The majestic street overarching mature elms of our western cities is something I hope we don't lose. It has happened in most eastern cities. When the new aggressive pathogen that a native gene pool has no idea how to deal with shows up, the results are frightening. Such is the case with DED, Dutch elm disease.
Our only defense is a model of mediaeval defense, a guarded fortress.That's what our western cities have become to DED, and a very good thing it is.
From the Maritimes through Quebec and on west to the Manitoba border was solid native elm country. They grew everywhere, in cities of course, but along all water courses and in any uncut forest remnant. The beetle didn't have to work at all, just fly to the next tree and begin feeding.
The island fortresses of the western cities stand a pretty good chance of keeping DED at bay. Through public awareness and a strict removal of the disease when identified, Winnipeg has kept their losses to a few percentage points each year. Work on resistant cultivars continues. My hope is that everyone learns to identity an elm and to be aware what a flagging branch means during the growth season. In a fully healthy tree a branch for no apparent reason turns yellow quickly and begins to die; that is a flagging branch, and a call to the city should be made that day.
Types of elms:
There are only two species that are viable in zone 3, the white or American elm and the Siberian elm. The American elm is the giant reaching from the sides of the street to meet high above, creating a beautiful tunnel. This tree is too big for residential use. The Siberian is not a good choice either. Weak wooded, very large, its one strong point is its low susceptibility to DED.
This leaves the Brandon elm, a clone from the American elm, a good tree, still large; if planted along the fence, you are asking your neighbors to have a large tree also.
Other problems: if you see an elm that has a black cast to the bark, you are looking at a tree infested with elm scale. As the scale feeds, it drips sap. A fungus, sooty mold, like a bread mold, feeds on the spilt sap and as the mold grows the bark of the tree turns black. The mold is harmless, but the scale can be quite dangerous. Old struggling, crowded and drought-stressed trees have been outright killed. Look for white/grey 1/4 inch spots on the bottom of branches. Any seriously affected tree is a candidate for some chemical assistance, and a rethinking of the amount of water the tree has been receiving -- not enough!
Woolly elm aphids, of which there are two species, are quite harmless. In mid season their activities either cause leaf curl or the formation of rosettes or leaf clusters.
Leaf miners can sometimes affect elms, but they never do any harm to a healthy well watered tree.
Architecturally, elms can be problematic; their multi-trunked form, usually all breaking up at one place is a study in included bark crotches. Most elms deal with this very well, but sometimes one or more of the large sub trunks will split off. Prevention is the only cure. Inspect closely the form and interrelationship of the major trunks or sub-trunks of your elm. If one is loose and a gap exists between the trunks where they should fit tightly together, don't ignore this! Any trunk that is losing its strong hold with other trunks and moving independently is a recipe for disaster. This is a good time to bring in a tree surgeon, someone accustomed to repairing trees with bolts and cables. There is quite an art to this work and I do not recommend most homeowners tackle this alone. Much better to have years of experience on the tree's side for this problem. A worst case scenario is the domino effect, meaning once a tight multi-trunked union is weakened or loses a trunk, as the total holding power is reduced you may lose another trunk or the whole tree.