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U of I research targets grapevine trunk diseases

By John O’Connell

University of Idaho

Idaho grape growers are apt to blame winter kill or herbicide damage when established grapevines die mysteriously within their vineyards.

Madeline Kinnear, a University of Idaho master’s student studying plant pathology, and her advisor James Woodhall, an Extension specialist at the U of I Parma Research and Extension Center, are targeting through their research an often-overlooked culprit — a group of fungal pathogens known as trunk diseases.

Grapevine trunk diseases are found throughout the world and attack the woody trunk and vine tissue of grapevines, entering through wounds such as pruning cuts.

A single vine may be infected by a plethora of trunk disease species. In California, trunk diseases have caused billions in estimated losses due to the replanting of sick and dead vines.

For her master’s thesis, Kinnear has taken samples from sick grapevines at several area vineyards, isolating and culturing the fungi they contain to diagnose which trunk diseases are present.

The project also entails inoculating grapevines in a greenhouse with those fungi to assess which trunk disease species are most aggressive, which ones may be innocuous and whether certain grape varieties may be more or less susceptible than others.

They intend to develop assays for the rapid detection of the most economically important trunk diseases they identify, and they’ll be trapping spores throughout growing seasons to pair with weather data, which they’ll include in modeling to provide growers with management guidance.

“Grapevine trunk diseases are kind of like a silent killer. You can’t really see them but eventually the whole plant dies over a protracted period. It may take a few years,” Woodhall said. “It’s really important we do this work locally for Idaho so we know what we get here and we know how the diseases will develop in Idaho conditions.”

Kinnear visited 10 area vineyards, taking tissue samples from 80 individual grapevines that showed signs of decline potentially associated with trunk diseases.

She found a wide variety of trunk disease associated with viticulture in both cold-weather and warm growing environments, including three of the industry’s top four trunk diseases of concern.

She grew cultures of about 200 fungi species, 16 of which appear to be pathogenic. Of those, 10 were confirmed as known trunk diseases affecting grapevines, and the remainder either affect the roots of the plant or haven’t previously been associated with losses in grape production.

Two of the species she isolated from several locations in southern Idaho are known to cause problems for grape growers in the Middle East but hadn’t previously been confirmed in the U.S.

“The wine industry in Idaho is only getting larger. Really the importance of this is for the longevity of the industry,” Kinnear said. “These growers are hoping to put a vine in the ground and keep it there for up to 40 years, and if we don’t have an idea about pathogens that could prevent our growers from reaching their goals, then we’re not really doing any favors for the industry.”

This past fall, Kinnear began inoculating grapevine cuttings in a greenhouse with many of the trunk disease strains she’s isolated, which should help her determine which ones are most aggressive and pathogenic.

A final component of the project will involve working with Oregon State University researchers to find better biological control options for combatting trunk diseases.

During surveys, the U of I researchers have isolated a genus fungi called Trichoderma that’s been deployed for biocontrol of trunk diseases.

Rather than using general Trichoderma strains supplied by biological chemical companies, they plan to evaluate local strains, which may be more active against trunk diseases.

The project began in July 2022 with a $100,000 grant from the Northwest Center for Small Fruits Research.

Their efforts to develop testing assays and best practices for managing trunk diseases have been funded through the Idaho State Department of Agriculture with $90,000 in U.S. Department of Agriculture Specialty Crop Block Grant funding.