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A patchwork of cover crops blankets a Sunnyside, Washington, vineyard floor during 2017 trials by Washington State University researcher Tarah Sullivan, exploring the connection between soil microbes and Concord grape vine chlorosis. (Courtesy Joan Davenport/Washington State University)

A patchwork of cover crops blankets a Sunnyside, Washington, vineyard floor during 2017 trials by Washington State University researcher Tarah Sullivan, exploring the connection between soil microbes and Concord grape vine chlorosis. (Courtesy Joan Davenport/Washington State University)



A Washington State University soil scientist has found a correlation between soil microbes and Concord grape vine chlorosis and now wants to figure out how to use that connection to manage the troublesome condition.

Tarah Sullivan’s work indicates that healthy grape vines recruit beneficial bacteria in the soil to help them take up nutrients, while chlorotic vines do not.

“What the picture started to shape up into was that the grape plants that were healthy were recruiting, actively recruiting, a plant-growth-promoting rhizobial community,” the soil microbiologist told growers at the Washington State Grape Society annual meeting in November in Grandview, Washington. “So, those organisms associated with the roots of the grape plants were promoting the health of the plant.”

Exploring the connections between complex soil biology and tree and plant health is a hot topic in horticulture and viticulture. Sullivan’s work, done in conjunction with Joan Davenport, a retired WSU soil scientist with a heavy background in viticulture, sheds a little light on that connection with regard to chlorosis.

Chlorosis is a condition of disrupted chlorophyll in juice grape vines that shows up in the form of yellow leaves, burning around the edges of leaves, lower yields and even vine death. It’s caused by an iron deficiency, made worse in cool damp soil in the spring. Growers often treat it with iron supplements, which can get expensive. In fact, one of the commercial vineyards in Sullivan’s field trials, which began in 2016, was pulled and replanted with watermelons because of chlorosis.

“It can be a big deal if you don’t do anything about it,” said Art den Hoed, a Sunnyside, Washington, Concord grower and former Grape Society Grower of the Year. It will gradually reduce production and eventually kill vines. He’s seen it happen to neighbors.

Den Hoed has found rhythms with irrigation timing to control it, and he also uses foliar iron supplements, but he would be interested in new ideas. “There’s always something on any given year that may not work,” he said.

The project

To study the problem beyond irrigation and iron, Sullivan and her cooperators gathered soil samples from a vineyard within 1 foot of vines that were either healthy or chloratic and analyzed the DNA for fungal and bacterial communities. The vineyard was flat with homogenous soil but had hot spots of chlorosis.

They found no connection between the biological makeup of the fungi and chlorosis, but they did with bacteria. At least 30 different bacterial families correlated to healthy vines, while three specific bacterial families associated with the chloratic vines.

“When we got onto the bacterial community, we did actually find a really strong pattern,” she said.

She also made what she considers a big discovery when she identified specific genes in soil microbiology that would “cheat,” or rob, some of the available iron from the soil after the plant had exuded acids that help it absorb that iron.

The next step, Sullivan said, is to figure out how to use the connection between soil microbiology and vine health as a management tool. She’s thinking of cover crops, specifically.

In 2017 and 2018, she tested cover crops of wheat, barley, rye and a mixture of wheat and vetch. By analyzing more soil samples, she determined that the cover crops definitely enhanced the microbial activity under the cover crops, which indicates overall healthier soil. Wheat alone and the wheat and vetch mix made the biggest difference.

However, nothing about the cover crops seemed to reduce chlorosis.

“We found that microbial activities were definitely different under cover cropping, but it didn’t seem to have much to do with chlorosis,” she said.

She still wants to take this line of research further. Her study was only two years long and she lost one of her vineyards partway through. She speculates that long-term soil stewardship will make a bigger difference.

That’s what she has proposed for her next research project and she is searching for funding now. The Washington State Concord Grape Research Council funded her work up until this point but declined the follow-up project this year.

“I’d like to better understand the evolution of a grape vine microbial community from the time it’s planted all the way through the health of the vine,” she said. •

—by Ross Courtney