While the emerald ash borer has been the big tree pest to capture headlines in recent years, it certainly isn’t the only thing that has done damage to local trees. Dutch Elm Disease has ravaged elm trees in North America since the 1920s, killing huge numbers of American Elm trees. The disease was first discovered in Eastern Canada in the 40s and decimated a vast majority of the elm trees in its wake over the next several decades. Quebec has few of these majestic trees left and Toronto saw about 80% of its elm trees infected and hence destroyed. One of the few intact stands of elm trees lie in Alberta and British Columbia, due to aggressive preventive measures.
What exactly causes Dutch Elm Disease though, you might wonder. Dutch Elm disease is a fungal infection spread by the elm bark beetle. Elm bark beetles burrow into infected trees and reproduce just under the bark. When the new beetles emerge the following spring, they are covered in spores from the infected tree, which are then spread to healthy trees when they feed on them. In an attempt to stave off the fungal infection, the tree plugs up its xylem tissues. The problem with that is that water and nutrients are also spread throughout the tree via its xylem tissues, so in effect the tree starves itself, causing its eventual death.
So what do you look for, if you are concerned that your elm tree might be infected? Some of the earliest signs of infection are wilted and yellowing leaves early in the summer, long before autumn leaf drop (June and early July). As the elm becomes further infected, wilting will spread throughout the tree. While the brown leaves will stay on the tree for earlier infections, later infections will show early leaf drop. A cut cross-section of a branch will also show a brownish-streak in the vascular tissues. The sad news is that a tree will often succumb to Dutch Elm Disease within a season or two.
That is all pretty grim news for elm trees. Is there a silver lining or any hope for these beautiful trees that once covered much of North America? Pruning and burning of diseased limbs can work on a small-scale basis, but when more than one tree is involved, it can get costly. In Alberta, there are bans in place restricting pruning of elm trees between April 1st and September 30th. Pruning must be undertaken from October to March, as that is when the beetles are not in their active phase. It is thought that the elm bark beetles are attracted to freshly pruned wood, and as Alberta is still DED-free, this practice makes sense. Some chemical interventions have also shown a measure of effectiveness, such as Arbotect. In the case of Arbotect, applications need to be injected every two to three years to prevent infection. Once a tree is infected though, its presence cannot be eradicated. There have also been studies into hybrid species, as well as extensive research on disease resistant species.
If you have an elm tree and are concerned about it, feel free to contact CLC Tree Services. We have been in the tree care industry for over 25 years and have seen our fair share of disease and insect infestations. We will gladly do what we can for your elm trees to make sure that you enjoy them now and into the future.
- Prolific Minnesota Dutch Elm Disease (fauxrhino.com)
- A One Percent Chance (thesciencebulletin.wordpress.com)
- Into the Garden: Historic elms get a dose of medicine (wvgazette.com)