Many woody ornamentals and most fruit trees are propagated by grafting. Sometimes the graft union fails, resulting in the main stem breaking off, dieback, poor growth or death of the top part of the plant. In contrast, the root system will often remain alive and may send up suckering shoots.
What is graft failure?
For various reasons, such as ease and speed of propagation, disease resistance, tolerance to certain conditions and for the control of vigour (predominantly in fruit trees), a large proportion woody plants are propagated by grafting and budding.
Unfortunately, the graft union sometimes ceases to function at a later period, possibly some years after planting. When this happens, the flow of nutrients from the rootstock to the top growth (scion) becomes limited or stops completely and the top growth will gradually or suddenly deteriorate.
The affected plant may start showing signs of poor growth and gradual deterioration; on the other hand, the demise can be quite sudden. Graft failure can occur in both young and fully-established plants with little prior warning.
- In some cases, the graft union does not display any obvious external symptoms of a problem being present
- In other cases, on closer examination the rootstock and scion can be seen to have become partly or completely separated
- There may be signs of decay of the wood around and below the graft union, but this is generally a secondary problem rather than the cause
- Also overgrowth (substantial thickening) at, above, or below the graft union can be a sign of graft failure
- The rootstock often remains alive and may produce suckers. Sadly, the rootstock is often of inferior quality to the plant that has died so it is seldom worth trying to grow it into a replacement plant
- When examining failed plant, if cutting vertically through the graft union you may find a dark line or corky tissue following the contours of the union between the rootstock and scion
Other reasons for plant failure
Remember, similar symptoms can be found on plants affected by bacterial diseases such as crown gall, which can cause decline of grape vines, roses and other trees and shrubs. Bacterial canker is also a common problem on both ornamental and fruiting Prunus.
Death of woody plants can be caused by root diseases such as honey fungus or Phytophthora.
In addition, establishment problems may be to blame for poor growth of recently planted trees and shrubs.
When buying plants select, healthy, strongly-growing specimens. Check the graft union carefully. It should be neat, well callused over (joined) with no signs of decay or suckers being produced from the rootstock.
Avoid mechanical damage to the graft union during transport, planting and in the plant’s final position, including accidental damage from strimmers and lawn movers.
Avoid over-deep planting. In the majority of plants, the graft should stay proud of the soil or be positioned just below the soil level (roses). One of the exceptions are tree peonies where the graft should be below soil level.
When mulching, keep the base of the plant free from much.
Graft failure occurs when the rootstock and scion become partly or fully separated and/or an impermeable, often corky, layer forms between the rootstock and the scion. However, it is often difficult to establish the exact cause.
Graft failure can be caused by factors such as:
- Poor formation of the graft union due to problems with anatomical mismatching (when the rootstock and scion tissue is not lined up properly), poor grafting technique, adverse weather conditions and poor hygiene
- Mechanical damage to the graft union
- Graft incompatibility
Graft incompatibility can occur for number of reasons, including:
- Virus or phytoplasma infection (see below)
- Genetic incompatibility or biochemical reaction of the rootstock and scion. The failure can be fairly immediate or delayed, perhaps twenty years or more
- When a weak cultivar is grafted onto an excessively vigorous rootstock
Phytoplasmas are single-celled organisms and, as opposed to bacteria they lack a cell wall. Previously known as ‘mycoplasma-like organisms’ or ‘MLOs’, they live as parasites of plants and cause symptoms that are similar to viruses. As with viruses, there is no remedy but to replace infected plants.