and if no one here has the answer... try the fruit forum

ken

Some want the dwarfing characteristics or disease resistance given by a different rootstock. Others need roots that are hardy enough to handle extreme cold winters. If you want a regular tree, in a relatively mild climate, air layering would work I think. Never tried it myself though....

They type of rootstock is very, very important. It determines or influences:

1. ultimate size of your tree
2. growth rate
3. amount and complexity of maintenance
4. ease of harvest
5. productivity
6. preciosity
7. senescence
8. hardiness
9. adaptation to different soils and soil conditions

10. drought tolerance
11. how well the tree will be anchored
12. whether the tree will need permanent or temporary support (not just how well it's anchored)
13. fruit size
14. resistance to certain diseases
15. resistance to certain pests
16. rootstock and scion graft compatibility
17. tendency to sucker
18. and I'm sure I'm leaving off something.

Also, grafting is faster and more reliable. Some apple cultivars are impossible or very difficult to layer (some are much easier).

Yep, it's all about what the rootstock can contribute. By the way, interstock selection can be just as important as the rootstock.