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Alan Kawakami, left, Lynn Mills, center, and Markus Keller work in WSU’s grape rootstock trial.  (Courtesy Washington State University)

Alan Kawakami, left, Lynn Mills, center, and Markus Keller work in WSU’s grape rootstock trial. (Courtesy Washington State University)

Most of Washington State grape growers plant vines on their own roots. But if a change to rootstocks was needed, growers could switch without worrying about affecting grape or wine quality, says a Washington State University researcher.

Data from a WSU rootstock trial show that five rootstocks resistant to nematodes and phylloxera performed well under eastern Washington conditions, with only minor effects on vine growth and fruit and wine composition.

Though most rootstocks could be used in Washington without concern, the trial highlighted the importance of avoiding those that have a long vegetative period because they won’t harden off in the fall, said WSU’s Dr. Markus Keller.

The rootstock trial was planted in 1999, and evaluation of scion performance began in 2002. Six rootstocks from Cornell University were planted: Teleki 5C, 140 Ruggeri, 99 Ritcher, 1103 Paulsen, 3309 Coudrec, and 101 CU (a Cornell ­selection).

“The Ritcher series of rootstocks have a long vegetative and slow acclimation period and didn’t harden off in time for our growing conditions,” Keller reported. “We eventually had to abandon 99R from the trial because of repeated scion dieback due to cold injury, leaving us with five rootstocks.”

Cold hardiness

The rootstocks were field grafted with Merlot, Syrah, and Chardonnay as scions. Chip budding was done in June and July of 2002. A freeze on October 31 killed some graft unions and required regrafting the following year.

It took until 2007 for the vines to produce enough yield to evaluate vine growth, and fruit and wine composition. Data were collected from 2007 to 2009.

Vines were field grafted at two different heights to find out which was best. Grafting 28 inches above the ground resulted in 99 percent grafting take and 71 percent survival after the Halloween freeze. At 10 inches off the ground, grafts had 91 percent take, but only a 20 percent survival rate after the freeze.

“The higher grafts had a better chance of surviving the cold temperatures,” Keller said. Once the grafts became established in the following year, hardening off was no longer an issue, he added. “The problem was that the grafts were still green and growing as we went into the first winter. In the first year, growers will have to turn off water early enough to harden off the graft union before winter temperatures arrive.”

The rootstocks used are crosses of North American Vitis species and are more cold hardy than the European wine grape varieties grown in the Pacific Northwest, Keller said, adding that 99R was the only rootstock that showed winter dieback.

Vine vigor and yield

Keller found no significant effect of rootstocks on the scion in terms of vine size and vigor. Syrah grafted on 3309C had a slight increase in pruning weight. Vine size (based on pruning weight) was near or below the optimum of 2.2 pounds of pruning weight per foot.

“If anything, the rootstock tended to reduce the vigor over time,” Keller said.

He found that the own-rooted Merlot and Chardonnay had greater growth capacity and grew more shoots than the grafted vines.

“Overall, there was more variation and more difference from the vine location within the vineyard than there was difference between own-roots and rootstock,” he said. “The location determined the vigor, not the rootstocks.”

Keller reported that Washington’s dry climate and use of regulated deficit irrigation influenced vine vigor, not the rootstock.

The highest yields were on 3309C and 5C for Merlot, 3309C for Syrah, and own-roots and 5C for Chardonnay. The rootstock effect was mostly seen in differences in the number of clusters per vine and berries per cluster, he said. Merlot and Syrah had larger berries on 3309C.

Fruit and wine composition

Fruit composition (Brix, tartaric acidity, pH, tannin levels of skin, seeds, pulp, anthocyanins) was not statistically significant between own roots and rootstocks.

“However, own-rooted vines on average had higher juice potassium and pH than rootstocks,” Keller said. “The berries had more potassium and that drove up the juice pH.”

As part of the rootstock trial, Dr. Jim Harbertson, WSU Extension enologist, made 90 different wines from the grapes followed standard winemaking practices and using research-sized stainless steel tanks at WSU’s research winery in Prosser.

When vine growth is controlled through regulated deficit irrigation, as it is in most eastern Washington vineyards, the grafted vines produce wines similar in quality to own-rooted vines, despite differences in growth and yield formation, Keller said.

Conclusions

Keller drew the following conclusions from the trial:
1. Rootstock effect on yield depends on scion, soil, and weather.
2. Rootstocks have minor effects on vine growth, and fruit and wine composition, though own-rooted vines have greater growth capacity.
3. Growth in dry climates is determined by temperature variation and limited by water availability (which is influenced by soil type and regulated deficit irrigation).
4. Fruit and wine composition are determined by scion cultivar and vintage effect.
5. In eastern Washington, rootstocks with long vegetative periods should be avoided.

“So if we ever have to use rootstocks because of phylloxera or nematodes, there’s nothing to be afraid of,” he said, noting that, overall, scion effects and differences due to yearly climate variation far outweighed any differences attributed to rootstocks. “We have rootstocks that will work for us.”

But the industry will have to solve the field grafting/winter damage issue if more field grafting is done. An option would be to use dormant planting material instead of grafting in the field.

Keller presented his findings during the Washington State Grape Society annual meeting. •