We had two mountain ash trees in our yard in the house where I grew up.
Beautiful when blossoming , beautiful berries. Being a kid, I don't know a lot about how dad took care of the yard, but I do remember problems with the trunks if these trees. I seem to remember him putting "tar" like substance on it to protect them from bugs due to damaged bark.
I think this tree has problems like this before the emerald ash borer.
If it is the beetle, there's not much you can do to stop it.