That sounds a lot like the Dutch Elm Disease that killed most of the Elm trees here in Minnesota years ago. The Elms were dying off at a pretty fast pace back in the 1970s here. The only positive was if you knew where a downed elm tree was you would find so many Morel Mushrooms that we would fill several large paper grocery bags full. We didn't know any better back then. Now we use net bags so the mushroom spores have a chance to escape and grow more mushrooms rather than be contained in the paper bag.