apple tree diseases
I grafted two apple cuttings onto a wild pear several years ago. It was successful and fruited. But this year the grafts snapped, revealing the cause: a brown/red rot in the heart of the wood. What is it?
It sounds like it is a wood rot of some kind.
Could be a bacterial disease or fungus.
A canker may have occurred at the graft site from an injury or even winter damage.
Here is a link with more information.