The Function & Future of Rootstocks

The Function & Future of Rootstocks

If the character of a wine is ordained by the ground in which its vines are grown, then why don’t we spend more time talking about rootstocks?

We love to talk about roots—how deep they are, what types of rock they interlace, how much they drink. Roots are sexy. But it’s almost as if we forget that the root is not the variety. With only a handful of exceptions worldwide, a vine’s roots are a foreign agent, a middle manager between earth and fruit. And like all middle managers, we do our best to ignore them.

But rootstocks are no VP of Marketing. Rootstocks have an essential role to play—multiple roles, actually. Not only do they mitigate problematic soil conditions (pests, lack of water, extreme pH), they also influence yield, ripening patterns, the timing and length of the growth cycle, and, ultimately, flavor. In this way, they are the stenographers of terroir. And they deserve a closer look.

The Root of the Stock

Rootstocks were born in the chaos of phylloxera’s global spread, the result of an intellectual race to save wine grapes from annihilation. The louse was first spotted in France in 1866 and within a decade had spread across nearly the entire country. To many observers at the time, France’s wine industry seemed doomed. But French wine didn’t die; it merely wriggled, bound and gagged, on the train tracks until American vines swooped in and saved the day.

That salvation—the grafting of European varieties onto American rootstocks—arrived in the late 1870s and early 1880s, more than a dozen years after phylloxera’s discovery. The long delay between diagnosis and cure was due in large part to the prejudice against American vines. Hatred of American vines was so strong, growers went to absurd lengths—from impractical measures such as packing their vines in sand to the application of extremely toxic chemicals—to avoid their use.

Ironically, American vines had already been considered in the war against phylloxera, but as outright substitutes for the dying French vines or as French-American hybrids, not as rootstocks. The direct-replacement strategy ultimately failed because the quality of the resulting wines was lacking. But for whatever reason, despite the fact that thousands of American vines were being imported by desperate French growers, and despite the common use of grafting in agriculture, fusing vinifera onto native American roots seemed somehow unholy. The French were convinced that the act would taint the wines, and the mere suggestion was long treated as sacrilege.

France’s growers were eventually assuaged that grafting would not alter the character of their grapes, but their problems were far from over. They soon found that not all American vines were immune to phylloxera, nor were all of them suited to France’s growing conditions. In response, a handful of top botanists were dispatched in search of proper stock, and all headed to America. This destination was selected not only because phylloxera itself was American in origin, but because North America is uniquely rich in indigenous vines.

The first rootstock to be deemed both effective and commercially viable was Vitis riparia. (Vitis is the genus; riparia and vinifera are species within that genus.) The problem with Vitis riparia was that it only really performed well in soils that were fertile and moist—not exactly hallmarks of France’s finest terroirs. Vitis rupestris, discovered in 1879, was something of an improvement as it did better in drought conditions. But vinifera scions on both rootstocks failed completely on chalk or limestone-rich soils. This was a major blow because scientists had believed until this point that phylloxera was the only subterranean menace. Now the soil itself was an adversary.

It would be several years before researchers understood why vines with riparia or rupestris rootstocks turned yellow, dried out, and died on chalky soils. Initially, the climate was blamed. But this condition, known as chlorosis, is now known to be caused by very basic (high pH) soils. The famous white soils of France contain an abundance of active lime (CaCO3), which impedes the uptake of iron by the vine. And below a certain level of iron, chlorophyll breaks down, interrupting photosynthesis and creating the jaundiced, dying leaves that are the hallmark of the disease. It would be a potent vine indeed that could thrive in such radical soils.

That rare vine was eventually found in the limestone cliffs of Texas.

Messing with Texas

The Vitis berlandieri vine, the missing piece of the rootstock puzzle, is named for Jean-Louis Berlandier, but it was claimed for wine by a different French botanist: Pierre Viala. Viala set out to the United States in 1887, charged with the task of finding a vine that was tolerant to high lime content. After a modest amount of exploring, he was drawn a treasure map by T.V. Munson, an American horticulturist who had catalogued an especially verdant species flourishing in the austere white hills of central Texas.

Viala hauled his bounty back to France, triumphant. Not only could V. berlandieri survive in France’s harshest soils, it was late budding. This would delay the growing season and help protect tender buds from spring frosts, a significant bonus. But he quickly found himself stymied. Ever the reluctant bachelor, berlandieri cuttings refused to set down roots! This meant the variety could only be propagated via seed, a problematic complication.

Prior to phylloxera, a new vine could be cultivated simply by training the cane of an existing vine into the ground where it would eventually sprout roots. This is a form of clonal, or asexual, reproduction, and it ensured that the new vine would be genetically identical to the mother vine. In the modern era of viticulture, genetic consistency is achieved via propagating cuttings—segments of dormant canes that, with most species, will sprout roots under warm, wet conditions.

Creating a seed requires the pollination of a flower, which is an act of sexual reproduction. Even if the fertilizing pollen belongs to the same plant as the flower, a genetic recombination takes place. This means that the new plant is not a copy of the mother plant, but an entirely new vine that may or may not possess the desired characteristics of its parent(s). This is obviously impractical for viticulture, where consistency is key.

Scientists tackled the berlandieri problem with an ancient technique: crossing. Using sexual reproduction, they introduced other species into the bloodline and prayed for favorable offspring. In 1894, nearly 30 years after phylloxera’s discovery, their devotion was rewarded: 41B, a cross of V. berlandieri with Chasselas (V. vinifera), was the golden child, a lime-tolerant vine that was easy to root and displayed at least a decent resistance to phylloxera. This rootstock is still going strong 125 years later; it remains essential in Cognac and can be found in roughly 80% of all of Champagne’s vineyards.

The Art of the Graft

So what exactly is a graft and how does it happen?

Put simply, grafting occurs when complimentary cuts are made in the dormant wood of two different vines and the pieces are held together until they are fused. The goal is to create a united vascular zone so that sap, water, sugar, and nutrients can flow uninhibited between the roots and the aboveground sections of the plant.

Successful grafting takes considerable skill and certain favorable conditions. First of all, the young rootstock and scion are typically quite slender at the time of grafting, so making the cuts is delicate, meticulous work. Secondly, the area surrounding the graft must be moist but not wet and the temperature ideally held around 80 to 90ºF for two to four weeks so that the wound forms a callus.

Historically, grafting was done in the field. Rootstocks would be planted directly into the ground and left for a few seasons or years to become established, at which point the trunk would be cut back and the desired variety grafted in situ. This technique is still utilized in certain regions, usually for specific reasons. In old vine vineyards, field grafting is useful for replacing individual vines as they die, especially if the grower is trying to preserve the genetic identity of the vineyard by pulling scions from neighboring plants. Field grafting is also occasionally employed in California vineyards that are intended to be dry-farmed. Minus irrigation, it is practical to give the rootstock a developmental head start before a graft is made.

For the most part, however, field grafting has given way to bench grafting, which occurs in the nursery. This shift is due in part to the specialized labor required to properly execute field grafting but also because most climates aren’t conducive to out-of-doors healing. For example, in George Gale’s book on phylloxera, he notes that the Nantais region in the western Loire Valley had an especially hard time because “the high humidity made it very difficult for field grafting to succeed since the graft join often became diseased before it had properly callused.”

Bench grafting is also considerably more efficient, and by the 1880s, machines had been developed to assist in the widespread recovery of the French national vineyard. This was essential as, again per Gale, a good worker could graft 300 or more vines in a day, but vineyards often contained over 5,000 vines per hectare. Furthermore, “The scale of the job was immense . . . the area to be covered with graft stock was roughly the size of the state of Massachusetts. The length of graft stock needed would go around the earth at the equator nearly 80 times.”

Bench-grafting machines have the added advantage of making more complex cuts than a person can with a knife in a field. Two of the more common shapes, “omega” and “revolving knife,” dramatically increase the surface area of the incision, and the interlocking that results makes it harder for the tissue to be rejected.

Some rootstocks graft more easily than others. As mentioned, because its cuttings won’t set roots, pure V. berlandieri is impossible to graft; its genome needs to be blended with other, more amenable species in order to work. But the fruiting part of the vine, the scion, can pose problems as well. Syrah is notoriously finicky, and Steve Chambers of Chambers Rosewood in Rutherglen reports that Muscadelle has difficulty forming calluses, perhaps because it is an especially low-vigor variety.

It is worth pointing out that there are corners of the world without predatory insects where grafting is still employed. Down in the Maule region of Chile, rolling hills abound in ancient, hulking País vines. And while the Chilean wine industry is waking up to the value of this historic variety, there are currently more vines than market appeal. Because of this, Bouchon’s Christian Sepúlveda is exploring using País as a rootstock, even though it is vinifera. “Why not?” he asks rhetorically, pointing to the phylloxera-free ground beneath his feet. “It is deep rooting, drought resistant, mildew resistant, and disease resistant.” Indeed, this past winter Bouchon started experimenting by grafting Chenin Blanc onto País, with the unusual strategy of appending two scions per trunk. “The País root is such a powerful engine,” Christian explains, “we need to give it an outlet for its energy.”

Rootstocks & Soil

Rootstocks make it possible for otherwise vulnerable varieties to grow in hostile conditions, but resistance to pests, or to extreme pH, is not binary. There is a gradient, and it can shift according to vine health.

Below the ground, vines have a complex system of large, carbohydrate-storing roots and smaller feeder roots that are constantly expanding and probing the soil around them. The pest-resistance of a given vine is directly related to where, if at all, the beasts are able to feed. With vinifera, phylloxera can dine on the main roots, which is a death sentence. But with so-called resistant rootstocks, phylloxera is incompatible with the main roots and can only nibble on the tips of the feeder roots. And because these root tips regenerate all the time, vine and bug can coexist.

At least for a while.

UC Davis’ Dr. Andrew Walker cautions, “More and more rootstocks that we thought were totally resistant are supporting rather large populations of phylloxera,” he says. “When a vine is stressed, either from excessive crop load or deficit irrigation, the root tips don’t regenerate as rapidly, which means the vine is less able to tolerate that pest pressure.” Toward this end, he’s found that organically farmed vineyards, such as certain plots at Fetzer, were able to survive for an extended period of time—though in the end, they too ultimately succumbed.

Nematodes are another major concern for grapegrowers. There are many different species found all around the world, some of which are vectors for viruses such as fanleaf. Adding further complication, research indicates that nematodes are able to adapt to their environment. “Pest pressure is increasing around the world and nematode damage is becoming more evident,” Dr. Walker explains. “This is one of the reasons you must never follow a vineyard with the same rootstock as was previously used.”

Another critical function of rootstocks is to modulate soil pH. Soil pH affects a vine’s ability to ingest certain nutrients, but individual rootstocks can also display nutritional preferences. Basic soils inhibit a vine from ingesting iron, but overly acidic soils can induce aluminum toxicity, or even prevent the uptake of phosphorous, an essential nutrient for plant growth, photosynthesis, and energy transport.

Furthermore, acidic soils that are volcanic in origin, like those widely found in Napa, tend to be rich in potassium. As potassium ions (K+) will displace hydrogen ions (H+) in the vine, and pH is the measure of the concentration of hydrogen ions, a surplus of potassium will decrease the acidity of the final wine. Because of this, Napa-based viticulturalist Kelly Maher likes to use 110R on volcanic sites, as it is a poor assimilator of potassium.

Rootstocks & Water

Rootstocks can also impact the way a vine consumes water. And though this mechanism is not completely understood, part of it has to do with the physical architecture of the plant. As seen below, riparia rootstocks tend to throw shallow roots that radiate horizontally outward. Rupestris, on the other hand, digs considerably deeper. This ostensibly exposes the vine to more moisture, but it doesn’t explain how or why a particular rootstock conserves and then portions its water throughout the growing season.

Caleb Mosley, a viticulturalist who works for grapegrower Mike Wolf in Napa Valley, feels that both the scion and the rootstock play a role in water conservation. “Grape vines, according to their genetics, are either in production mode or protection mode,” he explains. “If you give 101-14 a drop of water, it will use it up immediately; the stomata will open and it will effectively cool itself because it is passing humidity. It is in production mode.” He continues, “On the other side of the spectrum—in protection mode—is 110R. It will close its stomata to reduce the amount of water being lost in the leaves.”

Above the ground, varieties display similar tendencies. “For example,” Mosley explains, “Syrah will use the water right away and crash quickly, but Grenache is really thrifty with water.” For such extreme cases, pairing the right rootstock to the variety is essential. “If you have Syrah in a hot climate planted on 101-14 and you miss an irrigation by a couple of days, you’re done.”

Dr. Gregory Gambetta of the University of Bordeaux is the head of a team dedicated to the study of vines and water. “I’m going to be completely honest with you and tell you that we don’t really know how rootstocks modulate water,” he admits. “That’s what my research is focused on: do rootstocks really change the water use of a vine and, if so, how?”

“For example,” he begins, “when a grapevine is growing, where in the soil is the water being taken up from? This sounds like a simple question, and yet we don’t know the answer.” To address this topic, he is working with 3D imaging technology and is also studying the different isotopic ratios of water in various soil types. That last bit may sound overwhelmingly technical, but it is similar to the way doctors use contrast dye injections to enhance MRI or CT scans.

“That said, we do believe that drought resistance has something to do with the size of the root system,” he explains. “It’s not always the case, but 90% of the time, it’s the invigorating rootstocks that tend to be more adaptable to dry conditions. So we think maybe they can simply physically store more water.” But while this sounds reasonable, Dr. Gambetta is quick to admit his uncertainty.

“The thing is,” he says with a resigned chuckle, “rootstocks are kind of an enigma because they are underground and we can’t see down there. It’s a hell of a lot easier to study a grape cluster.”

Rootstocks & The World Above

Some of rootstocks’ effects can be measured by looking at the aboveground portion of a vine. Impacts on the timing of the growing season, on vigor (both crop load and canopy), cluster morphology, tannin maturation, and even virus uptake can be felt in the fruit. And all of these things either directly or indirectly influence the character of the final wine.

In the world of Napa Cabernet, tannin maturation is often the single biggest factor driving both pick date and wine style. So while farmers from other climates, such as that of Bordeaux, might be interested in moving the growing season forward to avoid weather pressure at harvest, some Napa growers prefer to delay things in order to capitalize on the late-summer sun. “Early season rootstocks like St. George and 101-14, the vines can shut down when you are waiting for the tannins to mature,” Caleb Mosley explains. “Why I’m a fan of 3309 is that the canopy hangs in there a little longer, through that last heatwave, to get those tannins over the line.”

Of course, that’s only one opinion. Jeff Wheeler, Chief Agronomist at grapevine nursery Novavine, thinks that 101-14 has been such a popular rootstock in California precisely because everything experiences accelerated ripening—both sugar and tannins. “It ripens fruit at a breakneck speed, but it also really drives the maturation of those small phenols and seed tannins.” Meanwhile, Dr. Gambetta believes that while rootstocks almost certainly shift the growing season forward or backward by some measurable amount, much of the anecdotal evidence overstates the case.

Vigor is another important consideration. High crop load can often result in increased income for growers, but fine wine production is more often associated with modest vigor. “We tell ourselves this story that low vigor concentrates a vine’s energy in the fruit, and that’s true,” Dr. Gambetta explains. “But there’s another side: overly developed canopies mean more hedging, thinning, and passes through the vineyard, which costs money. In addition, too dense of a canopy or fruit zone can also lead to rot.” Low-vigor rootstocks are also useful in cool-climate growing areas. Per A.J. Winkler’s book General Viticulture, “Weak-growing rootstocks are in demand in the cold northern areas of grape production since they tend to advance ripening. Owing to their more limited root system, these rootstocks do not supply so much of water or mineral elements in midsummer as do the strong-growing; growth is thereby checked, carbohydrates accumulate, and maturation is hastened.”

As far as cluster morphology goes, St. George makes an interesting study. “St. George throws really long, leggy clusters with space between the berries, which is why it’s good for tight-bunched varieties like Zinfandel—you’ll see less rot in the cluster.” Mosley relays. “But it’s a nightmare for clones or varieties like Malbec or Clone 6 Cabernet that have difficulty setting fruit.” Rootstock 420A behaves like the opposite of St. George, tending to form a lot of berries with very short stems. “A Zinfandel cluster would eat itself on that rootstock and become a botrytis bomb,” Mosley cautions.

St. George is also relatively virus tolerant, which can make it easier to graft. Often, if either the rootstock or the scion is virused, the healthy half of the plant will kill the tissue near the graft zone as a way to prevent cross-contamination. This is why so many growers place an emphasis on buying certified virus-free plant material from nurseries. But virus is often unavoidable, especially when working with old vine vineyards.

Of course, using a virus-tolerant rootstock may allow virus to express, which can retard fruit ripening and result in a slightly lower-alcohol wine. Similarly, using a rootstock that is a poor assimilator of potassium can preserve acidity in the wine, which not only alters the style but can help with microbial stability. All of which is to say, the effects of rootstock may often be subtle, but they are also profoundly multi-faceted.

Stock Selection

This year marks Bob Cabral’s 40th vintage, and 17 of those years were spent at Williams Selyem. Cabral was winemaker and general manager for the venerable brand when it first started developing its own vineyards in the late 1990s, and he played a major role in their design. “Soil and rootstock have a symbiotic relationship,” he tells me. “You can’t have one without the other.”

When planning out a site, Cabral would execute shadow studies to see where the sun fell, which determined row orientation. That settled, he would select the rootstocks that best fit the soils, and only then pick varieties and clones. “The first consideration when choosing a stock was soil composition—how deep was the topsoil, what was in the topsoil, the silt-sand-clay ratio? Then from there, I thought about the density of planting I wanted and the irrigation plan. Finally, vigor—how do you balance the vine so that it produces the highest quality?”

For the vineyard surrounding their winery, Cabral mapped 17 different blocks, each with its own rootstock. “Some of our preferred were 1103P, 101-14, 420A, and Riparia for a wet-feet block that had more clay.” Pest pressure soon became an issue. “We had more nematode problems than we anticipated in the flats, and after six years, we saw some failure and ripped out the 3309 and replanted to 101-14.” Cabral also found 101-14 to be a useful rootstock out on the Sonoma Coast. “If you are out on the coast and it used to be under redwoods, you’ll have a lower pH soil. So there, we use a lot of 101-14 because it tends to be more tolerant.”

In the cellar, Cabral found that having a range of rootstocks also increased his blending options. “Making the estate wine, I not only had Calera, Swan, and Pommard cones of Pinot Noir, I had them on different rootstocks, which gave me different spices and textures and aromas and mouthfeel.” He elaborates, “I really do feel that the interaction of soil, rootstock, and climate—that will really determine the expression of the site, especially if you are making Pinot Noir, Chardonnay, or Riesling. We really undervalue what rootstocks can do.”

Over in Napa, Caleb Mosley ranks his priorities in selecting rootstocks as follows: pest pressure, water availability, soil nutrients (“But really, mostly potassium—putting a low potassium uptake rootstock on a low site is a bad idea. You can do it, but then you have to spoon-feed the vine potassium for the rest of its life.”), and finally, the timing of ripening.

“The two most important decisions you can make when establishing a vineyard are vine spacing and rootstock,” he explains. “You can adapt almost anything else. You can mess with your training, soil amendments, et cetera. But if you want to change the rootstock, you have to rip the whole thing out.”

Earlier in his career, Mosely worked with Ridge’s Monte Bello Vineyard, where the rootstocks were mostly St. George and 420A, both selected for their ability to thrive in low-moisture environments. But drought resistance takes on even greater significance in the Old World, where irrigation is largely illegal.

At Château du Tertre in the heart of Margaux, the soils are primarily sand and gravel, with a maximum of 25% clay in any given parcel. Because the terroir is not calcareous, the estate’s technical director, Frédéric Ardouin, does not have to worry about lime tolerance. But his concerns over climate change and the poor water retention of his soils have brought drought tolerance to the fore. “We mainly use 101-14 because it is a little more vigorous than Riparia Gloire,” he admits. “We have also made some recent plantings with Gravesac and 161-49 because climate change imposes long dry periods (like this year), and the vineyard sometimes suffers too much.”

Beyond drought tolerance and vigor, Ardouin looks for rootstocks that can withstand the low pH and nutritional deficiencies of his sandy soils. Furthermore, he states that nematodes and the viruses that they carry are a “real scourge” for their region and laments that they are unable to leave their fields fallow long enough to be rid of the pest. “For obviously economic reasons,” he explains, “it is difficult for us to conceive of soil rest for more than two years.” This financial concern is not unique to du Tertre, underscoring the essential role of rootstocks.

Chilean-born Pedro Parra is a viticultural consultant for wineries across the globe. In his experience, rootstocks were historically selected less by the dictates of the soil and more by what someone’s neighbor had planted. “Often it was just copy and paste,” he explains, “and a bad decision was made.” For example, “Many vineyards were planted with rootstocks that were more superficial, shallow-rooting, or rootstocks were planted to avoid vigor, which is not working at all today. And because of that, people are realizing there is a rootstock problem.”

Parra is part of the new wave of viticulturalists that view rootstocks as critical to a vineyard’s success, especially in the long term. The problem, as he sees it, is that sometimes the best sites for quality wine are the most difficult sites for the vine, being well-draining and low in nutrition. And as climate change descends and drought spreads, many of those vines are failing. “Today in Sicily, with my client, we are using 140 Ruggeri and it is performing very well. But many of the others aren’t. They get too stressed, the leaves get yellow, they go down, and it’s horrible for the fruit—sunburn, dry tannins, lower yields. It’s a horror movie.”

Part of 140 Ruggeri’s extreme drought resistance has to do with its remarkable vigor: the stock is so productive that it actually requires punishing conditions to perform “normally.” But Parra finds that even more gentle terrains require precise selections. “I arrived in Oregon five years ago, and today there is already change because the soils are drier. So [rootstock selection] depends on your view and also the view of the vineyard owner. You have to ask, are we planting a vineyard for today or the next generation? If the next generation, you need to be prepared to have more dry. Otherwise, in 30 to 40 years, maybe the rootstocks we are planting today are no longer going to be successful.”

A Brief Guide to Selected Rootstocks
  • St. George (Rupestris du Lot): V. rupestris
  • Riparia Gloire Montpellier (Riparia, Riparia Gloire, RGM): V. riparia
  • 101-14 Millardet et de Grasset (101-14, 101-14 Mgt): V. riparia x V. rupestris
  • 110R: V. berlandieri x V. rupestris
  • 41B Millardet et de Grasset (41B, 41B Mgt): V. berlandieri x Chasselas
  • 3309 Couderc (3309, 3309C): V. riparia x V. rupestris
  • 1103 Paulsen (1103P): V. berlandieri x V. rupestris
  • 420A Millardet et de Grasset (420A, 420A Mgt): V. berlandieri x V. riparia
  • Gravesac: V. riparia x V. rupestris x V. berlandieri
  • 161-49 Couderc (161-49, 161-49C): V. berlandieri x V. riparia
  • 140 Ruggeri (140Ru): V. berlandieri x V. rupestris
  • Freedom: 1613 (V. solonis x Othello) x Dogridge
  • Ramsey (Salt Creek): V. champinii

Rooting for the Future

There is a fundamental problem with the rootstock industry today: lack of options. With few exceptions, research was effectively shelved once phylloxera and lime had been solved for in the late 19th century. Now, nearly 150 years later, the pressure created by relentless monoculture and a changing climate is laying that weakness bare. Climate, water quality, and pests are all evolving while rootstocks, for the most part, have not.

“Internationally, there are probably 30 major rootstocks that have decent market share, with another 20 that have tiny market share,” Dr. Walker laments. “But in the US and California, there are really only four: 1103 Paulson, 101-14, 110R, and Freedom in the Central Valley.” Jeff Wheeler’s experience at Novavine confirms this paucity. Though they work with around 20 different rootstocks, one always seems to dominate. “We graft around 6.5 million vines each year,” he tells me, “and about 3.5 million are 1103P.”

Fifteen years ago, 101-14 was the darling stock, but the California industry has been slowly drifting. “Why did people turn away from 101-14?” he asks, “It’s a great rootstock for ripening, but while phylloxera won’t kill it, it likes it quite a bit, and that can affect performance. But really, it’s because it is sensitive to water quality.” According to Wheeler, drought conditions draw mineral salts out of the soil, which is making irrigation and even ground water increasingly brackish. “1103P is a big producer, so it was typically seen in the Central Valley. Eight years ago, I never would have thought it would be the most popular option. But water quality is so much worse than it was 20 years ago, 1103P is now grown widely across the state, even in places like Napa.”

“Salt is a major future issue based on what’s going to happen with climate change,” Dr. Walker expounds. “It’s already a big issue in the San Joaquin Valley and in Chile.” The problem is not just that ground water is getting salty; rising sea levels and the need to irrigate with marginal waters are also of concern. A handful of existing rootstocks such as 140 Ruggeri and Ramsey (widely used in South Africa) are already touted for their salt tolerance. But a few more are in the works. “We are currently doing greenhouse trials on a rootstock that will grow in 12% sea water,” Dr. Walker reveals, adding, “I really hope we won’t need to use that.”

Currently, around 99% of all commercially available rootstocks are made of some combination of riparia, rupestris, and berlandieri. This is obviously limiting, but the danger runs deeper. “We looked at 60 different rootstocks and examined their parentage,” Dr. Walker details. “The surprising thing was that not that they all traced back to rupestris, riparia, and berlandieri but that most of them derived from the same selections—Rupestris St. George, Riparia Gloire, and Berlandieri Resseguier S2!” Dr. Walker and his colleagues found this highly alarming. “This means there’s zero genetic diversity, which makes them highly vulnerable to evolutionary events like phylloxera collapse and nematodes.”

Luckily, Asia and especially North America are crawling with all sorts of unexplored vine species. (Vines are almost exclusively native to the Northern Hemisphere.) Dr. Walker believes that some of these vines might possess special properties and has canvased the American southwest in search of novel species. He has already released five new rootstocks with significant nematode resistance (GRN 1 through 5), and his lab is currently evaluating over 700 more samples for their salt tolerance and pest, disease, and drought resistance.

Meanwhile, in Bordeaux, Dr. Gambetta’s lab is beginning to realize that while rootstocks clearly exert influence over the fruiting part of the plant, the road may run both ways. “The way vine species behave as rootstocks is not predictable; it is an interaction between the rootstock and the scion. And there are many effects on a rootstock when you change the scion.” The idea that grafting is bidirectional in nature is a very contemporary field. “As a scientist, that was frustrating to learn because you basically just doubled your work,” Dr. Gambetta exclaims with a laugh. “Honestly, I’m more confused than ever!”

He’s not alone. Today’s growers may be taking rootstocks more seriously than previous generations did, but even the most informed are still shooting somewhat from the hip. Or at the very least basing their selections on metrics that may no longer apply in an era of changing climate. So, unless we prefer to wait for nature to hand us the next plague, we should heed Dr. Walker’s advice and rejuvenate rootstock research. Otherwise, we may as well don our best 19th-century garb and strap ourselves back to the tracks.

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Bettiga, Larry J., Deborah A. Golino, Glenn McGourty, Rhonda J. Smith, Paul S. Verdegaal, and Edward Weber. Wine Grape Varieties in California. Oakland, CA: University of California Agriculture and Natural Resources, 2003.

Campbell, Christy. Phylloxera: How Wine Was Saved For the World. London: Harper Perennial, 2004.

Gale, George. Dying on the Vine: How Phylloxera Transformed Wine. Berkeley: University of California Press, 2011.

Novavine. “Characteristics of important rootstocks for California vineyards.” Accessed September 17, 2019.

Renouf, V., O. Trégoat, J.-P. Roby, and C. Van Leeuwen. “Soils, Rootstocks and Grapevine Varieties in Prestigious Bordeaux Vineyards and Their Impact on Yield and Quality.” Olivier Trégoat. Accessed September 17, 2019.

Sommerfield, Debra and Pamela Kan-Rice. “To save cabernet from climate change, UC studies rootstock and clone combinations.” University of California Agriculture and Natural Resources, August 20, 2019.

South Dakota State University. “Scientists analyze how rootstock affects grapevine characteristics.” Science Daily, October 25, 2016.

Stevens, Rob, Tim Pitt, Chris Dyson and Phil Nicholas. “Long–term properties of rootstocks.” Wine Australia, November, 2011.

Tandonnet, J. P., S. J. Cookson, P. Vivin, N. Ollat. “Scion genotype controls biomass allocation and root development in grafted grapevine.” Australian Journal of Grape and Wine Research 16, no. 2 (May 2010).

Zhang, Li, Elisa Marguerit, Landry Rossdeutsch, Nathalie Ollat, and Gregory A. Gambetta. The influence of grapevine rootstocks on scion growth and drought resistance. Theoretical and Experimental Plant Physiology 28, no. 2 (May 2016).