I'd try and relieve it of its concrete collar, trees continue to spread in the trunk throughout their life. Concrete up against it like that will cause rot as the bark reaches the hard stuff. I wouldn't worry about getting nutrients close to the tree, the feeding roots will be beyond that concrete.
If you can cut off that stub end with the hole it would be healthier for the tree. Hopefully the hole and rot won't go right through to the main trunk. Water collecting is not a good way to got.
If you've left any stub end like that in you pruning, cut them off right back to the branch they emerge from. Stub ends with no leaves on them, (leaf buds at this time of year) don't survive and that's where the rot gets in.
NB. I know nothing of growing apples but I can't see they'd be different from any other trees.