Has anything gummy or sap-like oozed out of the wounded area of the tree?

No, nothing gummy or runny, but the problem is only on the bottom of the graft. thank you for taking the time.

If it had been gummy or oozing, I would have suspected bacterial canker.

My suggestion is that you go post your photo and question at the Fruit and Orchard Forum here at GW/Houzz. I'll link that forum below. The folks there will have an answer for you about what it is, what caused it and what to do about it, as there is nothing they don't know about fruit. They are the fruit tree experts.

Hope this helps,

Dawn

Fruit and Orchards Forum

Thank You Dawn, I shall do this.

You're welcome.