Rootstocks are base for all commercially available apple trees. Apple cultivars or 'scions' are attached to the rootstock through a process called grafting. The picture below shows a young grafted apple tree.
You'll notice a bulge at the union between the scion and the rootstock. This is known is the graft union. It's important to keep this union above ground, otherwise the scion may root. Grafting is done at the nursery, and trees are sold to the consumer ready to plant.
There are dozens of varieties of rootstocks, but most of them are not hardy on the praires. Ottawa #3 (Ott 3) dwarfing rootstock has been used in prairie zones 2b and 3a for at least 20 years. Its susceptibility to fireblight has been behind a recent push by the U of S to find an appropriate replacement. Vineland #3 (V3) has been suggested as a possible replacement due to its resistance to fireblight. V3 has been tested in warmer areas of North America with very good results. On the prairies it has not been extensively tested. It has been observed that V3 is not as hardy as Ott 3 as a potted plant.
Dwarfing rootstocks like Ott 3 and V3 will allow tree densities between 545 to 726 trees per acre (1347 to 1793 trees per hectare) depending on row spacing, and will facilitate the most convenient cultural practices. This is based on 5 ft. (1.5 m) between trees and from 12 to 16 ft. (3.7 to 4.9 m) between rows.
Growers in zone 2a who don't want to take the risk on Ott 3 are advised to buy crabapple seedling rootstocks and plant at 12 by 16 ft. (3.7 by 4.9 m), which will be 227 trees per acre (560 trees per hectare).
New Dwarfing Rootstocks
The U of S has bred a number of new hardy dwarfing rootstocks for prairie growers. Before these rootstocks are released, they are being subjected to extensive testing. Field trials are planted and traits such as growth habit, yield and size are recorded. Rootstocks are even intentionally inoculated with fireblight bacteria to determine their level of resistance against the disease. Preliminary results show a number of potentially valuable rootstock cultivars.