I'd certainly try it if the cost wasn't prohibitive; mature American elms are such beautiful trees! You should be aware, though, that vaccination seems to have achieved (at best) mixed results -- at least one American study found that it had no real benefit in preventing the transmission of DED in trees in its study group.

If you do lose the tree, I suggest that you replace it with a hackberry. It's something of a lookalike that has edible fruit.

Thanks. It's so expensive to vaccinate every other year, when it's not a 100% bet! And it's irreplaceable. Giant, gorgeous tree - my tree-loving relatives are guessing it's 125+ years old?!

You might think about contacting the U of T gardening staff to find out how they've managed to keep their beauties alive. It's been a few years since I've lived in the Toronto area, but I enjoyed walking on the grounds of the downtown campus and admiring the mature elm trees scattered around it. I can't remember seeing any other areas of the city that had such large American elms. I find myself saddened by the fact that Alberta has become something of a refuge for threatened eastern species -- our urban forests here now have greater numbers of threatened species (elms, ash, butternut) than you can find in their native provinces!