Rootstock was at first a tool to fight Phylloxera, an insect that feeds on and destroys the wines roots system. Now, rootstocks have become the mediators between climatic and soil environment and the characteristics of the cultivar.
Rootstocks are grafted to scions in order to provide the final plants with properties to take on this role. They are obtained from the crossing of 4 species mainly: Rupestris, Riparia, Berlandieri, and Vinifera. Each has its pros and cons. Rootstock selection is the key decision before planting, as it will remain permanent until replanting and has serious implications on the ultimate profitability of the operation.
After vineyard evaluation for climate, soil and other factors, cultivars are selected based on a set of objectives and its potential ability to reach those. The rootstock’s compatibility to the cultivar will affect final quality, growth cycle, yield, and resistance to diseases. Several vineyard factors will influence the final choice of rootstocks: presence of Phylloxera or nematodes, soil conditions and nutrients availability, climatic factors, and planting plan.
The key consideration is presence or risk of phylloxera and/or nematodes. The vitis Vinifera, the grape species mostly involved with the production of quality wines, is especially prone to Phylloxera and has no natural resistance to it. American species like V.Riparia have a strong resistance to it and its roots are used for grafting onto a Vinifera scion. In some wine regions, conditions are such that Phylloxera can not grow. This is the case of Chile in particular where dry conditions, mostly sandy soils, and relative isolation from the rest of the world has enabled them to plant v. Vinifera species on their own roots. However, with the extensive use of irrigation, vineyards in Chile are replanting with rootstocks resistant to nematodes, a roundworm which can feed on roots or transmit diseases and destroy vineyards. Especially virulent is the “root knot” which can be kept at bay with riparia based rootstock which has a strong resistance to it. However, in modern vineyards, Phylloxera and Nematodes are not the only considerations when it comes to rootstock selection and other pests and diseases pressures need also to be considered.
Soil conditions is also key factors and rootstocks can influence their effect on the vine. In fact, rootstock offer the viticulturist the opportunity to influence some properties of the cultivar in totally natural ways. They can improve the performance of a cultivar versus a specific soil type. For example in Beaujolais, where the Rupestris Vialla is becoming more popular as it enables the Gamay cultivar to do well on granite soils. In most of the worlds best vineyard, soils are limestone based and the high content of lime in those soils can cause chlorosis to the vine. A v. Berlandieri and v. riparia crossing rootstock is especially effective to fight against this problem. Where problems of salinity exists, especially in those regions where irrigation is used extensively, rootstock selection based on Berlandieri and Rupestris, for example 140ru, is effective to deal with the issue. Soil depth and water retention abilities are other factors which can be dealt with rootstock, for example 3309c does very well in deep soils but 140ru is not adapted to wet soils. High fertility can cause problems with the vigor of the plant and the 140 ru is especially good to mitigate this issue while the 3309c is not so effective. If a vineyard has a large variation of soil types, as it is often the case in Italy, a mix of several rootstocks are used to plant the vines within its confine.
Included in the conditions of the soil is the availability of various nutrients. They can have a serious effect on the way a cultivar behave during the growing season. In fact, each cultivar has its own requirements and rootstock can also influence its properties and can modulate the intake of nutrients to the plant. A key issue to consider when selecting rootstocks is the dominance of a nutrient versus another. Indeed, some excess of one can antagonize the intake of another can thus create imbalance. Not all nutrients create difficulties, key concerns are potassium which affect growth, juice PH, and ultimately wine quality. Calcium, magnesium, and chlorine need also to be monitored carefully each rootsock has its own influence on nutrition. For example 140ru reduces the amount of magnesium intake while riparia gloire has minium effect on its intake.
Other than soil, climatic factors such as the risk of drought, climate type, and length of the growing season can be influenced by the choice of rootstocks. In the case of short growing seasons such as those found in cool climates, early fruit maturation will be favored and the use of riparia based crossings such as riparia gloire and so4 will help speed up maturation or grapes. Humidity is another factor of climatic concern and it can be dealt with as reviewed above in relations to the water retention of the soil. When a climate is especially cold and presents a high risk of exrtreme cold frost in the winter like can be found in Canada, and very high altitude vinyeards, several types of rootstocks can be used to reduce vulnerability. In very dry or hot regions, drought is a concern as the lack of water will shut down the plant and lowers ripening effectiveness. Especially efficient to deal with this issue is the 140c but not so is the so4.
Other than Phylloxera and nematodes, soils conditions and nutrition as well as climatic factors, the planting plan need also to be considered when selecting rootstock. As we have seen each cultivar behave differently in various conditions and rootstocks can help mitigate or improve the cultivar’s properties to deal with those conditions. Key will be their affinity with each other. The density of planting can be an issue and rootstock can help to create competition. Objectives such as yield requirements and quality objectives will also influence
Several factors influence the choice of rootstocks. Key among them are Phylloxera and nematodes, soil composition and nutrients, climatic factors such as risk of drought and length of growing season. Rootstock also influence the way the vine behave through the growing cycle. A regions regulations can dictate the use of specific rootstock. What is key is that the rootstock be selected in a way that it will be compatible with the cultivar chosen for the specific site so that objectives such as quality, yield, and economical health be achieved. However, not all rootstocks are created equal and each have their pros and their cons.