In orchards apples and pears are not grown on their own roots but are grafted onto rootstocks that control the growth of the tree. Clonal (genetically identical) rootstocks are used to promote early cropping, consistent tree size and freedom from some root pests and diseases.
Apple and pear trees are normally grown in a specialist fruit-tree nursery where the cultivar is grafted or budded onto the rootstock, and grown for another year before being bought by the fruit grower. The rootstocks are propagated vegetatively. Rooted stems are pruned and encouraged to send up many shoots, which are partly covered in sawdust to induce roots to form.
Traditional apple rootstocks
In New Zealand pipfruit were traditionally grown on large trees, on semi-vigorous clonal rootstocks planted at about 670 trees per hectare. The original apple rootstocks from the UK were found to be very sensitive to infestations of woolly apple aphid (Eriosoma lanigerum). A breeding programme in England in the 1920s produced the Merton (M) and Malling Merton (MM) apple rootstocks. They were resistant to this pest, and became the mainstay of New Zealand production for many years – particularly the rootstocks M.793 and MM.106.
Centre-leader trees and dwarf rootstocks
During the 1970s and 1980s apple growers changed from producing multi-leader vase-shaped trees to centre-leader cone-shaped trees, with one central trunk and distinct tiers of branches. These began cropping earlier than multi-leader trees, and picking and pest management were easier.
Dwarf rootstocks produce smaller trees which can be planted more intensively and are easier to manage. Growers began to use them, particularly M.9, in the 1990s. This trend continued into the 2000s, with 1,250 to 3,000 trees per hectare on dwarf rootstocks.
The rootstock M.9 is sensitive to woolly apple aphid, but the pest’s effect on the industry had already been reduced by the deliberate introduction in 1921 of the tiny wasp Aphelinus mali, one of the aphid’s enemies.
High-density apple orchards
Intensive planting with dwarf rootstocks allows earlier production, improved fruit quality and easier tree management. Much of the work can be done from the ground. Establishing an orchard is costly, but early production of high-quality fruit means that the orchard can reach break-even point in five years or less, particularly with new cultivars that fetch high prices.
Pear trees are normally grafted onto clonal quince rootstocks, which are easy to propagate vegetatively and allow the pear grower to control the size of trees and have them fruit early. However, not all pear cultivars are graft-compatible with quince, so a stem of a graft-compatible scion cultivar is grafted between the rootstock and the desired cultivar.