That doesn't sound promising :-(. 25-30 years is an average lifespan for a Mountain Ash - sounds like yours is on the far side of average. They can live to be 50-70 years under ideal conditions (acidic, peaty soil with excellent drainage), but most of us don't have those idea conditions - especially those of us in southern Ontario and Quebec.

Probably it is old and weak and something stressed it beyond its ability to thrive (insects, rot, fungus...). Given its age, and the fact that 2/3 of it is dying, I'd say it's best to hire an arborist to come take it down and replace it with a new tree.

One of the reason Mountain Ash is so popular is because it's a fast growing tree (that also means it's a short lived tree). A new one will quickly grow to a nice height. Or you could plant something else that's slower growing but longer lived.

Sorry!

BP

Hi Scamp:

Sorry to hear your tree is not doing well. If you were (much) closer I would give you one of our seedlings. I have two or three (volunteers) growing in the yard that will have to go soon.

BP: Good info as usual. I am probably one of the lucky ones for this particular tree as my soil is acidic and well drained. Here is hoping I get another 50 - 60 years (and I am around to see it;-) from mine as it is less than a decade old.

Cheers,
MG

Here is a link that might be useful: Our website

Sorry to hear about your tree. It's like losing a friend when a tree dies like that.
Mountain Ash trees are susceptible to a fungal disease called fireblight. I have seen healthy trees suddenly die as you described. There doesn't seem to be a cure, but sometimes if only a few branches are affected you can prune them out and the tree can then survive for a long time. You have to cut back to healthy stock, and make sure to disinfect your tools. If you are able I would get an arborist to give you a diagnosis. Good Luck.