Irrigation Method Can ‘Trick’ White Grapes

Washington State University scientists believe they have discovered a sweet spot for irrigating white wine grapes.

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Dr. Markus Keller, a professor of viticulture, and Dr. Yun Zhang, a postdoctoral research assistant, found that partial root drying can trick vines into believing they are drought stressed without negatively affecting grape quality. And the method significantly cuts back on water usage.

“With PRD, you’re imposing a water deficit over space but not over time, and the plant doesn’t seem to interpret that as water stress,” says Keller, who with Zhang works at Washington State’s Irrigated Agriculture Research and Extension Center.

Red varietals can do well under deficit irrigation. The technique reduces canopy volume and stresses the grapes, improving their quality for winemaking. But in white grapes, the same practice can create bitter flavors and off aromas.

Irrigating only one side of the root zone, and alternating each time you irrigate, however, can reduce competition between canopy and fruit in white varietals, Keller and Zhang says.

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“The (side of the) root zone being dried down senses the drought, and the plant communicates with the canopy, to the leaves,” Zhang says. “It thinks there is a drought, but at the same time it has access to water.”

The result is that the roots on the dry side of the plant send signals to stop or slow shoot production, while the water available on the other side is enough to keep the fruit healthy and free from off flavors and aromas.

Keller says water consumption was also down with the PRD method.

“We conserved a considerable amount of water,” Keller says. “We used about the same amount of water as with deficit irrigation.”

Zhang says during trials, researchers alternated irrigating sides each two or three weeks, but the key was to monitor soil moisture and keep an eye on upcoming weather patterns to determine when to irrigate.

Growers would have to install a second drip irrigation system to implement the PRD program. But Keller says the reduction in water and irrigation management could make up for that cost over time.

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