Dutch elm disease is a fatal disease of elm trees caused by the fungus, Ophiostoma novo-ulmi. This fungus grows only in elm trees and is responsible for the death of millions of elms.
Originally from Asia, the Ophiostoma novo-ulmi fungus was imported to Europe, where Dutch researchers were the first to isolate it (hence the Dutch elm disease name). It was later imported into the United States where it has spread throughout its range of native elms, and is now also widespread throughout Canada.
Which Elm Species are Vulnerable?
Most species of elms that grow in North America are susceptible to this disease, including American, Slippery (red), English, European, and Winged elms. Less susceptible species include Siberian, Chinese, and Cedar elms, but these trees have been known to contract the disease as well.
How the Disease Spreads
Dutch elm disease can spread to a healthy tree in two ways:
- Over land by bark beetles
- Underground through root grafts
The most common way the disease is spread is by elm bark beetles. Spores from the fungus in an infected tree are picked up by the bark beetles and transferred to a healthy tree when the beetles stop by to feed on its twigs.
Root grafts (a situation where an elm tree’s roots are fused underground with a nearby elm) can also transfer the disease as the fungus grows from one tree to another.
Most elm trees get the disease via the beetles, but knowing the fungus can grow into nearby elm trees is important when managing properties with multiple elm trees.
How the Fungus Kills the Tree
The fungus that causes Dutch elm disease is not a blight that lives on the leaves, nor is it a wood-decaying organism that consumes the heartwood of the tree. Instead, this fungus lives inside the tree’s water-conducting tissue (know as the xylem) of a susceptible elm.
A characteristic stain on the xylem of an elm tree infected with Dutch elm disease is caused by the tree producing gum-like substances called tyloses in an attempt to stop the spread of the disease. These tyloses actually are what cause the tree to wilt and die as they block the xylem and prevent water transport to the top of the tree. In effect, the fungus stimulates the elm to kill itself.
Dutch Elm Disease in Canada
According to reports, Dutch elm disease reached eastern Canada during the Second World War, and spread to Ontario in 1967; Manitoba in 1975; and Saskatchewan in 1981. In Toronto, 80% of the elm trees have been lost to Dutch elm disease; many more fell victim in Ottawa, Montreal and other cities during the 1970s and 1980s. Quebec City still has about 21,000 elms, thanks to a prevention program initiated in 1981.
Alberta and British Columbia are the only provinces that are currently free of Dutch elm disease. (In an isolated case, an elm tree in southeastern Alberta was found to be diseased in 1998 and was immediately destroyed.) Today, Alberta still has a large number of elms unaffected by Dutch elm disease. Many streets and parks in Edmonton and Calgary are still lined with healthy, mature trees. Aggressive measures are being taken to prevent the spread of the disease into Alberta, as well as to other parts of Canada.