Dutch elm disease

Overview

• Invasive – native to Asia and Europe.
• Dutch elm disease is an invasive, highly infectious fungal disease, first confirmed in Ontario in 1946.
• There are two species of this fungus present in Ontario, and the disease occurs throughout the range of elm in Ontario.

“Invasive” refers to a species that has moved outside of its native habitat and threatens the new environment, economy or society by disrupting local ecosystems.

Host species

All native elms are susceptible: white or American elm (Ulmus americana), rock elm (Ulmus thomasii) and slippery elm (Ulmus fulva).

Characteristics and life cycle

• The fungus is spread mainly by two species of beetles: European elm bark beetle (Scolytus multistriatus) and the native elm bark beetle (Hylurgopinus rufipes).
• The disease can also spread through root grafts from infected trees to healthy trees.
• The beetles dig galleries beneath the bark of infected trees where they reproduce; in spring, the bark beetles which have become covered by fungal spores, emerge and attach to healthy trees and begin feeding.
• The fungus develops in the sap-conducting tissues under the bark and causes wilting and browning of the foliage.
• Infections usually are fatal and the tree dies within one to three years.

Symptoms and Damage

• Symptoms of an infection usually appear in mid-June, with wilting and yellowing of foliage at the end of branches.
• Leaves turn brown and curl but remain attached, eventually causing a bare area in the crown when the leaves fall.
• The following year, the leaves fail to develop or are under-sized and soon wither.
• Infections cause brown discoloration in the outer sapwood.
• Typically the disease is more widespread in the tree than external symptoms may indicate.

Control measures

Monitor trees for symptoms of infection throughout the summer. Prune and remove all dead branches and trees from the site. Don’t use elm for firewood.

An insecticide can be used in spring and fall to control bark beetles, and highly valued elm trees can be injected with a registered fungicide.