Dutch Elm Disease

He that plants trees, loves others besides himself. Thomas Fuller

Photo by Mike Allen

Identifying Your Elm

Dutch Elm Disease can infect all types of elms in Saskatchewan. American, Siberian & Manchurian elms are all susceptible and must be treated equally. American elms are the most common shade tree found in the older neighbourhoods of Saskatoon. In fact, there are as many as 100,000 in the city. American elms are native to Saskatchewan and therefore ideally suited to our cold, harsh climate. They can live for 250 years or more in our urban setting, providing beauty and shade for generations to come.

American elms have a characteristic "umbrella" shape when seen in profile, growing to 23 metres (75 feet) in urban conditions. The canopy of mature trees will often meet across a street creating an attractive "tunnel" of branches and leaves. Its dark green leaves are oval in shape and asymmetrical (having halves that are not even in size and shape) at the base. The leaf edges are double-toothed.

Siberian elms (often called Manchurian elms) are less graceful and majestic than the American elm. A hardy, weedy, and relatively short lived species, the Siberian elm can have a variety of forms depending on location and pruning – from a shrubby hedge to a single tree. Their leaves are similar to, but smaller than, those of the American elm. Siberian elms are fast growing, typically reaching heights of 12 m (40 ft). They are unpopular with gardeners as they produce an abundance of seed and their branches break easily. They are less likely to die from DED, but can be a source of infection.

The following drawings illustrate the leaves of each tree; American elm leaves are typically larger than Siberian elm leaves.

Illustration by Paddy Tutty

Dutch Elm disease

Tree populations in Saskatchewan are subject to numerous stresses. Currently the most serious threat comes from Dutch Elm Disease (DED), a deadly disease of American elm trees.

  • Cause: a fungus under the bark of infected elms
  • Carrier: elm bark beetles that feed on, and breed in elm bark.
  • Victim: American elms, our most magnificent shade tree.
  • Symptoms: Leaves that wilt, curl up and turn brown in summer.
  • Control: Frequent pruning to remove dead wood, immediate removal of dead elms and careful disposal of prunings and firewood.

How Does Dutch Elm Disease Spread?

Elm bark beetles spread the disease along natural corridors of American elms, such as the Saskatchewan, Qu'Appelle, Souris and Frenchman river valleys. Eventually they reach a community like Prince Albert or Saskatoon. Carrying the fungus on their bodies, the beetles infect elms as they feed. DED kills the tree and the beetles are forced to find other elms. These new elms will in turn die and the cycle continues.

People spread DED by transporting infected wood. By this means DED can strike your community overnight. For this reason it is illegal to transport or store elm firewood in Saskatchewan.

How can we protect Saskatoon from Dutch Elm Disease?

Saskatoon is almost the last major city in North America that has a significant population of American Elms and no DED (until July 2015). American elms have no resistance to DED, so early detection is essential to prevent an outbreak. We are surrounded by sites that have active disease. It is only a matter of time before we have to battle DED in our city to save the 100,000 elms that provide charm and cool summery elegance to our streets.

Be aware if one of your elm trees suddenly has wilting leaves on one of its branches.

  • Flagging is a common early sign of Dutch elm disease noticeable when the weather is hot and dry.
  • Other diseases also cause wilting, reducing the overall health of the tree.
  • For images of diseased elms, go to this page from the City of Saskatoon.

What should you do, if you see an elm tree with a wilted branch?

  • Contact Saskatoon City Hall (306-975-3300) or the Ministry of Environment 1-800-567-4224.
  • The city will send a crew to remove the branch, and have it diagnosed. You do not have to do this yourself, and there is no cost to you. It is far less expensive to do this than to deal with tens of thousands of dying elms.
  • You might be preventing a catastrophic outbreak of Dutch elm disease.

Do not prune elm trees between April 1st and August 31st.

  • Elm bark beetles are most active at this time and they are attracted to the open wounds created by pruning, which are prime infection sites.
  • There may already be spores in the air.
  • During the period when pruning is prohibited, new cuts often ooze sap that can trap spores, as well as attract beetles which might be carrying DED as well.

Remove dead elm branches, and have them burned or buried.

  • The fungus can grow on dead wood, and produce air-borne spores.
  • Do not bring elm logs into the city for firewood – it is illegal!
  • Logs may have fungal spores or overwintering beetles or both.
Photo by Paddy Tutty