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What works well on red wine grape varieties might be too much of a good thing for whites.

Grape growers in Washington State have learned how to manage deficit irrigation on red varieties to produce high quality fruit, says Washington State University viticulturist Dr. Markus Keller. But he sees need for improvement on irrigation strategies for white varieties.

Most growers tend to dry down the soils in white varieties too soon, said Keller.

Moderate deficit irrigation is desirable for red wine grapes in most situations because it can help control canopy vigor, reduce berry size, and increase skin tannins and anthocyanins (color pigments). However, growers often apply the same deficit mindset when irrigating white varieties, which can lead to a host of problems.

Most of the research for regulated deficit irrigation has been done on red varieties, Keller said, adding that there hasn’t been a lot of data to guide growers in ­irrigation whites.

“Berry size and sun exposure are less important in white wine grapes than red,” he said. Using a mild deficit in red grapes  to increase phenolics (tannins and anthocyanins) is important for red wine flavor and color, but less so for white varieties, especially delicate varieties.

He thinks that most growers are stressing white varieties too early.

In a three-year trial with Sauvignon Blanc grapes, Keller imposed two irrigation treatments with the aim of creating two different wine styles in the same vineyard—one with more vegetal flavors similar to Sauvignon Blanc wines produced by New Zealand and one with more tropical fruit flavors. At fruit set, he took over irrigation from the grower-­cooperator, installing his own drip lines and meters to monitor the amounts. The vegetal-style block received 100 percent water through veraison in an attempt to grow a full, vigorous canopy. The other treatment received 50 percent water through veraison.

“The idea was to grow one canopy without any stress and develop vegetal flavors in the fruit, and use stress in the other to open up the canopy, have more sun exposure and more tropical fruit flavors,” he explained. “We got the tropical flavors, but we got them in both treatments. Data showed very minor differences between the two treatments in terms of canopy vigor and growth, yields, and aromas. We got no vegetal flavors in the treatment that should have had a full, vigorous canopy.”

Wines made from both treatments have been very good with fruity, tropical flavors, he said, but he didn’t achieve the vegetal flavor and aroma that he was after in the 100 ­percent water treatment.

Keller said that by the time his water treatments were imposed at fruit set, the soils were already too dry. The grower-cooperator in the trial followed his normal irrigation strategy of watering only once or twice before fruit set to help control canopy vigor.

“The canopies in both treatments were small because of the early dry-down period, resulting in low vigor in the canopy and high exposure of the clusters,” he said. By imposing deficit irrigation on white varieties early—before fruit set—the vines just got too dry too early in the season, and they stayed too dry, he added.

For white varieties, he advised growers not to let soil moisture levels go as low as they would in red varieties. “Don’t let the soil dry down. Don’t be afraid of upping the irrigation quantity during the hot periods. You don’t want to stress whites too much.”