We have four established apple trees here that suffer terribly from canker. As a result, they are a really poor shape. They also don't produce much, since once I prune off the diseased branches every year, there's not much of last year's growth left to produce fruit in the coming year!
However, before I cut them down and plant new trees, I want to find out if I could cut off the old tree and graft a new, more canker resistant variety onto the existing root?
Any thoughts as to whether this is worth doing, and if so, how and when to go about it?