Discuss the factors influencing the choice of rootstock.

Rootstock selection is determined firstly by survival, resistance to soil pests or diseases, then to environmental suitability, to the chemistry of the soil and climate, as well as to the objectives for production and performance of the chosen scion-rootstock pair. Rootstocks are most commonly used to prevent soil pests or diseases, the most important are phylloxera and nematodes. Adaptation to a particular soil and climate conditions, such as tolerance to drought or wetness, as well as to lime, acidic or saline levels in soils will also determine rootstock selection. A rootstock will be selected for production and performance compatibility with the chosen grafted variety, for control of yields, mineral conditions and ripening time. Sourcing of the rootstock is important and could be mandated by law. Some argue that rootstock choice is more important than scion selection. If the rootstock cannot root and grow, then it will never be strong and thus, economically viable.

Rootstocks are frequently used to prevent soil-borne pests or diseases such as phylloxera or nematodes. Widespread phylloxera devastation in France in the 19th century lead to the use of resistant American species and now more commonly, hybrids. One hybrid erroneously thought to be phylloxera resistant was AXR1, which succumbed to phylloxera in the 1980s, requiring the replanting of large amounts of land in Napa and Sonoma. The term "nematode resistant" is virtually meaningless as there are different nematodes such as dagger, rootknot and ring; the nematode resistance needs to be specified. Resistance to one nematode does not mean the rootstock is resistant to a other species of nematodes. SO4 is used widely in Europe and not as frequently in the US. SO4 offers high phylloxera, medium to high rootknot and low to medium dagger resistance. In some parts of Chile, Argentina, Australia and California nematode resistant grape rootstock need is thought to be on a par with, if not more important than, phylloxera resistance. At Errazuriz's vineyards in Casablanca Valley, Chile, nematodes thrive in the sandy soils, severely decreasing vigor and sometimes, killing vines. One of the rootstock in use is Harmony, with a high nematode resistance but with no protection to phylloxera. Keeping the rootstock healthy and free from soil pests and diseases is paramount.

Despite the importance of disease resistance, environmental adaptation has been one of the main reasons for continued rootstock trials. Rootstocks are selected for their suitability to particular soil conditions relative to the chemical levels of lime, acidity, pH or salinity in soils and climate conditions such as drought or wetness. Some rootstocks are better suited to certain soil types than others. SO4 has low to medium to tolerance to drought, low to medium tolerance to salinity and medium tolerance to lime. Colome in the Salta region of Argentina is currently in the process of replanting rootstock as the previously planted SO4 had a poor uptake of magnesium that resulted in a deficiency that inhibited photosynthesis. Lime-rich soils can induce lime-induced chlorosis in areas such as Champagne and Burgundy, where limestone content is high. SO4 and Fercal are popular choices. Acidic soils such as those found in Oregon need rootstocks that can tolerate acidity although liming the soil is an alternative. A study in Australia showed that vines grafted to Ramsey did not have any effect on yields in areas of high salinity. In the hot and arid Cotes-du-Roussillon, rootstocks like 110R have a high drought tolerance. Drought tolerance rootstocks may become more important with global warming but 110R may be too vigorous if the climate becomes warmer and wetter. In Inner Mongolia in China, winter hardiness is important for survival due to extreme sub-zero temperatures. Rootstocks such as Beta may be used. The complex puzzle of rootstock selection needs to be matched with soil and climate.

Rootstocks can affect vigor, mineral nutrition and ripening time and need to be paired accordingly with the scion. Many growers think that controlling vigor is the most desirable rootstock trait, second only to phylloxera resistance. Rootstocks at Errazuriz were chosen with a dual purpose to both to combat nematodes but also to control vigor. With chardonnay, high vigor rootstocks in low vigor sites can reduce the time needed to train vines. However, high vigor rootstocks like St George can adversely affect quality by producing overly dense canopies and high yields. SO4's main drawback is its low vigor; however, it is more productive on high vigor, irrigated and fertile sites. Cabernet Sauvignon has both a high magnesium and potassium demand and should not be grafted onto rootstock that is susceptible to those deficiencies, especially when levels are low in the soil. The effect of rootstocks on fruit ripening is particularly important in cool-climate viticultural regions such as Oregon and New York. In years when rain or frost looms, advancing fruit maturation by a few days could result in a significant economic benefit to the grower. Selecting an earlier ripening rootstock may allow for greater optimal grape maturity before harvest. Matching the appropriate rootstock with the varietal can positively affect quality.

Growers should seek certified virus-free plants with no record, according to Channing Daughters Winery in Long Island. Reputable nurseries sell certified virus free plant material from the Foundation Plant Services(FTS) at UC Davis with similar programs available from Geisenheim, Montpelier and Bordeaux. Other local growers, a nearby cooperative extension and consultants may have local experience and are helpful in determining the choice of rootstock. In addition, appellation rules in the EU may specify the usage of rootstock, limiting the choices.

Choosing more than one rootstock, even for the same conditions, is a method to hedge one's bets. Cain Vineyards in Napa uses an assortment of rootstocks as they feel they are still discovering which rootstocks work best with their sites and produce the best tasting grapes.

In conclusion, there are many factors influencing the choice of rootstock. Prevention of soil pests such as phylloxera and nemotodes are the first priorities. Chances for success can be increased by matching rootstock attributes to soil and climate conditions of the site. Finally, matching scion and rootstock to obtain optimum yield, mineral levels and ripening is another factor in rootstock selection. Rootstock selection is both an art and a science as multiply remedies are available, depending on the objectives of the grower.