Davy DeKrey samples for trunk disease in a vineyard.

May 20, 2020
Researchers eye grapevine trunk disease in northern Midwest

Midwestern growers can take a few cautionary steps to reduce risk from grapevine trunk disease that research is now confirming to be present in Midwest vineyards.

A recent project at the University of Minnesota showed pathogenic fungi causing grapevine trunk disease (GTD) to be found in vineyards in the Midwest where researchers collected samples.

photo of Annie Klodd“We have found GTD in every vineyard we’ve sampled so far,” said Annie Klodd, an Extension educator for the University of Minnesota. “We’ve looked at over 170 samples from 35 vineyards in Minnesota and Wisconsin.”

She added that some cases can be managed.

“Grapevine trunk diseases cause grapevines to decline gradually over a number of years, but healthy vines can also survive and produce fruit successfully even when infected. Therefore, GTD is not a death sentence and can be managed.”

In many cases, it’s difficult to distinguish how much of a cordon’s partial dieback is due to grapevine trunk disease and how much is actually from winter injury.

“It is fair to say that winter injury still contributes to a lot of the vine die-back that we see in cold climate vineyards,” she said. “If vines die suddenly as many did after the 2019 polar vortex, it makes more sense that that would be due to cold injury, because GTD acts gradually, not suddenly. But if a vine has partial cordon dieback, it could be due to either GTD or cold injury, and since the vines almost always have GTD, it is difficult to distinguish the cause.”

Positive ID’s

A grapevine trunk cross section with trunk diseases.
A grapevine trunk cross section with trunk diseases, though the typical symptoms are barely visible. Photos: Annie Klodd

Klodd first presented some results from the study in a February 2020 webinar for the Northern Grapes Project.

“Grapevine trunk disease is a category of diseases that affect woody tissues, and they survive year to year and they gradually spread through the vine,” she said. ‘So, they’re gradually infecting the vine and damaging the plant as they grow through and spread.” Spores enter through an opening in the bark – through cracks, pruning injuries, or injuries caused by rubbing up against a trellis wire.

To test for the disease, the University of Minnesota team collected woody samples and grew fungi from the samples in test cultures.

“As far as we know, this is the largest grapevine trunk disease survey in the U.S.,” she said. A total of 176 disease isolates were sequenced during the study, including 79 non-pathogen fungi and 42 unknown species.

The top 3 pathogens found were diaporthe, cytospora and phaeoacremonium; others included cadospora and diplodia.

Cytospora was an unexpected discovery for the team.

A cross section of a vine that has trunk disease, indicated by the brown discoloration.

“For a plant pathologist, that was very exciting for them to find,” Klodd said. “They were not previously aware that the species of Cytospora (cytospora viticola) was present on grapevines in Minnesota. It had just recently been ID’d on vines in the Eastern U.S. and Canada.”

Many pathogens were found in otherwise healthy vines.

“The implication of this may be that these vines can survive sometimes just fine if they have grapevine trunk diseases depending on the overall health of the vine,” she said.

Management practices

Current University of Minnesota’s tips for vineyard management include cutting out non-productive vines and burning them or removing them from the vineyard to help in reducing the amount of disease in the area.

Other tips include:

  • Removing cuttings from the field and burning them.
  • Considering pruning earlier, before temperatures drop below 35˚ F.
  • Keeping vines healthy to slow the effects of GTD.
  • Dabbing large cuts with latex paint.

While the use of protective sprays and latex paint is widespread in California, “that idea doesn’t go over well in Minnesota,” Klodd said. Minnesota winegrape growers range from hobbyists to commercial, but “regardless of who I’m talking to, they’re not excited about going out there and painting all of their pruning cuts.” So, instead, Extension is emphasizing painting over the biggest cuts.

The University of Minnesota team is continuing to fine-tune suggestions for reducing the risk from GTD.

Future projects include studying GTD in nursery stock, rapid diagnostic methods and developing recommendations for GTD’s in the Midwest. What’s the best time to prune, how to protect pruning wounds, Hot water treatment for nursery stock.

Recommended practices in California don’t always work for colder climates, in part because spore release times are different. Research in British Columbia showed 80% fewer wound infections in Chardonnay vines pruned Jan.15 as opposed to those pruned on April 15.

“You might consider pruning earlier right now, if it is feasible, if that makes sense for your area,” she said. “But we’re still doing research on that.”

— Stephen Kloosterman, associate editor; top photo: Davy DeKrey samples for trunk disease at a vineyard. He was the plant pathology graduate student on the project, titled “Determining Distribution, Impact and Management Options for Grapevine Trunk Disease in Minnesota Vineyards.” Photos: Annie Klodd

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