The stately American elm was once one of the most common shade trees lining streets across Ontario. Then came Dutch Elm Disease (DED), and it changed our urban forest forever. The good news: if you can spot it early, you have a far better chance of saving an elm, or at least protecting the healthy elms nearby. Here's what to look for.
What is Dutch Elm Disease?
DED is a fungal disease that clogs an elm's water-conducting vessels, essentially starving the tree of water. It spreads two main ways:
- Elm bark beetles, which carry fungal spores from infected trees to healthy ones as they feed.
- Root grafts, when the roots of nearby elms naturally fuse, the fungus can move directly from one tree to its neighbour underground.
Because of that second route, one infected elm can put every elm around it at risk.
The warning signs
Watch your elms through late spring and summer for:
- Flagging. A single branch or section of the canopy suddenly wilts, with leaves turning yellow then brown, while the rest of the tree still looks green. This is usually the first visible symptom.
- Curling, browning leaves that often stay attached to the branch rather than dropping.
- Brown streaking in the sapwood. If you peel back the bark on a recently affected branch, you may see brown or grey streaks in the wood just underneath.
Flagging in mid-summer is the classic red flag. If you see it on an elm, act quickly.
What you can do
- Don't wait. DED can move fast. Early removal of an infected limb, well below the visible symptoms, can sometimes stop it before it reaches the whole tree.
- Practice good sanitation. Dead and dying elm wood is where bark beetles breed. Promptly remove and properly dispose of infected wood, and never store elm firewood with the bark on.
- Prune at the right time. Prune elms in the dormant season (late fall to late winter). Fresh cuts in spring and early summer attract the beetles that spread the disease.
- Protect high-value elms. Healthy, important elms can sometimes be protected with professional fungicide injections, a job for a certified arborist.
- Consider resistant varieties when planting new trees; several DED-resistant elm cultivars are now available.
When to call a certified arborist
Diagnosing DED correctly, and distinguishing it from other causes of wilting, takes a trained eye. An arborist can confirm the problem, advise whether the tree can be saved, prune or remove it safely, and help protect the rest of your trees.
If you have an elm that's flagging, or you're not sure what's wrong with a tree on your property, request a free assessment and we'll take a look.