The Hard Graft of Grapegrowing
Grafting is the connecting of pieces from two different plants so they develop and grow as one. In wine production shoot system material from the grape variety from which one wishes to produce wine (the scion) is combined with the root system from another vine, usually a different species.
Here the focus is on bench grafting of cane cuttings, as occurs in commercial nurseries (i.e. at the workbench). This method has the benefit of being scalable, and largely mechanized. The alternative is to graft in the vineyard (field grafting).
History
The widespread use of grafted grapevines was prompted by the need to combat pests. The practice spread in the late 19th and early 20th centuries (alongside hybridization) to increase resistance to Phylloxera, the aphid-like bug devastating the vineyards of Europe. It had been noted in the USA that, unlike vitis vinifera, native American species were not susceptible, so rootstock grafting was swiftly taken up with positive results. American viticulturalists such as Texan Thomas Volney Munson were lauded in Europe.
The cause of the Phylloxera blight is not certain. Paradoxically, the bug may have travelled across the Atlantic in the 1850s in American vine cuttings brought over for earlier grafting trials. It was likely not realised that the resistant American vines were still carriers of the bug. The invention of the Wardian case, a mobile sealed container, in 1842 had (along with nautical advances) greatly increased the global transport of plants. It allowed them to be carried in the sun on the deck of ships, sealed from saltwater in a controlled environment.
The choice of rootstocks has narrowed since the early 1900s. Testing and experience, particularly in France, identified 20 or so which between them provided the best match for conditions and reliable performance. Of these probably around half account for 90 percent of global plantings.
This limited genetic diversity carries major risks, should a rootstock's resistance prove inadequate. This seems to be particularly a problem with rootstocks with vinifera in their parentage, such as the vinifera–rupestris hybrid AXR1. The failure of the latter was behind major phylloxera outbreaks in California in the 1980s. Here the bug mutated (as Phylloxera biotype B) and AXR1 succumbed. Around two-thirds of Napa's vineyard were replanted in the 1990s, and the fallout from this episode continues.
It should also be emphasized that most rootstocks – even without any vinifera – are only resistant to the bug, and not immune. Thus vines may still grow well but infestations can build up. This has caused problems in Oregon where there are a relatively high number of vines on their own rootstocks alongside grafted ones.
Some regions can still use their own rootstocks in confidence, because of factors such as geographic isolation, climate, weather or soil type. Sandy soils deter phylloxera; the Colares region of Portugal is located on sand dunes which are intolerable for the bug. The ungrafted Ramisco vines found here are among the oldest in Europe. Hill of Grace, the famed Barossa Valley Shiraz vineyard owned by Henschke, has a topsoil of silty and sandy loam. Its greybeard vines are more than 150 years old, pre-dating the phylloxera blight and older than any commercial plantings in Europe.
Windy sites are also less attractive to phylloxera, and the bug is no mountaineer. It has not, under its own power, crossed the Andes from Argentina (where it is present) to Chile (where it is not). Some varieties may be more resistant, though this can be hard to prove in isolation from terroir.
Some growers, notably in Australia, believe their own rootstocks give them good quantities of higher-quality wines. Own-root plants from a field nursery can cost a fraction of a rootstock cutting, and have a higher success rate in establishing themselves. However there is some concern in Australia that rootstocks are not more widely used.
Choosing a rootstock
The selection of available rootstocks can vary between countries, based on the degree to which local evaluations have been carried out, bureaucracy connected to biosecurity, and so on. However there are various common factors which must be considered by all viticulturalists when choosing a rootstock variety.
Of these factors, degree of resistance to pests, soil-borne pathogens and nematodes (microscopic worms) is just the start. Rootstock varieties vary in their reliability to establish themselves, and in their compatibility with any intended partner grape variety. Root systems can vary in depth, and so have to be matched to the geology of the site. Tolerance to drought and water-logging also varies, as does suitability for specific microclimates. Rate of nutrient uptake and tolerance of soil acidity and salinity are all factors which can dictate the choice.
Rootstocks can also affect the vegetative growth of the vine, as well as the yield. The market positioning of the intended wine may therefore also come into consideration. Some rootstocks are more closely associated with super-premium wines than others, for example.
The same clone of a grape variety can give different results when grafted to different rootstocks. Many growers vary rootstocks with the same scion variety (and clone) in order to spread risks, hoping to prevent losing the whole crop. Others use multiple rootstocks to add complexity to the end product. This can be a positive by-product of an "off-the-shelf" purchase from a nursery to quickly fill a couple of extra rows.
Most available rootstocks are now based on three American vine species. The Texas native Vitis berlandieri is late maturing, has good phylloxera resistance and very good resistance to lime-induced chlorosis – important in chalky areas. Berlanieri crosses include SO4, 1103 Paulsein, and 99 Richter.
Vitis riparia is widely found east of the Rockies in the USA and Canada. It is cold-hardy, prefers rich humid soils, grafts easily and has excellent Phylloxera resistance. Its low vigor limits yield and it is correspondingly associated with quality. The pure Riparia Gloire (de Montpellier) is widely used in cooler locations such as New Zealand, where early ripening is required. Riparia crosses can be more vigorous.
Vitis rupestris from the Midwest USA has a long growing cycle and suits warmer climates. Pure Rupestris rootstocks are less common than crosses such as 110R and 99R.
Variants of bench graft
Detached scion grafts are the most familiar option in viticulture (when not grafting in the field). A cane cutting from the target grape variety (the scion) is detached from the donor plant and joined to a rootstock cane. The scion typically contains multiple buds. Apical grafting is another term associated with grapevine grafts. It refers to the union being made at the top (apex) of the rootstock material.
A key concern with any method of grafting is to create an effective alignment between the cambium layers of the two elements. The cambium layer is the main engine of radial growth of many plants including grapevines. It produces phloem cells (to transport compounds for photosynthesis) on its outside and xylem cells (transporting water nutrients and moisture up the plant) on the inside.
The formation of a callus in response to the damage caused by grafting is crucial to the process. The undifferentiated cells of the callus establish the union between the vascular and cortical tissues of the scion and rootstock.
Viticulture nurseries tend to use whip or Omega cuts. A simple whip or splice cut forms a diagonal union which needs very careful fastening. If the scion is smaller in diameter than the rootstock then they are lined up on one side so that the vascular cambium layers match up on one side. This technique is useful for wood which is too inflexible for more complicated cuts.
A whip-and-tongue cut starts with the diagonal cut. It then adds an additional cut in each diagonal face, creating interlocking tongues. This approximately doubles the surface of the cambium union.
The omega cut uses a special machine to create a junction which looks like the Greek letter, increasing contact area. The two pieces lock together like a jigsaw piece. The scion and the stock material must be of the same diameter.
Types of plant
Beyond the choice of [variety x clone] x rootstock, commercial nurseries often offer further variables. These can include plants in different stages of development, and/or grown using different methods.
Dormant field grown vines are most popular across the industry. Rootstocks are grown in blocks and harvested in winter when the vines are dormant. Canes are cut and buds are removed to avoid sucker growth, and to ensure that roots grow in a controlled manner from the base.
Then the scion plant is harvested, and sent to the nursery shed for grafting. Connections may be stapled to add stability. The graft is then dipped in wax to protect the wound and conserve moisture. The cutting is then stored until required for planting.
The graft union calluses in the late winter or spring. The vines are planted out in fields in rows, then dug up after one growing season. After sorting, grading and bundling they are either placed in a cutting mix or placed in cold storage for up to 2-3 months. They are supplied as bare rooted vines ready to be planted in the vineyard.
Without drip irrigation plants usually need to be planted in the early spring. This allows plants to develop sufficient roots to supply enough water and nutrients once leaves have developed. In vineyards with drip irrigation planting may be able to happen later in the year with less risk of stressing the plant.
They offer the benefit of a solid, established graft union, and well-developed roots. But because they take a season to produce, nurseries carry little off-the-shelf stock. Therefore a grower needs to decide on scion/rootstock combination(s) plus the number of plants, 15 months before planting.
Some nurseries offer second-year dormant vines. These follow the above process, being lifted and graded and having their roots trimmed before being replanted in the nursery for a second year. They cost more, but as off-the shelf purchases bring production forwards one year. They could be commissioned in advance to allow extra time for vineyard landscaping and other preparatory tasks. Field grafting, in some situations, can also be an option for speeding up vineyard establishment (this will be discussed in a further article).
Potted vines, also known as green-growing or dormant bench grafts are grown in a sleeve or tube. The larger the sleeve, the greater the root mass, and the earlier the start of shoot growth. After a period outside the greenhouse to air-harden, they are delivered green and growing, around 20cm (8 inches) tall in a small pot. They can allow a vineyard to be established one season quicker.
While field grown vines must be planted in Spring, potted vines can go into the ground from late Spring through the following summer, though they do need more care and nutrition during this period. They are not field-grown, which can reduce risks of pests or pathogens, and are not placed in cold storage. The plants begin growing from the next Spring. Some vineyards also supply lignified potted vines, designed to replace damaged vines. They can quickly be integrated into normal operations for the rest of the row.
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