My mountain ash tree died last year after only having it for six years

http://gardening.yardener.com/Solving-Problems-Of-Mountain-Ash

it may be ash tree borers .

There was a beautiful mountain ash in a neighbor's yard that died last year. It was healthy for about 30 years before that. Many trees around here have gotten diseased and died, including some of mine. I did lots of research on this, and concluded that it was due to a fungal-like growth under the bark (phloem layer), stealing the tree's nutrients. You can see this fungus if you look at a twig cross-section with a 400 power microscope, which I did to confirm the fungal infection. It looks white in color. It's spores are spherical, about 50 microns in diameter, and grow out of the bark.

Yes, after some experimentation, I found a fungicide that was effective against this fungus. Actually, two - propiconazole and mancozeb. You have to spray either on the foliage every 3 to 4 weeks to keep this fungus at bay. Do NOT use these fungicides as a "ground drench", as they will instead kill off good bacteria in the soil, negatively impacting the health of the tree/shrub (I tried it).

I do the spraying myself, but you can hire a tree or lawn care company to do the spraying for you (I've done both).

Emerald Ash borer. We've lost thousands of Ash trees here in Southern Ohio!

Emerald Ash borer doesn't infect Mountain Ash trees.