What is graft failure and what are the causes?
- Grafting is a method of propagation that joins a scion (upper portion) and rootstock (lower portion) of two similar species of plants.
- Graft incompatibility, improper technique, or environmental conditions, may cause graft failure.
- Graft incompatibility is not clearly understood but may be the result of genetic differences between the grafted parts.
- Abnormalities may develop in the vascular tissue at the graft union.
- Disease-causing organisms such as viruses or phytoplasmas are other possible causes of incompatibility.
- Care should be taken to use disease-free plant material when grafting.
Graft failure. Photo: R. Heflebower
Symptoms
- The most pronounced symptom of graft failure is a smooth, clean breaking off of a tree at the graft union. This may occur one, two, or many years after the graft is made.
- Other symptoms of graft failure include general ill health of the tree or shoot dieback.
- Foliage may yellow in late summer, followed by the early leaf drop.
- Vegetative growth of scion (part of the graft that produces growth) and rootstock may begin or end at different times. There is often a distinct difference in growth rate between scion and rootstock.
- Overgrowth may occur above, at, or below the graft union, and results in a visible difference in the trunk diameters. Suckers can develop from the rootstock.
- While it is possible for trees to survive with one or more of these symptoms, a combination of many symptoms may result in the premature death of the tree.
- Weakened trees may have to be removed.
Normal bulging at the graft union
- A slight bulging, swelling, or "crook " in a tree trunk, two to three inches above the soil line, is the result of normal healing at the graft union.
- The site where the scion and rootstock meet is called the graft union.
- Swelling occurs as callus tissue (tissue that forms around a "tree wound") is formed and new vascular cambium tissue develops in the callus bridge area.
- The graft union should remain above the soil line to prevent the scion from developing roots, losing the influence of the rootstock.
Normal graft union. Photo: University of Maryland
Grafts and tree production
- Nurseries frequently use grafting as an excellent way to propagate plants not easily grown from seed or cuttings, especially the cultivated varieties.
- It allows the grower to produce a saleable plant more quickly.
- It also allows nurseries to select and control desirable plant characteristics such as growth habit, growth rate, size, hardiness, and time required for fruit production.