pecks,
yellow jacket holes, and other common pests.

The second component necessary for sour rot is yeast. While there are several yeast varieties that can play into it, Hall says the most common is Metschnikowia spp.

The third component is an acetic acid bacterium, and the fourth is a problem all on its own: common fruit flies, Drosophila spp. Note that this drosophila is not to be confused with spotted wing drosophila, an invasive pest that growers despise for entirely different reasons. The latter drosophila has not been shown to cause sour rot.
The fact that four different components need to combine to cause sour rot sounds like good news at first, but on some level, the odds are stacked against growers.

Yeast, for example, is already present in the grapes. The yeast is what converts the grapes’ sugars into ethanol. When grapes become wounded, there’s already some ethanol built up inside the grapes. Bacteria and microbes will then oxidize the ethanol into acetic acid. Hall says the smell of acetic acid is what attracts the fruit flies, prompting them to lay their eggs inside the grapes.

Unlocking The Cause
The million-dollar question, then, isn’t what causes sour rot, but what are the flies doing? Hall is an academic advisor to Patrick Kenney, a graduate student. With Hall’s guidance, Kenney is trying to solve the mystery.

One thing they do know is that after drosophilae enter the grape and lay eggs, they eat through the pulp. Hall says the flies are doing something to the pulp in order to create sour rot symptoms.

It’s unclear what happens during this process. It’s also unclear at what stage the flies cause the issue.

In their experience, the researchers have found the larvae seem to be more problematic than the adults. They are working on finding evidence to support this theory, but there’s no data to support a definitive conclusion, just yet.

Sour Rot Management
In the meantime, Hall says, the best way to treat the problem is with insecticides. During her Ph.D. work, Hall also focused on management strategies. The real-life implications of this issue are what drove her research.

“How growers can control sour rot has been a big motivation factor for me,” she says. “I want to make sure that what we’re doing can be conveyed to growers.”

For three years, the research team conducted spray trials in Geneva, NY. The results of these trials have helped inform the current treatment recommendations. “Controlling the flies is the most important aspect of controlling sour rot,” Hall says.

The trials showed the combination of an antimicrobial and an insecticide was the most effective way to prevent sour rot. Spraying at 12-13 Brix, before the symptoms arise — around the 15-Brix point — is a grower’s best bet for minimizing crop loss. Hall says the optimal spray schedule after the initial application is weekly up until harvest.

Hall also recommends scouting. Once a grower sees flies, it’s time to spray the grapes. Although it may be tempting to drop the bad fruit, Hall says this will only encourage the flies to attack the healthy fruit. Infected grapes must be removed from the vineyard.

Rotate, Rotate, Rotate
With any pest-related problem, it’s essential to rotate the Insecticide Resistance Action Committee (IRAC) groups used on the crop. Delegate (Corteva Agriscience), Oxidate (BioSafe Systems), Mustang Maxx (FMC Corp.), and Venom (Valent USA) are options, but growers will need to check what is approved for use in their area and have backup options.

Hall can’t stress this bit of advice enough. She’s seen what happens when the flies build a resistance to certain chemistries. The sour rot spreads like wildfire, and the results are devastating.

“This happened at a vineyard in the Finger Lakes,” Hall says. “The grower couldn’t use the insecticide anymore and didn’t have any tools to fight the problem. It was a terrifying moment.”

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