Dutch Elm Disease
Once upon a time, the American elm tree was the urban tree of choice for Canadian and US cities under temperate climate. It was the species of choice because of its ability to tolerate compacted soil, its rapid growth and its beautiful crown and relatively compatible with the infrastructures. The American elm was planted extensively and provided countless benefits to urban populations during the first half of the 20th century.
This king of the urban forest, however, was abruptly dethroned as Dutch elm disease first appeared in North America in the 1930s through infected elm wood from Europe. In only a few years, several cities and towns have seen almost all their elm quickly decline in health and were forced to cut down those big beautiful trees. Ever since the Dutch elm disease has been around, the bark beetles have been dispersing this disease at their will and, up until now, some beautiful specimens of elms have been spared. These beautiful specimens deserve to be protected and preserved for the long term but they should be vaccinated or they might catch the disease from one summer to the next and may die quickly.
The disease is caused by two fungal pathogens or fungi called Ophiostoma novo-ulmi or Ophiostoma ulmi that develop in the vascular tissues of infected trees and hamper their effectiveness. In addition, the immune response of the infected tree often time causes a thickening of the sap, making it even harder for tissue circulation. The combination of fungus development and the response of the tree tend to block the conductive elements, which causes rapid dieback and tree death. Fungal spores are transmitted from one elm to the other by the elm bark beetle (Hylurgopinus rufipes), a native insect and the smaller European elm bark beetle (Scolytus multistriatus); a cousin species from Europe. The adult beetles, which move by flying, lay eggs on the bark and the larvae tunnel under the bark, contaminating precious vascular tissues of the tree with the pathogenic fungus.
Early symptoms include yellowing of leaves on the tips of branches. Then, there is a browning and a wilting of leaves. In a more intrusive manner, a brownish to blackish discoloration can be observed under the bark of infected branches. Entire branches will begin to lose their leaves until the tree is completely dead.