Rootstocks for Size Control in Apple Trees
Modern orchards are being developed with small (dwarf) trees at close spacing. The proper distance for setting the trees varies with the variety selected and the fertility and water-holding capacity of the soil.
A dwarfed tree is produced by grafting or budding a desired variety onto a special type of root system, commonly known as the rootstock, which restricts or dwarfs the growth of the scion variety grafted upon it. Some rootstocks preferred for dwarfing apple varieties were selected at the East Malling Research Station, Kent, England, and are generally termed East Malling (EM). (Often only an “M” is used, and East Malling is usually shortened to Malling.) Other rootstocks resulted from a cooperative breeding program between the John Innis Horticultural Institute, Merton, England, and the Malling Station; they are named Malling Merton (MM).
Four density systems are commonly used—low,108 trees per acre maximum; medium, 220 trees per acre maximum; high, 600 trees per acre maximum; and ultra-high, more than 600 trees per acre. The producers’ need for knowledge, time, and management skills increases the increase the tree density of their orchards. Also, the more dwarfing rootstock should be grown on the better soils and sites.
Many advantages have been given for close-planted trees. The small size makes them much easier to prune and pick. The use of ladders, at least tall ladders, may be eliminated. Pruning and spraying costs can be lower than those for orchards with fewer trees. Less pruning is needed on close-planted dwarf trees, and they can be pruned from the ground. Less spray material is required, and small spray equipment can be used successfully. Quality of fruit may be better with the close-planted trees. Annual yields, on a per-acre basis, may be similar after the trees come into full production, because yield is more a function of plant cover than number of plants. Although close—planted orchards cost more to establish, they come into production earlier, so that planting costs are recovered much more rapidly than with traditional plantings. This advantage is maintained throughout the life of the orchard.
Research reports indicate that some of these rootstocks, under certain conditions, are subject to various root rots. Other problems include: nontolerance of various soil types, scion/rootstock incompatibility, poor anchorage from weak root systems, suckering of some rootstocks, and lack of precocity in semivigorous stocks. The MM stocks are said to be resistant to woolly aphids, whereas the EM stocks are not. Since they were selected in a climate far different than ours (less extreme), they should not be expected to perform ideally under our many soil and climate conditions. This is not to say that East Malling stocks are “problem trees” in all areas.