Napa Valley, Calif.—In more than a century of fighting phylloxera, winegrowers have tended to select grapevine rootstocks that resist the destructive pest. With progress, other considerations have come to light: Viruses; nematodes; limited vine life spans; crop productivity and soil issues have become major concerns. Most recently, water usage and conservation have emerged as vitally important issues.

In many areas where grapes grow well, it’s only because they’ve been irrigated—and in some cases are protected from drought by spraying. This is true in most of California, Washington and many other areas. But future restrictions on water for environmental reasons and because of increasing demand seem likely to make drought-resistant vines and rootstock key, along with tolerance for salt and other minerals.

Balancing all the requirements including drought tolerance can be tricky, however.

Water usage is not as big an issue in Napa Valley as in many other areas, since it receives just enough rainfall on average to sustain vines—although much seems to come in the wrong season. But it is an emerging issue there, too. Dr. Andy Walker from the Viticulture & Enology department at the University of California, Davis, recently discussed rootstocks for dry climates and saline soils at a workshop sponsored by Napa County and local agencies and organizations, the seminar welcomed attendees from the whole North Coast.

Back to basics
Walker first discussed the history of rootstocks. They were developed by French researchers to address grape phylloxera in the late 1800s. French scientists came to the United States to collect species resistant to phylloxera, a native American bug that devastated much of Europe’s grapevines when American grapes were exported to Europe in hopes of developing hardier vines.

The scientists collected many cuttings, but only Vitis riparia and V. rupestris rooted well from dormant cuttings. These remain important components of today’s rootstocks.

Vitis riparia is native to North America from the Rocky Mountains to the Atlantic Ocean, and from Canada to Texas. It naturally grows in alluvial soil in riparian habitats, and climbs in trees and shrubs. It has shallow roots and low vigor, but matures rapidly. It resists phylloxera and is easy to propagate, but is susceptible to lime in the soil. One example is Riparia Gloire.

Vitis rupestris grows from Texas to Tennessee, but is relatively rare now: Much of its habitat has been destroyed. It’s a shrubby plant found in rocky creek beds, and it rarely climbs.
 
Its deep roots induce vigor, but are not very drought tolerant on shallow soils. It resists phylloxera and is easy to propagate, but has variable nematode resistance or lime tolerance. One example is St. George rootstock, which is virus tolerant but susceptible to nematodes.

In early tests, vineyards grafted onto these species grew well initially but began to decline on limestone-based soils. Un-grafted vines of these species thrive on lime-based soils, but scions grafted on them do not take up enough iron, so the French scientists returned to the U.S. and with the assistance of T.V. Munson, collected Vitis species from the limestone plateau of central Texas.

Vitis berlandieri grew on limestone soils in Texas. It is considered a subspecies of V. cinerea, var. helleri, and climbs on trees. It is found on deeper soils between ridges, and is deep rooted, with some drought tolerance, variable phylloxera resistance and good lime tolerance; it is difficult to propagate

The scientists also experimented with a few other species for breeding rootstocks, including V. longii (V. acerifolia), V. champinii and V. cordifolia.
 
None propagate as easily as the V. riparia- and V. rupestris-based rootstocks, but V. berlandieri was the best of these, and it was hybridized with V. riparia and V. rupestris in Europe to produce hybrids that were easy to propagate and lime tolerant.

Rootstock use in California
Vinifera moved up from Mexico into California (and New Mexico) in the 1700s during the Gold Rush era. Vinifera later came to California from Europe and the eastern U.S.

Phylloxera arrived on both from European and eastern American vines. Devastated by phylloxera, California growers began trials with European rootstocks by the 1890s. They tested about 60 rootstocks for adaptation to a large array of sites and ability to produce vigorous vines with good crops, but didn’t establish controls for phylloxera resistance.

In those days, most vineyards used limited flood irrigation or were dry-farmed. Growers used a limited range of rootstocks from an initial set of about 60. AXR#1 and St. George predominated, but Freedom and Harmony, 5C, 3309C and 110R were also planted.

Vinifera x rupestris rootstocks like AXR#1, 1202C and 93-5C thrived, as did St. George.

Now, there are many more rootstocks to choose from. About 20 are available, but Walker said that 101-14Mgt and 1103P predominate.
 
The close-spacing trend continues and 420A, 3309C, Riparia Gloire and 1616C are used in those cases.

As interest in drought adaptation grows, 110R, 1103P and 420A are suitable, while Freedom and Harmony are popular in the San Joaquin Valley. Walker said that 5C and 5BB are under-utilized.

V. berlandieri x V. rupestris rootstocks were developed for drought and lime tolerance in warmer, drier parts of Europe. Most are shrubby mother vines that produce short canes and many laterals. They have deeper root systems to avoid drought, good phylloxera resistance but limited nematode resistance. Some are difficult to root and graft.

110R is an example of a V. berlandieri x V. rupestris hybrid. It is a brushy mother vine with short canes and abundant laterals. It is moderate to high in vigor but the lowest of the berlandieri x rupestris group. It is susceptible to nematodes and slow to establish, but shows more vigorous growth after four to five years; best on low-vigor sites. It is intolerant of the viruses that induce graft failure. It exhibits moderate rooting and grafting success.

1103P, another V. berlandieri x V. rupestris hybrid, has relatively high vigor—between 110R and 140Ru—moderate nematode resistance and good salt tolerance. It is widely used in California because it roots and grafts well, and produces more graftable canes. It is adaptable, but better on low-vigor sites or large canopy trellis and spacing systems.

140Ru, another V. berlandieri x V. rupestris hybrid, has the highest vigor of the group but is susceptible to nematodes. It is rarely used in California, but is good for shallow limestone soils in drought-likely soils where high vigor is needed. It has brushy growth with short canes and abundant laterals. It roots and grafts like 110R.

420A Mgt is a V. berlandieri x V. riparia hybrid with low vigor, but is relatively drought tolerant. It can respond well to deficit irrigation, but has poor nematode resistance and roots and grafts with more difficulty. The mother vine grows well, with long canes and few laterals. The leaves are thicker, bumpier and occasionally three-lobed at the base.

Root architecture and drought tolerance
The profile of root systems of rootstocks can have major impact on drought tolerance. The most obvious is that deep roots enhance drought tolerance by seeking underground water, but root system of rootstocks can be both deeply penetrating and shallow, a dichotomy that reflects their water needs and utilization.
 
The density of roots in the soil profile also varies. Some are primarily deep, some primarily shallow and some evenly distributed.

The root architecture and density of the deep and surface roots relate directly to nutrient and water uptake. Some plants hydraulically lift water from deep in the soil profile to keep the surface roots active. These characteristics vary, allowing rootstocks to be more or less drought tolerant and impacting their ability to take up nutrients.

The most deep-rooted rootstocks include Ramsey, 140Ru, 1103P and 110R. Broadly distributed roots are found in 1103P, Freedom, Harmony, St. George, O39-16, 5BB and 420A Mgt; 101-14 Mgt, Schwarzmann, 101-14 Mgt, 5C and 16161C  have primarily shallow roots.

In collaboration with Andrew McElrone, Walker is breeding rootstocks to tolerate drought and control growth and their phenology—the cyclic and seasonal natural phenomena, especially in relation to climate. They are trying to optimize root architecture from shallow to deep-rooting angles (to the surface), root density from two-tiered to even distributions, hydraulic lift and water use efficiency/productivity,  associating characteristics with drought tolerance and salt tolerance.

They have been sourcing wild grapevines in the southwest U.S. for further evaluation. These plants may be adapted to hot, dry, saline conditions. They’ve found many promising individual vines, but the samples are very diverse and require complicated grouping.

Walker also discussed the impact of the rootstocks on phenology of vines. He noted that rootstocks can impact crop ripening, which is more pronounced under limited irrigation or on shallow soils. Some studies have found differences of seven to 10 days between early rootstocks like 5C, 101-14, 1616C and Riparia Gloire, compared to late-maturing rootstocks like 420A and 140Ru. Other rootstocks lie between them.

Finally, Walker commented about sustainable viticulture. Dry farming provides the truest expression of terroir, but brings inherent yield and quality issues.