Tree diseases in Quebec: how to recognize them and protect your property

A maple tree losing its leaves in the middle of July. An ash tree whose bark is starting to crack and fall off in patches. These are not signs to be blamed on heat or a difficult summer. Trees in Quebec face a number of serious diseases and pests, including maple tar spot, Dutch elm disease and emerald ash borer. Here’s how to recognize what’s going on and know when to act.

Key points to remember

• Tree diseases often progress silently for several years before becoming visible.
• The emerald ash borer is one of the most destructive threats to urban trees in Quebec.
• A diseased tree can pose a safety risk to your property
• Quick action by a certified arborist can save a tree or avoid costly felling.
• Prevention and regular monitoring are the best ways to protect your arboricultural heritage.

What exactly is a tree disease?

A tree disease is any condition that disrupts the normal growth or survival of a tree. It can be caused by a fungus, bacterium, virus or insect pest.

What complicates the management of these diseases is that they advance without making a sound. A tree may look perfectly normal while the disease settles under the bark or in the roots. By the time symptoms really become visible, the problem is often well established.

Among the most widespread diseases in Quebec are anthracnose, which creates spots on ash, oak and maple trees, black cherry knot, Dutch elm disease and root rot.

Emerald ash borer: the number one threat in urban areas

If you have an ash tree on your property, this is the issue to watch out for first. The emerald ash borer is an exotic insect native to Asia, whose larvae burrow under the bark, destroying sap ducts and gradually killing the tree.

The figures give an idea of the scale of the problem. In general, 99% of ash trees in a forested area have been killed six years after the start of an infestation. Of the 255,000 trees listed in the City of Montreal’s public spaces, over 50,000 are ash trees.

In Montreal and Gatineau, more than 40,000 trees were felled between 2012 and the end of 2020 due to this pest. In 2025, Quebec City plans to cut down 4,315 trees, a faster pace than the 3,250 felled the previous year.

What signs should you look out for on your trees?

The table below lists the most common symptoms and what they may indicate.

When should you really worry?

Not all visible signs on a tree require emergency intervention. Some diseases, such as pine needle rust, cause little damage to large trees and generally do not compromise their survival.

Other situations are more serious: a rapidly receding crown, fungus at the base of the trunk, an ash tree whose bark is peeling off with galleries visible underneath, large dead branches above a frequented area. An EAB-infested ash showing signs of dieback will not recover. In this case, every season lost reduces the options available.

Common mistakes to avoid

Waiting for things to sort themselves out is the most common mistake. Fungal diseases and insect infestations do not regress without intervention. The longer you delay, the less room you have for manoeuvre.

The timing of ash tree pruning is also important. We strongly advise against felling or pruning an ash tree between April 30 and October 1: a pruned tree releases a pheromone that attracts the insect and accelerates its spread in the area.

Moving infected wood should also be avoided. EAB larvae live under the bark and can survive in cut wood. The movement of logs from one area to another accounts for much of the species’ rapid spread.

What to do to protect your trees

Having any tree showing unusual signs inspected is the first step, especially if it’s an ash tree. A certified arborist can make an accurate diagnosis and propose the appropriate options depending on the tree’s actual condition, whether it’s treatment, preventive pruning or safe felling.

In the medium term, regular maintenance makes a big difference. Preventive pruning, aeration of the soil and monitoring of overall condition help to spot problems early, before they become difficult to manage.

For the long term, diversifying the species planted on your property is a useful reflex. According to the Quebec government, prevention aims to increase tree resistance and gradually reduce the extent of disturbance. The less you depend on a single species, the less damage a targeted epidemic can do to your property.

Prevention: habits that make a difference

The best maintenance practices are simple. Inspect your trees in spring and fall, avoid injuring roots and bark when landscaping, and ensure good drainage around trunks to limit root rot. Don’t leave firewood of unknown origin on your property. consult a certified arborist as soon as something seems abnormal. A preventive visit costs much less than an emergency felling.

Your trees deserve a professional look

Tree diseases often advance unnoticed, but their effects eventually affect the safety of the land, the value of the property and the immediate environment. Most problems can be solved by early intervention. A certified arborist can take stock of the situation and help you make the best decisions for your property, without pressure.

In doubt about the condition of a tree?

Our team can assess the situation and explain your options. Whether you need a diagnosis, preventive pruning or safe felling, contact us.