Despite the discovery near Kennewick in 1894 of the destructive grapevine root pest phylloxera, the vast majority of Washington State’s vineyards continue to be planted to wine and juice grapes grown on their own roots. While juice grapes are relatively tolerant of the insect, European wine grapes can succumb within a few years of infestation of their roots. The only long-term solution to this threat is grafting to rootstocks derived from North American grapevine species that have coevolved with the insect and have thus learned to tolerate or resist attack.

Luckily, but for reasons that are not entirely clear, phylloxera has not spread in Washington to the point of leading to widespread vineyard infestation. Leaders of the rapidly expanding wine industry nonetheless became sufficiently concerned about this potential threat to form a task force in 1992 that recommended that Washington State University begin a research program to evaluate international rootstocks under local conditions. The common rootstocks Teleki 5C (5C), 99 Richter (99R), 140 Ruggeri (140Ru), 1103 Paulsen (1103P), 3309 Couderc (3309C), and an unnamed rootstock from Cornell University were chosen for evaluation.

Rootstock trial

A rootstock field trial was planted at WSU’s Irrigated Agriculture Research and ­Extension Center at Prosser in 1999. In addition to the six rootstocks, the three wine grape cultivars that were used as scions (Merlot, Syrah, Chardonnay) were also planted on their own roots to permit testing of vines grafted to rootstocks alongside own-rooted vines. In most other wine regions, the presence of phylloxera precludes inclusion of own-rooted vines in rootstock trials. Yet this direct comparison was important in our case, because eastern Washington growers are concerned about winter survival and the influence on wine quality of grafted vines. The trial block was established with nine feet between rows and six feet between vines. Vines were spur-pruned and trained to the industry standard of loose vertical shoot positioning. They were drip-irrigated, using ­regulated deficit irrigation after fruit set, to control shoot growth, canopy density, and berry size.

Following field-grafting using the chip-budding technique in 2002 and 2003, it quickly became apparent that 99R did not permit successful graft survival. Both the rootstock and the scion cultivars grafted to it suffered dieback during winter. The main reason for this problem seems to be the long vegetative cycle of 99R, which results in late cane maturation (shoots turning from green to brown) and, consequently, late cold acclimation. We have since noticed that those vines that survived the first few years have had no further problems with winter survival, probably because once established, the woody rootstocks and grafts no longer suffer from the late maturation that characterizes their green shoots. Nevertheless, from an economic perspective, planting of 99R or other rootstocks with very long vegetative cycles, such as 110R, Rupestris du Lot, and others, cannot be recommended in eastern Washington because too many vines would likely have to be replanted repeatedly during their first few years in the vineyard. The remaining ­rootstocks had no difficulty with winter survival, and there were no signs of graft unions being more vulnerable to cold temperatures than were the scions themselves. Thus, in collaboration with WSU’s enology program at Prosser led by Dr. James Harbertson, ­winemaking trials were added to the evaluation from 2007 through 2009.

Yield differences

One of the key findings of this study was that implementation of regulated deficit ­irrigation prevented the rootstocks from promoting vine vigor (estimated in terms of pruning weight). Overall, own-rooted vines were similar to or slightly more vigorous than grafted vines; the former grew more and, sometimes, heavier shoots than the latter. Although this is contrary to some findings in more humid regions, this result is not all that surprising. The wild progenitors of the European wine grape cultivars evolved in the dry habitats of western Asia and are probably more drought-tolerant than are most of the North American rootstocks. In all cases, however, the pruning weights were near the low end of the optimum range of 0.2 to 0.4 pounds per foot (0.3 to 0.6 kg/m) of canopy. Average cane weights also were near the low end of the optimum 0.7 to 1.4 ounces (20 to 40 g). This indicates that, as intended, regulated deficit irrigation effectively controlled shoot growth after fruit set of both own-rooted and grafted vines.

Minor effects on wine

The rootstocks generally did not impact vine phenology and vine water status, which were estimated in terms of midday stem water potential. Nonetheless, Chardonnay grafted to 5C and own-rooted vines had the highest yields as a result of the higher shoot numbers on these vines. By contrast, in Merlot and, particularly, Syrah, 3309C was associated with the highest yields. This was mainly because this rootstock led to higher cluster weights and, especially in Syrah, more clusters per shoot. Despite these differences in yield, the rootstocks had only minor effects on fruit ripening and on fruit and wine composition. No consistent differences were found in terms of soluble solids, color (anthocyanins), and titratable acidity. However, own-rooted vines tended to be associated with higher pH in the fruit. Additionally, Merlot, but not the other cultivars, usually had the lowest pH on 140Ru. It therefore should come as no surprise that the wines made from the various scion-rootstock combinations also differed very little with respect to their chemical composition. Variation in temperatures among years turned out to be a far more important driving force of variation in fruit and wine composition within cultivars than were rootstock effects.

The WSU rootstock trial has shown that among the rootstocks tested only 99R is not suitable under local conditions, because of its lengthy vegetative cycle. For most rootstocks, however, fears that the graft union might somehow compromise winter survival appear to be unfounded. Moreover, the old European practice of hilling up around the graft union during winter could also be used here to minimize loss of entire vines due to an especially severe freeze event. The study suggests that most available rootstocks should lend themselves well to commercial use in eastern Washington wine grape production. Whereas regulated deficit irrigation, not rootstock, controls vine vigor in this arid climate, seasonal variation in temperature, rather than rootstock, is responsible for most of the differences in fruit and wine composition.

Over the past few years, we have been adding rootstocks to the Northwest Grape Foundation Service. These rootstocks are available for distribution to certified nurseries and will form a clean basis should Washington’s wine industry ever voluntarily or involuntarily choose to make the transition to grafted vines. For a list of the selections ­currently available, visit the Foundation Service Web site at http://wine.wsu.edu/research -extension/plant-health/nwgfs.