If you’ve ever packed fruit snacks in a lunch, enjoyed a glass of grape juice, or made a PB&J, chances are the grapes used in those products were grown in Washington’s Yakima Valley. That’s because household brands like Welch’s Smucker’s, and Tree Top get most of their grapes from Washington state, the largest producer of Concord grapes in the nation.

Researchers at Washington State University are working hard to keep it that way by helping grape growers fight iron chlorosis, a pervasive and destructive disease that threatens Concord grapevines throughout the state.

Chlorosis is often easy to identify by its yellowing-effect on leaves and vines, but its specific causes have proven difficult for scientists to pinpoint, especially when growing conditions are the same, vineyard-wide.

“You can have this very homogenous soil type, and very flat land,” said Tarah Sullivan, WSU assistant professor and soil microbiologist. “It’s all irrigated the same way, and with the same water. And yet, you have a series of healthy vines, then one sick vine, and more healthy ones, and so on.”

Tarah Sullivan examines the leaves of concord grapevines for chlorosis.
Soil microbiologist Tarah Sullivan examines grapevines for chlorosis.

Sullivan is working with Joan Davenport, a now-retired WSU soil scientist with three decades experience studying grapevine chlorosis, to find out what accounts for these differences.

The scientists began researching soil microbes beneath chlorotic and non-chlorotic grapevines in 2016 and discovered a possible link between microbial communities in the grapevines’ rooting zones and chlorosis.

They found certain microbes can express genes that allow them to “cheat,” or steal available iron from the grape rooting system for the benefit of the microbe, potentially making the plant vulnerable to chlorosis. Overall, they correlated over two dozen bacterial families to healthy vines, while linking three particular bacterial families to vines with chlorosis.

In an attempt to deter growth of the harmful microbes, Sullivan and Davenport started introducing cover crops such as barley, wheat, rye, and a vetch-wheat combination to vineyard floors to fortify soil health. While the initial tests produced mixed results, there was some room for optimism.

“Our work did show some positive impacts from the vetch cover crops,” Davenport said.

Vetch, a legume related to the pea, is a spindly, flowering plant often used as fodder for cover crops.

The Washington State Concord Research Council has just funded Sullivan to continue her research in the hope that her work could provide a viable alternative to conventional management tools like foliar applications that have proven labor intensive and cost prohibitive, leaving many affected growers with few if any options.

For example, one grower Sullivan worked with was faced with such a severe case of chlorosis, he was forced to uproot an entire vineyard and replant it with watermelon. These kinds of repercussions not only jeopardize the individual grower, but they can affect supply chains, eventually jacking up prices on the jams, jellies, juices, and snacks Americans love and have come to rely on.

“I am very hopeful that the work Dr. Sullivan has proposed will further support these efforts,” Davenport said.