Posted: 11/03/2013 at 16:47
One thing I can tell you, Plums cannot be grafyed on to Crab apples. they are different , Prunus for Plum and Malus for Apple. they are both Rosaceae but not compatible for grafting.
Does sound like the tree has reverted to wild plum which is what the original tree would have been grafted on to.
The only other thing is a condition called' pocket plum' But in this the faulty plum is mishapen rather than small.