Ask any five growers or viticulturists what their favorite rootstock is and you'll probably get five different answers--unless they know one another, in which case you'll probably get the same answer five times.
Ever since the downfall of AxR, rootstock selection and usage seems to have been propagated more by hearsay and anecdote than by scientific evaluation. (Some would say the same about AxR usage!) There is a good reason for that: not everyone has the time, money or ability to conduct a rootstock trial on their own property, let alone on all of the various soil types that may exist within their plantable acreage. So, "looking over the fence" at the neighbor's vineyard is often the means by which such decisions are made. Nevertheless, there are good reasons upon which to base rootstock decisions. This column is not intended to spell out those guidelines for you but to discuss some of the factors that others are considering for rootstock selection and for breeding of rootstocks of the future.
The American Society for Enology and Viticulture (ASEV) held a Rootstock Symposium on June 23, 2009 in Napa with the subheading of "Rootstock Characteristics and Implications for Selection." Half of the day was devoted to hearing from rootstock breeders and the other half about more applied observations and experiences in the selection and use of rootstocks. Jim Wolpert of UC Davis ended the symposium with a discussion about rootstocks and vine spacing influences on vine balance. Because I plan on writing a column about vine spacing in the near future, I'll save Wolpert's discussion for that time.
Peering into the near and far future: research and breeding
In my past columns, I've probably mentioned Andy Walker of UC Davis more frequently than any other academic in the universe. That is probably because Walker seems to be present at seemingly all of the scientific symposia even remotely associated with rootstocks. And for good reason: Walker is extremely prolific in his research and breeding efforts, as well as in being a top-notch general viticulturist. But as I have done many times before, much of what he discussed at this meeting I have already discussed in prior columns.
Briefly, Walker's lab has been devoting most of its resources into developing rootstocks intended to address one or more pests/diseases of grapevines. Most famously, he has been working on rootstocks with broad and durable nematode resistance--resistant to several different species of nematodes. This is important as nematicides and fumigants become more restricted and unavailable, or at least frowned upon as being incompatible within a "green" viticulture framework. Current rootstocks are resistant to only some nematode species. For example, Freedom and Harmony are resistant to root knot and lesion nematodes, but not to dagger nematodes.
Walker's lab has released six different GRN (Grapevine Rootstocks for Nematodes) rootstocks, which will complement the Ramming/McKenry RS-3 and RS-9 rootstocks. As is true for any program devoted to developing disease-resistant materials, the viticultural aptitudes (e.g., drought tolerance and vigor potential) are not clearly known and must be sorted out right away. This is probably the job of us in the private sector, though I would assume that these rootstocks will find their way into public trials as well.
Obviously, any nematode-resistant rootstock must also be resistant to phylloxera, but that is not an automatic given and screening must be performed. Besides nematodes and phylloxera, resistance (or tolerance) of fanleaf degeneration disease commands high importance in his lab, and he is now working on some fundamental aspects of salinity tolerance in addition to embarking on a search for Pierce's Disease-tolerant rootstocks.
Peter Cousins of USDA and Cornell University added that crown-gall resistance is an important consideration for rootstock breeding in the Northeastern U.S. Currently, there is no rootstock recommendation for crown gall resistance. Additionally, any rootstock that provided resistance to the grape root borer, a devastating pest in that region, would have positive economic impact. He did mention that a few existing rootstocks, including 110R, 5A, Harmony and the generally undesirable 44-53, had some root borer resistance. There is not any rootstock known for its cold-hardiness, an important consideration in many parts of the northern US and Canada and a reason why many northern latitude vineyards are ungrafted.
Nathalie Ollat of INRA in Bordeaux discussed their objectives for rootstock breeding, which were largely similar to those of the western US: Resistance of Phylloxera, the dagger nematode Xiphenema index and tolerance of fanleaf degeneration disease, drought and limestone soils. The latter is quite important as 25 percent of their vineyards are on calcareous soils. With climate change, drought tolerance is becoming a trait of higher importance in their country, especially since irrigation is not allowed in many of their finest growing regions. Their efforts into drought tolerance are in their early stages. Regarding X. index, they have found that Vitis rubra has the potential for resistance and produces very low-vigor plants (lower than Riparia Gloire, which means very low vigor).
Rob Walker from CSIRO of Australia discussed his country's approach to rootstock R&D. In a sense, Australia has been behind the rest of the world in rootstock evaluations, as most of their original vineyards were planted on their own roots. However, as phylloxera has spread throughout the grape growing regions of their land, many new and replanted vineyards have been on grafted vines. Depending upon region, between 2 percent and 40 percent of vineyard acreage is on rootstock.
In a large way, the Aussies have an advantage in that they can tap into the world's knowledge base regarding pest and disease resistance while focusing on breeding and selection of rootstocks to suit their own conditions. Faced with a monumental drought on top of an already arid climate, water use efficiency/drought tolerance and salinity tolerance tops their needs for rootstocks. Most vineyards are now planted on the relatively drought- and salinity-tolerant 1103P stock, while salt-tolerant Ramsay is also widely planted, in addition to smaller amounts of 140Ru and 101-14 (the latter of which is not very drought tolerant). Schwarzmann had been commonly planted but is now in decline due to its poor drought adaptation.
They have realized that some of the so-called drought adapted rootstocks are also those that tend to have high vigor, and that high vigor is generally associated with higher potassium uptake and poorer color and phenolics development. Hence, they have set out to develop some drought-adapted rootstocks that produce less vigorous scions. Three selections have been developed to this end, called Merbein 5489, 5512 and 6262.+
R. Walker discussed the concept of transpiration efficiency (TE), defined as the grams of dry matter (carbohydrate) produced per kilogram of water transpired. Merbein 5489 seems to have better TE than 1103P. However, it must be tested on grafted vines to be sure, since the rootstock leaves do not play a part in the physiology of the final product.
On the salinity front, they found that 140Ru has the ability to exclude chloride from entry into the xylem. It also performs similarly for sodium, and accumulates the ions within the root system while preventing them from entering the parent vine. They found that chloride exclusion is a heritable trait, and that 1103P has shown reduced chloride exclusion over time, though a concomitant reduction in yield has not been seen.
Back to the here and now: rootstock performance and selection
The symposium organizers made a truly great effort into making this symposium into one with an international perspective. In addition to the researchers already discussed, David Saayman, from beverage producer Distell in South Africa discussed his region's selection criteria for rootstocks. His general premise was that rootstock genetics was less important from a soil-matching standpoint and that roots will generally grow where soil allows them to grow. That is, if the soil provides little resistance to root growth, the roots will venture forth regardless of whether they are 101-14 or 140Ru. He mentioned that soil resistance of less than 2 MPa (about 290 psi) is required for root penetration. Hence, they focus mostly on site preparation and worry less about specific rootstock selection, requiring that phylloxera and nematode resistance be the primary screen.
While I'm not sure that I completely agree with their methodology, their procedure is to heavily amend soils to bring their pH levels into proper range while loosening the soil to allow for deep root exploration. Most of their soils are very old and very acidic. They will correct with up to 30 tons per acre of lime to bring the soil to 5.5 pH. Wow! That's a lot of lime on a per-acre basis. They also incorporate the lime very deeply, ripping down to four feet and using delve plows to create raised beds above grade for planting. Because the soil physical and chemical condition have been heavily modified, their choice of rootstocks are not as critical. However, drought is still a consideration and 140Ru, 1103P and 110R are used in consideration of their drought tolerance. 101-14 is not used where drought is a risk, though it is appreciated as a rootstock associated with high wine quality. Any remaining soil variation (e.g., textural) is accommodated not with rootstock changes but by changes in vine spacing within a row. The variable in-row spacing concept is something that I will write about in a column coming soon.
I found the approach that Saayman descriobed rather brute-force and geared, it seemed, towards larger production vineyards. Heavy, deep soil manipulations is something that is generally done less often in California Coastal vineyards as it destroys soil structure, raises erosion potential and produces overly-vigorous vines in this climate, which features high rainfall totals. However, in more arid climates where water supplies are limited, deep soil preparation may indeed be a valid means to aid in soil-based water storage and accessibility by the vine's root system. I have to imagine, however, that such heavy manipulation essentially negates the soil component of terroir for any given location.
Closer to home, Larry Bettiga, UC Cooperative Extension for Monterey County, discussed some results from a rootstock trial conducted in the Salinas Valley. He made an important point that is sometimes lost on viticulturists: His early results differed greatly from the results gained a few years later from the trial. It is important not to draw conclusions too early on any rootstock trial or comparison.
I once looked at some data sets from long-term rootstock trials and found that it took about 5 years before annual data (such as yield, pruning weight, cluster counts, etc) began to level off for just about all rootstocks. There was one exception. 420A took about seven years before the data settled into a consistent pattern. So early data from any rootstock trial may be very misleading and should oftentimes be ignored, unless information is needed about relative rates of early development.
Bettiga looked at cold-damage susceptibility, forced to do so as the vineyard was exposed to some unusually low temperatures. He found that the 1103P-grafted vines recovered much better than 3309C or 110R-grafted vines, the two of which were most greatly affected by the cold snap. Vines on 5C and SO4 rootstocks were similarly least affected. Vines on Freedom, Salt Creek (Ramsey) and 101-14 were all stunted by the low temperatures, but they later recovered.
Rhonda Smith, UC Cooperative Extension for Sonoma County, showed results of two rootstock trials in two very different sites, both with Chardonnay scions. One site had a very clayey soil and is located in Sonoma Carneros and the other site has a lighter top soil but only to 18 inches in depth, below which was a clay subsoil and water table at 36 inches.
She evaluated 101-14, 5C, 3309C, 110R and 1103P rootstocks at both sites. There were pronounced differences in the results at the two sites. Looking for similarities between them was more difficult than pointing out differences. One similarity: 1103P had higher tissue phosphorus levels in the scion at both locations. Both 110R and 1103P had lower tissue potassium levels, but only at the Carneros site. In the very heavy soils of the Carneros site, 1103P and 110R had the highest amount of vegetative growth amongst the five stocks. I have seen similar results in some very heavy Carneros clay soils, indicating very poor performance of 101-14 and 5C rootstocks in that extreme type of soil.
Smith's results point to the important site-specificity of rootstock selection. Because soil types may vary over just hundreds of feet, it is important to match the rootstock to the local soil conditions, not just to those of one's neighbor! Creating blocks based on soil types, not just convenience, will allow for proper matching of rootstock to the soil, producing a more uniform vineyard. This will later reduce costs involved with differential irrigation, uneven ripening and reduction in overall quality. wbm
