A graft incompatibility is the failure for a graft to create a strong union. The incompatibility results in unhealthy growth or inability for growth, which results in premature death of the graft. The reason for such results are due to incompatible and biological differences between the scion and the root-stock.
Causes of graft incompatibility
There are three different factors for graft incompatibility. These are as follows-
1. Anatomical factors: Anatomical differences might contribute to structural weakness in inter-specific grafts. The distortions of vascular tissues between the scion and the rootstock may disturb the movement of minerals, water and metabolites resulting in characteristics overgrowth and poor union. Lack of lignification of the adjoining cell wall of the graft combination is also an important cause of incompatible graft combination.
2. Physiological factors: Physiological incompatibility is due inability of the stock or the scion to supply the other component with the necessary amount or quality of materials for normal functioning. There is some evidence that in certain graft combinations one component (scion or stock) produces chemicals that are toxic to the other, killing the entire plant. An example of the physiological causes of incompatibility is that when Jonathan apple is grafted with Mailing-Ix rootstock, the portion develops molybdenum deficiency while in the rootstock no such symptom is evident. It is said that the inability of Mailing-lx rootstock to absorb molybdenum insufficient quantity is the reason for this type of physiological incompatibility. However, the exact cause of incompatibility due to nutritional and hormonal misbalance is not clearly understood.
3. Disease factors: The third factor governing incompatibility in certain combination is attributed to virus disease. For example- Baslette pear grafted on Pyrus pyrifolia – indicates incompatible situation due to virus susceptible rootstock. Baslette pear grafted on Pyrus communis —compatible due to virus resistant.
Symptoms of graft incompatibility
Incompatibility in a graft may be manifested in a number of ways-
1. Complete failure to form a graft union.
2. Very low percentage of graft success.
3. Union takes place, growth occurs, but eventually the tree dies either in the nursery or in the field.
4. Degeneration of the tissues at the graft joint, yellowing of the foliage and premature defoliation.
5. Small stunted growth and general ill health of the graft.
6. Deficiency symptoms or nutritional disorders.
7. Overgrowth or undergrowth of the rootstock or scion.
8. Excessive swelling at the graft joint.
9. Breaking off of the tree at the point of graft-joint and the break is clear and smooth.