Grapevine grafting 101
Grapevines are mainly top worked to change variety. Grafts can change one fruiting variety to another, or from a rootstock to a fruiting variety. Grafts can be used to convert existing vines to a variety of premium or remove varieties of low demand.
Top working can be an expensive process if undertaken without careful preparation, the risk of graft union breakage or failure increases as grapevines age. Grapevines that are three or more years old with trunk diameters greater than 2.5 – 4 cm and up can typically be grafted.
Advantages of top-working include:
- Fast response to market changes, only one production season lost and up to full production in second year post grafting.
- Less impact on cash-flow.
- Less lead time to obtain planting material.
- Lower cost than full redevelopment.
The main problems that can occur with top-working are:
- Graft union vulnerable to being broken by wind or machinery.
- Variability in grafting success resulting in increased vineyard management costs due to regrafting or replanting.
- Reduced yield and long-term decline in vine health.
- Rapid vigorous growth may put pressure on the graft union to keep the vine hydrated.
It is critical to carry out a health and business assessment before undertaking top working in the vineyard. Existing vines must be assessed to determine if top working is a viable option over replanting.
Grapevine health must be assessed first before grafting, as poor rootstock vine health is a significant cause of grafting failure. Both bud wood and rootstock sources should be screened, especially for viruses.
If vine or rootstock health is sound, a cost benefit analysis should also be done to evaluate if top working is the best financial decision for a commercial vineyard. Grafting is an expensive exercise and should only be undertaken to meet certain market requirements or if replanting is unviable.
Health assessment
Top working can directly transmit viruses, fungal, bacterial diseases and other pathogens to new grafts. Virus and virus associated diseases are the primary concern regarding vine health and grafting success.
Varietal susceptibility and sensitivity to pathogens varies greatly and is a major cause of graft failure within the scion.
The health assessment consists of three parts:
1: Assess the past performance and characteristics of the vineyard
2: Visual assessment of the vineyard
3: Submit vines sample(s) for virus or pathogen screening
Step one: Vineyard Performance and Characteristics
In a vineyard where nutrition, soil, pest, or existing diseases are already an issue, grafting may be ineffective and/or fail. This is due to the added stress vines may experience in addition to the grafting process.
Certain varieties with existing virus may give no indication of infection. It is crucial to ensure no new viral load is introduced through scion or rootstock material. Both scion and rootstock can have varying sensitives and responses to viruses. Vines or graft material afflicted with diseases should not be used for grafting.
Step two: Visual Assessment
Many grapevine viruses do not show physical or visual symptoms of infection. Vines may only be symptomatic during certain phenological (growth) stages. This is especially true for white varieties. Viruses, such as the ‘grapevine leafroll-associated virus’ group, give away visual signs that a grapevine has a virus. According to Vinehealth Australia, visual assessment for leafroll viruses is best done in April – May. For more information on visual health inspections of vineyards, contact the Department of Primary Industries and Regional Development or Vinehealth Australia.
Step three: Virus Testing
There are thought to be more than 15 grapevine viruses present in vineyards across Australia. These viruses can be transmitted from bud wood, often resulting in multiple viral infections as new viruses are introduced. This can be detrimental to the vine, causing graft failure and accelerated vine death.
Laboratory testing is the best way of determining the presence of viruses in vine grafting material. However, virus testing alone may not provide a definite answer to whether top-working should be undertaken. Contact Richard Fennessy at the Department of Primary Industries and Regional Development regarding submissions of samples for virus testing.
Business assessment
Grafting is only one possible strategy used to improve vineyard profitability in the short – medium term. It should only be undertaken if certain market criteria are met and, on a larger scale, if risk of graft failure is acceptable. Top-working may lower the impact on cash-flow and return vineyard to profitability more quickly if successful.
Top working grapevines
The following is an extract from Vinehealth Australia's text: A grower’s guide to top-working grapevines for more information on grafting and top-working, follow the link to this guide and other references.
1. Chip-Bud Grafting
This is the most common method used to top-work vines because of its high success rate. It is most successful in early spring after budburst through to flowering – or when bark slip starts to occur. The budwood used is taken from vines when fully dormant. It is usually taken as a cutting and stored in a cold store at approximately 2°C until required. Chip buds are then taken from each dormant cutting and stored in cool water until grafted, which is best done within 12 hours of bud preparation. The chip-budding technique can also be used later in the season using a green bud from the current season, or a lignified brown bud from the current season. Green buds are more susceptible to hot weather than the lignified brown buds.
2. Cleft Grafting
Cleft grafting can be done high or low on the trunk, or even below the soil surface. Low level cleft grafting is normally only done when disease needs to be removed from the vine. This type of graft is done from budburst to bark slip. Cleft grafting is best suited to mature vines with a large trunk diameter over 10cm. Thinner trunks are not able to hold the budwood firmly enough to ensure callusing. Grafting ideally occurs within 30 minutes of removing the vine framework. The budwood, which is cut to a wedge at the base, is inserted into a split made into the trunk to a depth of 5cm. This grafting technique requires the use of a pruning wound sealant because of the size of the wound.
3. T-Bud Grafting
T-bud grafting is normally used when top-working is done late spring to summer once bark slip occurs on the trunk. The bark on the trunk is cut in a T-shape under which the bud wood, cut into the shape of a shield, is inserted. The union is then wrapped with grafting tape. The budwood is taken from fully dormant vines and stored in a cold store at approximately 2°C. Chip buds are then taken from each dormant cutting and stored in cool water until grafted, which is best done within 12 hours of bud preparation.
Management implications for top-worked vines
In the first season of growth, top-worked vine nutrition, water, and training needs to be managed carefully to ensure graft union does not break or die. Replanting will be required if top-work grafting fails. The greater the number of grafting failures, the less uniform the vineyard will become due to replanting. This may impact fruit yield and quality, making management more costly across the vineyard.
For further information please contact Colin Gordon or Jesse Bowman