Hey, I'd really appreciate some help with a few problems I've been having with my plants.

First of all something killed off all my elm trees within a year or two, in a kind of dutch elm like way. They died from the top down in large sections of branch at a time which turned brown and stayed on the tree. It started with the oldest tree and a month after it spread through the roots of the smaller ones. What is confusing me is that I'm pretty sure it just spread to a different species of tree whose roots were coming out of the same area. There were a few seedlings of the tree at the time and I noticed a few dying out but i thought it might be a coincidence but it's managed to kill the final tree of that species this year, I think five years after it came, but it kills quickly. I have to assume it's been spreading one tree at a time through these little understory trees all this time. I'm not sue what they are by the way but I think buckthorn. I'm worried that it's managed to survive this long if it is the same thing and that it's infected more than one species so I'd really like to know what you think it could be. Here are some pictures I think might be helpful:

An elm I saved in a container of the species that died:

All these died within a year...they're surrounded by unharmed maples, black walnuts, and oaks:


This is the other species, it's died a little more slowly but the pattern is the same:


Section of branch dies at once:


Typical remaining branch of tree is at the base but will soon die:


Any advice is appreciated, thanks!