Most Common Reasons Trees Die (And How You Can Keep Yours Thriving)

You’d think those massive trees in your yard require little to no maintenance. In reality, trees are like any living thing: tough on the outside, a little needy on the inside. 

From hidden diseases to brutal winter injuries, a lot can go wrong (quietly). And most of the time, by the time your tree looks sick, the damage is already well underway. 

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The good news? With the right care and a sharp eye on the signs of tree decay, you can keep your trees standing tall for decades. Let’s check out the most common reasons trees decline and how to stop them from happening in your yard.

Why Do Trees Die?

Most tree deaths aren’t sudden. They’re the result of slow decline caused by a combination of factors that pile up over time. Here are the biggest culprits:

• Poor planting practices: Choosing the wrong species or planting too deep can set a tree up for failure from day one.
• Drought or overwatering: Inconsistent moisture weakens roots, especially in young or newly transplanted trees.
• Soil compaction: Heavy foot traffic or machinery can suffocate roots by squeezing out vital oxygen.
• Mechanical damage: Lawn mowers and string trimmers are repeat offenders when it comes to bark wounds.
• Nutrient deficiencies: Poor soil or lack of fertilization can lead to weak growth and increased disease risk.
• Pest infestations: Insects like borers and aphids stress trees and open the door to infection.
• Delayed winter injury: Sudden cold snaps or dry winters can cause tissue damage that shows up months later.
• Unnoticed disease: Fungal and bacterial infections often creep in quietly and go untreated until it’s too late.

Common Tree Diseases That Can Lead to Death 

1. Bronze Leaf Disease

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Bronze Leaf Disease is bad news for your poplars, especially Swedish Aspen and Tower Poplar. This fungal disease causes leaves to turn a coppery-red or bronze colour, usually starting at the edges and moving inward. By the time the colour change is noticeable, the tree may already be stressed and shedding leaves early. Left untreated, it can cause severe dieback and even tree death.

The disease spreads through spores, often carried by wind or rain splash, and thrives in moist conditions. Infected leaves and branches must be pruned and destroyed, not composted. And timing matters: Prune only during the dormant season to avoid spreading the infection.

How to prevent it

To prevent BLD, sanitize pruning tools, rake up leaf litter, and keep trees well-spaced for airflow. This disease isn’t curable, but early intervention and consistent maintenance can help infected trees recover and prevent them from spreading to others.

2. Dutch Elm Disease

Dutch Elm Disease (DED) is one of the most devastating tree diseases, and it has hit Western Canada hard. Caused by a fungus spread by elm bark beetles, it blocks water movement in the tree’s vascular system, leading to sudden wilting, yellowing, and browning of leaves, usually starting in one section and spreading quickly.

Once infected, the disease moves fast. Left untreated, it can kill a mature elm in a single season. It spreads not only by beetles but also through root grafts between neighbouring elms, making containment especially tricky in older urban areas.

How to prevent it

Never prune elms between April and September since this attracts beetles. Remove and destroy infected trees or branches promptly. For high-value trees, fungicidal injections may offer protection, but timing and professional application are critical. Cities like Winnipeg have strict DED monitoring, so if you spot symptoms, call an arborist immediately. 

Our plant disease diagnostic laboratory can also help you confirm if your tree 0is infected by DED. 

3. Oak Wilt

Oak Wilt is the silent killer of one of Canada’s most iconic trees. Caused by a fast-moving fungus (Bretziella fagacearum), this disease essentially shuts down a tree’s vascular system, preventing water from reaching the leaves. The first sign? Leaves suddenly browning from the edges inward and dropping mid-season. It can kill red oaks in a matter of weeks.

The disease spreads through root grafts (underground connections between neighbouring oaks) or via sap beetles attracted to fresh pruning wounds. That’s why we recommend never pruning oaks during the growing season; open wounds are an invitation.

Note: Oak wilt has been detected in Ontario since 2023, but there is no confirmed case in the Prairies yet. 

How to prevent it

There’s no cure once a tree is infected, which makes early detection and prevention critical. Avoid injury to oaks, space trees apart to prevent root contact, and consult a certified arborist if you spot symptoms.

4. Anthracnose

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Anthracnose is a common fungal disease that affects several tree species across Western Canada, especially maples, ashes, and oaks. It’s not always deadly, but repeated infections weaken trees over time, making them vulnerable to other pests and problems. 

Symptoms include brown or black blotches on leaves, wilting shoots, and premature leaf drop, especially during cool, wet spring weather when the fungus thrives. You’ll often see affected leaves looking tattered or falling off altogether by early summer. While a healthy tree can usually bounce back, ongoing infections can stress it into decline. 

How to prevent it 

The best treatment? Rake and destroy fallen leaves, prune infected twigs during the dormant season, and ensure your tree has good air circulation. Pesticide treatment is very effective. Avoid overhead watering, which encourages fungal spread. 

5. Cytospora Canker

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Black Knot Fungus is easy to spot and hard to forget. This fungal disease affects plum and cherry trees (both wild and ornamental) and shows up as thick, black, tar-like galls wrapped around branches. It’s not just ugly; it slowly chokes out healthy limbs and can eventually kill the tree if left untreated.

The fungus spreads through airborne spores in spring, especially during wet weather. New growth is particularly vulnerable, and infections often go unnoticed until the knots grow large and woody over time.

How to prevent it

Prune out infected branches at least 6–8 inches below the knot, ideally during late winter when the fungus is dormant. Always sanitize your tools between cuts, and never compost infected material—burn or bag it instead. 

6. Root Rot (Phytophthora)

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Root Rot is the ultimate sneak attack. Caused by water-loving fungi like Phytophthora, it strikes beneath the surface. The roots begin to decay, starving your tree of nutrients and stability. Above ground, symptoms can look vague at first: stunted growth, yellowing leaves, or dieback in the crown. 

By the time you notice the decline, the roots may already be mushy, brown, and beyond saving.

The most common cause? Poor drainage or overwatering. If your soil is clay-heavy or your yard has low spots that stay soggy, your tree could be at risk.

How to prevent it

There’s no reversing root rot once it has set in, but you can prevent it by planting trees in well-draining soil, avoiding overwatering, and using mulch carefully (never pile it against the trunk). For high-value trees or suspect areas, it’s best to call in professional arborists who can help assess root health before problems start underground.

7. Delayed Winter Injury

This one’s tricky because the damage doesn’t show up until months after the weather that caused it. Delayed Winter Injury happens when fluctuating winter temps, sudden cold snaps, or dry conditions cause cell damage in branches and buds. Come spring, your tree may fail to leaf out, show random dieback, or produce distorted growth.

Young or newly planted trees are especially vulnerable, but even mature ones can suffer if fall care was missed or the weather was unusually harsh. It’s particularly common in prairie cities like Saskatoon or Regina, where freeze-thaw cycles are unpredictable.

How to prevent it

Prevention is all about preparation: deep watering in the fall, proper mulching to insulate roots, and avoiding late-season pruning. If damage occurs, prune dead limbs in spring, feed the tree to boost recovery, and keep it well-hydrated going into the next winter.

Don’t Wait for Trouble to Take Root

If you’ve spotted any of these signs or just have a hunch something’s off, don’t ignore it. Trees often suffer in silence until the damage is done. 

At Green Drop, we offer professional tree health assessments, disease diagnosis, and year-round tree health care to keep your trees strong and thriving. 

With our team of ISA-certified arborists and deep local experience, we proudly serve Calgary, Edmonton, Red Deer, Saskatoon, Regina, and Winnipeg. 

Get your free tree care assessment.