When most closely related plants are grafted together they unite readily and continue their growth as one plant. When entirely unrelated plants are grafted together, the usual result is the failure of the graft union. The inability of parts (stock and scion) of the two different plants when grafted together, to produce a successful graft union and the resulting single plant to develop satisfactorily is termed as graft incompatibility.
Ex. Apple on pear, pear on quince and apricot and almond.
Types of incompatibility:Graft incompatibility in fruit trees has been classified in to two types.
1. Translocated incompatibility.
2. Localized incompatibility
1. Translocated incompatibility:
This type involves phloem degeneration and this can be seen by the development of brown line or necrotic area in the bark. This is due to the movement of toxic materials through the phloem. This cannot be overcome even by inserting an intermediate stock.
2. Localized incompatibility:
Incompatibility reactions are seen at the graft joint. This incompatibility can be overcome by inserting a mutually compatible interstock in between them by avoiding direct contact between stock and scion. This may be due to repulsive action of stock and scion.