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.
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.

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.
The table below lists the most common symptoms and what they may indicate.
Observed sign | What it may indicate | What to do |
Leaves spotted black or brown | Anthracnose, tar spot | Consult an arborist |
Leaves turn yellow in summer | Emerald ash borer, root stress | Urgent inspection |
Peeling or cracked bark | Larvae infestation under the bark | Contact a professional |
Mushrooms at the base of the trunk | Root rot | Recommended safety assessment |
Dead branches at the top | Advanced decline | Don’t wait |
Black growths on branches | Black cherry nodule | Corrective sizing possible |
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.

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.
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.
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.
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.
Our team can assess the situation and explain your options. Whether you need a diagnosis, preventive pruning or safe felling, contact us.
Not necessarily, but it’s a signal not to be ignored. Premature leaf fall can indicate water stress, fungal disease or infestation. An arborist can make an accurate diagnosis and recommend the next steps, depending on the actual condition of your tree.
Look out for a receding crown, leaves turning yellow out of season, or growths at the base of the trunk. Bark can also crack and peel. A certified arboriculturist can confirm the infestation and guide you according to your municipality’s regulations.
Yes, in many cases. Some fungal diseases can be treated with targeted pruning or appropriate products. In the case of emerald ash borer, preventive treatments are available to slow infestation. It all depends on the stage of infestation and the species. A professional diagnosis is essential before taking any action.
Yes, a dead or weakened tree can detract from the attractiveness of your property and, in some cases, make you liable if a branch causes damage. A healthy, well-maintained tree contributes positively to the value of your property.
For mature trees or high-risk species such as ash, an annual inspection is recommended. For young, apparently healthy trees, a visit every two or three years may suffice, in addition to personal observation in spring and autumn.
