Dutch Elm Disease

Dutch elm disease (DED) kills elm trees and has been a problem in Manitoba since 1975.  DED is caused by a fungus which blocks the tree’s vascular tissue; preventing it from taking up water and nutrients. Elm bark beetles are unwitting carriers of DED as they carry the sticky fungal spores from tree to tree.  DED can also spread underground if the roots of an infected tree are touching another tree.

 

What to Look For

DED can progress quite quickly and these 3 symptoms can be seen during the summer:

    • Sudden wilting or drooping of the leaves on one or more of the major branches in the crown of the tree. This stage is subtle and can sometimes be difficult to notice.
    • Leaves turn yellow, then brown and shrivel, but stay on the tree; referred to as “flagging”.
    • DED advances quickly and the affected branch will die and more of the tree becomes infected. You might see dead leaves falling out of season.
    • Brown or red streaks in the wood when bark is peeled away.

If a tree is infected in late summer, it can be hard to tell the difference between DED symptoms and normal fall colour-change, but you might notice smaller-than-normal leaves next spring.

DED symptoms can also be seen under the bark of infected branches. Healthy elm wood is cream-coloured, but when a tree is diseased, dark brown streaks can be seen when the bark is removed. There are two other diseases (Verticillium wilt and Dothiorella wilt) which can mimic the symptoms of DED so it may be necessary to have a branch sample tested to confirm if DED is the culprit.

 

Preventing Dutch Elm Disease

Don’t Move Firewood – Burn it Where you Buy it!

DED spores are spread by elm bark beetles. These tiny beetles breed in recently dead or dying elm wood, so it’s important to dispose of any elm branches (5 cm thick or larger) or elm firewood on your property. The City of Winnipeg recommends chipping the wood into mulch or taking the wood to Brady Landfill.

If you really want to keep your elm wood, it can be treated for safe storage. The City of Winnipeg has more details under Common Questions: Can I Store Elm wood for Firewood?

 

Basic Tree Care & Maintenance

Regular tree care (watering and pruning) can help reduce the general stress on trees and can help make them less susceptible to disease.  Removing dead branches can help prevent DED and other diseases, especially if pruning cuts are made correctly and the tree can heal over them properly.

Please note that Tanglefoot© Bands or “sticky bands” are used for cankerworm control and have not been proven to prevent DED. 

 

Observe the Elm Pruning Ban

Elm bark beetles are highly attracted to freshly cut wood in spring and early spring pruning can increase the likelihood of DED infection. For this reason, elm pruning is banned in Manitoba from April 1 to July 31. Complete elm tree removals (even infected trees) can be done at any time of year.

 

Fungicide Treatments

Elm tree injections are an option for those who wish to protect healthy, high-value elm trees on their property. There are a variety of products available; the treatments are most effective when applied to healthy trees and must be repeated every few years, depending on the product. These treatments should be done by a professional arborist. For tips on how to find an arborist, click here.

 

Planting Elms

Dutch elm disease (DED) can be managed successfully, so elms are still being planted in Manitoba. A number of new elm varieties are now available which have proven to be more resistant to DED than our native species. The University of Minnesota provides a detailed list of DED-resistant elm species that are now available. For more information about our hardiness zones, general tree selection, and planting, click here.

 

Caring for City-Owned Trees

If you wish to have boulevard or other city-owned elms treated or pruned at your own expense, you can apply for permission by filling out a property owner’s agreement and you can hire from a list of arborists recommended by the City of Winnipeg. For the agreement form and more information, click here.

 

How to Identify an Elm

American elm (Ulmus americana ) is native to Manitoba and has been planted in our communities for hundreds of years.  Siberian elm (Ulmus pumila) is native to Asia and is much less susceptible to DED. This species is similar to the American elm, but the leaves are smaller and it is known for its delicate, wispy branches and heavy seed crops. American elm can be identified by the following characteristics:

Leaf: Oval or egg-shaped with a pointed tip, with double-serrated leave margins (meaning the “teeth” have “teeth”). The base of the leaf where it connects to to the stem is asymmetrical.

Bud: Alternate arrangement, brown and pointed.

Bark: Deeply grooved in older trees, often easily broken off to observe alternating red and cream “wafers” in cross-section.

Form: The American elm, the susceptible species in Winnipeg, often has an umbrella-like canopy, arching over streets and parks to be almost as wide as the tree is tall.

 

History of Dutch Elm Disease in Manitoba

Dutch elm disease (DED) has devastated elm populations around the world. It was first recorded in Canada (in Quebec) in 1944.

DED entered Manitoba via diseased elm firewood from the United States and was found in Winnipeg in 1975. Homeowners and government officials sprang into action and the DED Management Program was initiated. The program is still ongoing today and includes DED surveillance, removing DED-infected trees, elm pruning, public education, research, and tree planting.

For decades, the annual loss of elms to DED had been kept to a level of less than 2% of the total tree population, which is considered a great success. More recently the loss rate has crept up to 3-5% (or approx. 5000 trees/year) and we are currently working with the Province of Manitoba and City of Winnipeg to reassess the DED program and revise the management practices.

It is thanks to this collaborative effort between the Provincial and Municipal governments, private industry, the academic community, and the general public that healthy, mature elms are still a part of Manitoba’s landscape and communities.

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