Rootstocks don’t affect wine

Markus Keller says growers in eastern Washington now have no reason to fear using rootstocks.

Photo by Melissa Hansen

A major rootstock trial spanning more than a decade found that rootstocks generally did not impact vine phenology, fruit set, and plant water status, nor did they impact wine quality when compared to wines from own-rooted grapes. Variety, location, and vintage are the driving factors of grape quality, say a pair of Washington State University researchers.

In short, WSU viticulturist Dr. Markus Keller said the research dispels the fear of using rootstocks in eastern Washington. “There’s always been a fear of growers that rootstocks would induce too much vigor and the vine would have difficulty hardening off in the fall before cold temperatures hit,” he explained. “But because the industry uses regulated deficit irrigation and can control vine growth through soil moisture, the research shows that such fears can go away. That was the big finding of the rootstock research.”

The rootstock research was unique in that it also evaluated wine quality, making wine from the rootstocks and own-rooted vines for three vintages. With regards to wine ­quality, Dr. Jim Harbertson, WSU extension enologist, said that winemakers have no need to fear that grafted vines will impact wine quality.

“We did all that winemaking—90 wine replicates each year for three years—and we didn’t find anything to be that particularly different,” Harbertson said. “We thought we would find some small difference because it was such a huge project. We certainly did our best to make sure that we got the right answers, but there just weren’t a lot of ­differences in the wines.”

Monumental trial

Keller’s predecessor Dr. Bob Wample started the rootstock trial at WSU’s Roza research vineyard in 1999 to evaluate a handful of rootstocks obtained from Cornell University and learn which might be best suited for eastern Washington. When Keller took over Wample’s project in 2001, he expanded the trial by field grafting three scion cultivars (Chardonnay, Merlot, and Syrah) onto the rootstocks and planted the same own-rooted varietals for side-by-side comparison.

“Rootstock trials have been conducted for hundreds of years, but not at this level and scope,” Keller said. “Most rootstock projects don’t compare own-rooted vines because the research is in a location where they can’t grow them because of issues like phylloxera. And almost none of the previous rootstock trials have carried the ­evaluations through to the wine quality aspect.”

The extensive trial included three scion varieties grafted on six rootstocks: 5C, 99R, 140Ru, 1103P, 3309C, and an unnamed Cornell University selection dubbed 101CU, plus the own-rooted vines. 101CU is thought to be a sibling or seedling of 101-14 Mgt. The experiment had ten field replicates, which meant that WSU enologist Harbertson made nearly 300 wine replicates from 2007 to 2009 in WSU’s research winery.

The research vines were irrigated in the same manner that eastern Washington growers follow, using regulated deficit irrigation. By carefully applying irrigation before veraison as Washington growers do, vigor of the grafted vines were kept in check and rootstocks were not ­invigorated as has been reported in other research.

Keller said that if he were starting the rootstock trial today, his list of rootstocks would be different, including some that have been developed since the project began. One rootstock, 99R, was discarded early in the trial because of repeated scion dieback from cold injury. And though the trial was designed to include a drought-­tolerant rootstock, two of the rootstocks sent by Cornell were ­mislabeled and not what they were supposed to be. The drought-tolerant rootstock was one of the mislabeled casualties.

He noted that the experiment confirmed what many growers and winemakers already know—variety, vineyard location, and vintage are the dominating factors of grape and wine quality.

Overall, he found that the scion effects and differences caused by yearly climate variation far outweighed any differences on account of rootstock. “The rootstocks had only minor effects on fruit ripening and did not consistently alter soluble solids, titratable acidity, potassium, or anthocyanin pigments, but the pH was higher in fruit from own-rooted vines compared with grafted Merlot and Chardonnay,” he stated in his research abstract.

Harbertson reported that rootstock caused few significant differences in fruit and wine composition. Rather, the dominant factors were scion and, to a lesser extent, vintage. Rootstock had no effect on grape anthocyanins and tannins. However, significant but small and variable effects were observed for wine anthocyanins and tannins. He found that wines from own-rooted vines tended to have somewhat higher pH, potassium, and total tannin than did wines from grafted vines, an observation counter to other research that has found higher potassium in wines from grafted vines.

Both Keller’s and Harbertson’s research findings were published in the March issue of the American Journal of Enology and Viticulture.

Own roots

Washington grape growers are some of the few in the world who can grow wine grapes on their own roots and not be forced to use phylloxera-resistant rootstocks. The tiny pest, which feeds on the roots of vines, reducing yields and weakening the plants, has been found in a few vineyards in the state, but the isolated infestations have been monitored for years and have not spread, perhaps due to soil or climate conditions. However, western Washington vineyardists are beginning to experiment with early fruit-maturing rootstocks, and several growers in Walla Walla are interested in planting small rootstock ­trials.

“For the most part, we’ve stayed away from rootstocks,” said Keller, “and there’s no big reason for us to switch.”

When cold temperatures occasionally cause winter injury to eastern Washington vineyards—like the 2010 Thanksgiving freeze—own-rooted vines that are damaged can be retrained from the ground and back in production the following year. The same cold damage to a grafted vineyard requires replanting the vine or regrafting in the field, a somewhat dicey option in Washington because it has yielded inconsistent results.

Another bonus of own-rooted vines is cost—they are cheaper in cost than grafted vines that are more ­expensive for nurseries to produce.

But use of rootstocks in Washington could change in the future.

Keller thinks nematodes, not phylloxera, will probably force Washington growers to adopt rootstocks in the future. “Nematodes will likely be more of an issue as vineyards go through second and third generations,” he said, adding that the wine grape industry is still very young in many locations in the state, with some areas just being planted for the first time.

But as vineyards get planted a second and third time, he fears nematodes will be a growing problem, and ­nematode-resistant rootstocks may be necessary.