Stone Fruit Propagation by Grafting
Not all stone fruit scions are compatible with available Prunus rootstocks, although incompatibility between rootstock and scion (graft incompatibility) may not be apparent for several years after establishment. The use of a compatibility bridge or “interstock” might be necessary to form a successful graft union if the desired rootstock is incompatible with the budwood variety. Interstock compatibility bridges can be costly because of additional labor costs and royalty fees associated with trademarked varieties.
Graft union success is evaluated by the vigor, productivity, and longevity of the scion. Some rootstocks might have undesirable influences on the scion including reduced fruit size, delayed leaf growth, and delayed ripening. Typically, several years after peach scions are grafted onto plum rootstocks the graft union develops a “shoulder” and trees topple over in high winds. Other signs of incompatibility include low scion vigor, shoot dieback, premature leaf drop, or excessive root suckering (Macdonald, 1986). Grafting can be used to “patch” damaged or broken scaffold branches, or to repair damaged graft unions, in stone fruits. Additionally, when it is necessary to change the fruiting variety in an orchard grafting can be used to “topwork” existing rootstock.
The most common method for producing finished trees is by budding fruiting varieties onto established rootstocks. Most peaches and nectarines are June budded from May to early June. Summer budding, from late July to September, is also common in peach and nectarine trees grown for retail nurseries because a tree with a larger caliper is produced. Spring budding of plum and apricot on fall planted hardwood cuttings of ‘Marianna’ plum produce a tree in 1 year. Plum is generally T- or chip budded in fall on ‘Myrobalan’ seedling rootstock.
T-Budding (a.k.a. Shield budding)
Budwood should be collected from the current season’s growth and budded onto one-year old rootstock. Rootstock should be ¼ inch to 1 inch in diameter and growing actively with “slipping” bark to ensure successful budding. Place buds 3 to 5 inches above the soil line (Lockwood & Ferree, Propagation, 2011). Insert the shield (scion bud) into the T cut on the rootstock and wrap the bud with a budding rubber or parafilm to hold it firmly in place. See the General Propagation section for details on T-budding techniques.
Chip budding can be used throughout most of the year because it uses either active or dormant plant tissues and has a greater success rate than T-budding. Chip budding ca be done by hand, or mechanized, in the field when rootstocks are between ½ in to 1 inch in diameter. See the General Propagation section for details on chip budding techniques.
If an existing orchard has low yield, a change in market demand makes the current variety unprofitable topworking can be used to reduce costs of changing fruiting varieties when rootstocks have been established for only a few years. Topworking generally reduces the cost of orchard establishment by eliminating the costs associated with: complete tree removal, fallow periods (2 years in peach), purchase or propagation of new planting material, and planting. Topworking is most successful when conducted in the dormant season, but can also be performed in the early part of the growing season. Although a budded scion typically contains only one bud, a grafted scion contains several buds.
Most often topworking is conducted in mid- to late winter using scion wood gathered during the dormant season. Use budwood immediately to prevent buds from drying out. If it is not possible to use budwood immediately lay it in a moist paper towel, newspaper, peat moss, or sawdust, wrap it in a plastic bag, and place it in cold storage at 34° to 36°F (Lockwood & Ferree, Propagation, 2011). The age of the tree from which scions are collected will determine the type of graft that will be used. For young trees, whip grafting is the best method. For older trees, the branch that will be grafted should be cut 3 feet above the ground, and the cut should be 6 to 12 inches away from the crotch. If the entire tree will be grafted with a new scion then all scaffolds should be cut back to 3 feet above the ground except for one “nurse” limb that will provide photosynthates to the new growing scions until they are established. Nurse limbs are only necessary in trees four, or more, year old and can be removed at the end of summer. Cleft and wedge (saw kerf) grafts are most commonly used in older trees in the dormant season. Bark grafting can be used during the growing season when the bark is slipping with stored, dormant budwood scions. See the General Propagation section for details on grafting and topworking techniques.
Micropropagation is increasing in popularity in the tree nursery industry because it enables production of virus and disease free certified plants and rapid propagation of plants with small amounts of plant tissue. Micropropagation of rootstock clones is conducted in vitro. Peach, and other fruit and nut rootstocks, are produced in a variety of micropropagation techniques, some of which are proprietary.
To micropropagate most fruit and nut tree varieties excise shoot apical meristems and plate them on a medium with a combination of plant growth hormones, nutrients, and vitamins that promotes shoot proliferation. Next, remove new shoots, replant to allow them proliferate, and either root tip cuttings in agar plates or greenhouse liners. If rooted on agar plates, plants will need to be planted in liners, and then hardened in a greenhouse before planting in the field in late February to early March for June budding.
Additional online resources about grafting and budding include:
Propagation (Lockwood & Ferree, Propagation, April) http://www.ent.uga.edu/peach/peachhbk/cultural/propagation.pdf
The following is an article concerning breeding trends for new cultivar development.
Trends in stone fruit cultivar development (Byrne, 2005) http://horttech.ashspublications.org/cgi/reprint/15/3/494