regime. For these reasons, it is best to choose several rootstocks to plant out a vineyard to spread the risk. If the vineyard needs to be redeveloped in the future, the best performing rootstocks can then be used.
stages, as they have to develop an entire root system. Alternatively, the cuttings can be grown in a nursery for a season, dug out in the winter and planted in the vineyard as dormant rooted cuttings.
Grafting
The previously discussed methods produce own-rooted cultivars, normally only used in phylloxera-free regions or with phylloxera-resistant cultivars.
Most vines planted in major grape-growing areas are grafted due to the presence of a soil pest or to take advantage of rootstock effects on scion growth.
The majority of commercially produced vines are propagated by bench grafting. A rootstock section of about 35 cm long is cut from dormant vines of the selected rootstock mother plant and all buds removed either manually or by a disbudding machine, which uses stiff-bristled brushes to remove the buds.
The scion section is cut from healthy vines of the selected variety in lengths of approximately 35 cm for ease in handling but, before the actual grafting takes place, they will be cut into individual nodes.
If cuttings are not to be used immediately they should be buried in moist soil or treated with a botryticide and stored in high-humidity refrigerated storage near, but not below, 0°C, as the fungicide used may reach toxic levels if the water on the vines turns to ice (Nicholas et al., 1992). These cold and moist conditions prevent desiccation of the cuttings, development of disease organisms and slow respiration in the tissues.
Before grafting the rootstock and scion, cuttings are soaked in water and the scion wood cut into one-node sections. These are matched in diameter with a rootstock cutting and the ends slotted with a special blade, giving a complementary ‘V’-shaped or omega (Ω)-shaped cut. The two pieces when fitted together provide optimum contact between the cambia of the rootstock and scion, which will then ‘knit’ together the two pieces of wood. They may or may not be wrapped with special tape or dipped in wax to keep the union from drying out or coming apart too easily (see Plate 23).
The joined cuttings are placed in a moist medium (e.g. perlite, sphagnum moss, sand) and incubated at approximately 28°C for up to 4 weeks to encourage the development of undifferentiated cells (callus) at the graft union and roots at the cutting’s base (see Plate 24). When the grafts have healed the cuttings are unboxed and can be planted outside, closely spaced. Maximum rooting will occur if the vines are well irrigated and the soil covered with plastic to prevent weed competition. Typically, the grafted vines are grown for a full season in the nursery, and in the winter dug out mechanically by undercutting, graded as to quality, roots trimmed and then bundled for delivery. Although they can be stored, the best results are from planting as soon after digging as possible.
Another grafting technique commonly used to change varieties in an established vineyard is field grafting. In this procedure, a small portion of a
dormant cane (that includes a bud of the variety desired) is cut so that its cambium can be closely matched to the cambium on the trunk of the parent vine (see Fig. 5.8). Methods of doing this include T-budding and use of a cleft graft. This method has the advantage of enabling a vineyard to be converted to another variety quickly with very little loss in production, due to the existing and established vine root system backing up the newly grafted scion. However, it also has the disadvantage of its success rate being highly dependent on the weather and skill of the team doing the grafting (Nicholas et al., 1992).
Other methods
Green, softwood cuttings can be rooted in mist beds in a greenhouse. An advantage of this is that many cuttings can be rooted in a small space, but it
Fig. 5.8. Top-grafted vine, showing the developing new shoot and the white tape used to keep the cambia of the cane and trunk closely associated as well as decreasing water loss.
does require special facilities and the softwood cuttings are more susceptible to disease than dormant cane cuttings (Dirr and Heuser, 1987). This method can also be used to produce grafted vines. The rootstock is rooted in a mist bed and then a section of the scion variety grafted to the rooted understock. These are grown in pots under controlled conditions until acclimated and then planted directly in the vineyard.
A new variety or clone that is originally selected from a single plant is propagated asexually to provide a small number of vines for further testing.
The rapid production of large numbers of vines for commercial planting can then be accomplished by growing some of the vine tissue in sterile culture through micropropagation. Following the induction of many shoots in the culture tubes, each is harvested and transferred to sterile test tubes containing growth media. After rooting, the buds are acclimated to greenhouse conditions and then transplanted. These ‘test tube plants’ can be further acclimated to nursery conditions and then planted in the vineyard.
It is notable that vine health and, increasingly, pedigree, is an important consideration in the production or purchase of vines. Vine stock that has any of a multitude of diseases can result in low vigour and performance or, at worst, complete failure of vines (see Chapter 9), so using certified stock is a worthwhile investment.