We are having the same problem, & the whole tree will probably have to be cut
down in a year or two. Mountain ash is not particularly long lived. I don't
think trying to grow it from a sucker or cutting would have helped, because it
appears to be dying from an opportunistic fungus. I would not plant another
mountain ash in the same location.
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
Robert Lowell (1917-1977)