Dutch Elm Disease (DED) is an invasive fungal disease caused by a vascular wilt fungus, Ophiostoma ulmi and/or Ophiostoma novo-ulmi.
Ophiostoma ulmi was introduced to North America in the 1930s as a result of a shipment of lumber that was carrying the fungus.
DED made it to Eastern Canada in 1940.
It was first recorded in PEI in the late 1970s, and first discovered in Charlottetown in 1996. Since then, the disease has continued to spread across PEI.
DED is an issue across Canada, except in BC and Alberta where the disease is absent.
One DED-infected tree was removed in 1998 in Wainwright, Alberta, and two were found in Lethbridge, Alberta in 2020.
These are considered isolated incidents, and eradication is considered successful.
Montreal streets were once home to over 32,000 elms. Today only a few remain, as a result of DED.
Once you can identify signs of DED, it is likely too late for the tree, as there is no known cure for DED and the tree usually dies within 3 – 5 years.
It is still important to know the signs of DED, and regularly monitor elms on your property for symptoms to prevent spread.
This is especially true if you have multiple elms on your property, or many in the general area, as these trees are at risk of infection. Symptoms of DED begin to appear in late spring.
Symptoms of DED include:
Flagging (browning/yellowing and wilting of leaves at the ends of branches in the canopy).
Dead or dying branches.
Elm bark beetle galleries under the bark have a unique appearance. They have a starburst or centipede-like shape.
Insect exit holes in the bark.
Epicormic shoots (new shoot growth arising from the tree’s trunk or branches)
Brown staining of internal wood (if a healthy elm is cut down, the wood should be a creamy beige).
Feeding by woodpeckers can indicate the presence of elm bark beetles
Presence of DED-infected elms near healthy elms (DED is transported from tree to tree through root grafts).
What it does in the ecosystem
DED affects the conductive tissues of elm trees.
Conductive tissues in trees are like human veins. They carry water, minerals, amino acids and sugars to every part of the tree, like our veins carry blood. Without the flow of these liquids, diseased trees begin to wilt and eventually die.
Smaller elms can be killed within weeks, while older trees will usually be killed within 1-2 years.
Sources of spread
Through the feeding activity of elm bark beetles who transfer disease-causing fungal spores from diseased to healthy trees.
Bark beetles burrow into dying elm trees to reproduce in the fall and remain there for the winter.
In spring, the beetles go in search of healthy elm trees to feed on, carrying Ophiostoma ulmi spores with them and furthering the spread of DED.
Pruning from tree to tree without cleaning pruning tools can facilitate spread.
Pruning infected trees during the season when elm bark beetles are active creates wounds that attract more beetles (March-October).
Storage and movement of DED-infected wood, whether for firewood or for general use.
Not removing DED-infected trees promptly.
Improper disposal of DED-infected wood.
Through the trees’ interconnected root systems (root grafts).
Root grafts and DED
Root grafting is the merging of the roots of two or more trees underground in an area. This is done by trees to communicate and share nutrients & water. Unfortunately, DED can travel between trees via these grafts.
Trees that are infected via root graft tend to die more quickly than those infected by other vectors.
If you have multiple elms near one another and see root grafting as a potential risk, you may want to consider trenching.
Trenching is the process of creating a trench to sever the root graft connections between trees.
This practice may save uninfected elms near an infected elm from transmission.
Stressors that Increase the Spread of DED
Extreme weather events such as drought, or storms which cause damage to the elm trees.
Mild winters lead to a lower rate of elm bark beetle mortality.
The presence of a high percentage of DED-infected elm trees in the area.
Outdoor storage of untreated elm wood.
Early detection and prevention of DED are key to managing its spread.
Knowing how and why the disease is spread will help make you an effective manager.
Monitor your trees for signs of DED regularly, especially in late spring and early summer.
If you believe your tree is infected, contact a professional for guidance on how to proceed.
Keeping trees healthy is the best way to prevent disease onset and pest damage.
If you place a high value on your elm tree, you may want to consider hiring an arborist to inoculate the tree with fungicide. Fungicide treatment makes the tree more resistant to DED, as a preventative measure.
These must be applied professionally, as improper application can negatively impact the tree’s health.
This is not recommended for trees infected by root graft or those with >5% infected canopy.
Once a tree is infected with DED, there is little that can be done to save the tree.
If less than 5% of the canopy appears to be affected, pruning may extend the tree’s lifespan.
Pruning is a risky choice, as any damage done to the tree has the potential to attract Elm Bark Beetles, a carrier of DED spores.
If the Elm Bark Beetles that visit your tree, came from a DED-infected Elm they can transmit the disease to your tree.
Pruning should only be done when the Bark Beetle is inactive, from November to March.
Prune the tree ten feet below the symptomatic area.
Disinfect your tools between cuts and immediately after pruning is finished. Dirty tools can spread DED to other trees if left unclean.
Again, this practice risks spreading DED and will ultimately NOT save the tree.
removal, disposal, and Preventing spread
The best way to prevent an infected tree from spreading DED is to remove the tree promptly.
If you have only one or two infected elms, immediate removal can be undertaken, as leaving the infected elms standing for even one growing season can further spread significantly.
Just like pruning tactics, larger removal efforts should be undertaken only when the elm Bark beetle is inactive, from November to March.
Proper disposal of infected elm wood is very important.
Infected elm wood should be disposed of promptly after tree removal.
Burying the wood and ensuring that it is covered with 30 cm of soil will prevent any elm bark beetles from escaping.
Elm wood can be burned, but this increases the risk of the elm bark beetles leaving the wood before being burned.
Burning may be prohibited seasonally or based on some other measured risk.
With any fire comes risks. Fire can spread quickly, especially during times of drought. It is important to assess the applicable safety risks associated with a burn before undertaking one.