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.