Tree Diseases in Ottawa: Signs Your Tree Is Sick and What to Do

A diseased tree usually shows it in the leaves, bark or crown well before it fails: leaves that are spotted, wilted or too sparse, bark that is cracked, oozing or peeling, fungus growing on the trunk or roots, and dead branches scattered through the crown. In Ottawa, the cause can be a fungus, an insect, or plain stress from drought, road salt or construction, so the first step is working out which. A tree disease evaluation by an arborist tells you whether the tree can be treated, managed or should come down.

Signs a tree may be diseased

No single sign proves disease, but these are the ones worth acting on:

• Leaves that wilt, yellow or drop early in summer while neighbouring trees look fine.
• Spots, blotches or a powdery coating on leaves. Many of these are cosmetic, but heavy repeated infection weakens a tree over the years.
• Dead branches that appear one after another, especially if they start at the top or the tips.
• Cankers: sunken, cracked or swollen patches on the bark, sometimes weeping a dark liquid.
• Fungus on the trunk, at the base or on the roots, such as shelf-shaped brackets or clusters of mushrooms. Fungus often means decay is already happening inside the wood.
• Bark that falls away in sheets, leaving bare wood.
• Hollow or soft spots in the trunk, or sawdust-like material at the base, which can point to insects.

Common problems in Ottawa trees

Ontario trees face a mix of diseases and pests. Some you may come across:

• Dutch elm disease. A fungal disease spread by elm bark beetles that blocks the tree's water-carrying tissue. A typical sign is one branch whose leaves wilt and turn yellow or brown in summer, called flagging, followed by more branches.
• Tar spot on maples. Black, raised spots on the leaves in late summer. It looks alarming, but it mostly affects the leaves and is usually cosmetic for a healthy tree.
• Anthracnose. A group of leaf diseases that cause brown or tan blotches and early leaf drop, most common after cool, wet springs. A healthy tree usually recovers.
• Powdery mildew. A white, dusty coating on leaves, often late in summer. Generally a cosmetic issue.
• Canker diseases. Infections that kill patches of bark and can girdle a branch or trunk. The branch beyond the canker dies.
• Root and butt rot. Decay fungi that attack the roots and base of the trunk. The crown may look thin, but the real danger is a tree that becomes unstable.

Emerald ash borer is not a disease. It is an insect that tunnels under the bark and kills ash trees. Thinning crowns, D-shaped exit holes and woodpecker damage are the clues. The signs overlap with disease, which is one reason an arborist should look.

Disease, insects or stress?

Many "diseases" are really stress. A tree can show yellow leaves and dieback from:

• Drought or poor watering, particularly in the first few years after planting.
• Road salt on trees beside streets and driveways, which burns leaf edges and damages roots.
• Soil compaction or grade changes, such as driveways, patios, trenching or building work near the roots.
• Planting too deep, or mulch piled against the trunk.
• Mower and trimmer wounds at the base.

Treating a stress problem as a disease wastes time, and treating a disease as stress lets it spread. That is the value of having the tree looked at in person.

What a tree disease evaluation involves

An arborist looks at the whole tree and its setting, not just a leaf. Typically that includes:

• Checking the crown for dieback, sparse foliage and dead branches.
• Examining the trunk and root flare for cankers, cavities, wounds and fungus.
• Looking at the soil, drainage, nearby construction and anything else stressing the roots.
• Taking samples when needed to identify the cause.
• Deciding what the tree needs: monitoring, pruning out affected wood, improving growing conditions, treatment where one exists, or removal.

The outcome often involves a safety question as well as a health one. A diseased tree that is not a risk can be managed. One that is structurally compromised near a house, driveway or play area needs a different answer.

What you can do yourself

• Take photos of the leaves, bark and any fungus, and note when you first saw it.
• Pick up and bag diseased leaves in fall instead of composting them, which reduces some leaf diseases.
• Water deeply in dry spells, especially young trees, and keep mulch a hand's width away from the trunk.
• Avoid pruning heavily in response to a problem you do not understand. Removing the wrong wood can stress the tree more.
• Do not move firewood from a tree that died of disease or insects. Spreading wood is one way pests travel.

When a diseased tree needs to come down

Removal is the right call when the tree is dead or mostly dead, when decay has made it structurally unsafe, or when the damage cannot be reversed and it threatens a building or a target below. In Ottawa, many trees are protected under the City's Tree Protection By-law, and the rules differ for dead trees, ash trees killed by emerald ash borer, and healthy ones. Our guide to Ottawa tree removal permits explains what applies, and our post on arborist reports and Tree Information Reports covers what the City asks for when a permit is needed.

Frequently asked questions

How do I know if my tree is dying or just stressed?

Look at how much of the crown is affected and how fast it is changing. A few yellowed leaves after a dry spell is usually stress. Large dead limbs, bark falling away or fungus on the trunk point to something more serious. When in doubt, have an arborist look.

Can a diseased tree be saved?

Sometimes. Many leaf diseases only need better care. Some conditions can be managed by pruning out the affected wood and fixing the growing conditions. Others, such as advanced decay or Dutch elm disease that has reached the trunk, cannot be reversed.

Is fungus on a tree always a bad sign?

Fungus growing out of the trunk or at the base usually means wood decay is under way, although how serious it is depends on the tree. Fungus on the ground nearby may be harmless. Have a tree with fruiting bodies on the trunk looked at.

Can a disease spread to other trees?

Some do. Dutch elm disease can pass between elms through connected roots and by beetles, and fungal decay can affect trees in the same area. Others, like most leaf diseases, are specific to one kind of tree.

Do I need a permit to remove a diseased tree in Ottawa?

It depends on the tree. The City's Tree Protection By-law covers distinctive trees and has exemptions, including for trees confirmed dead and ash killed by emerald ash borer. Check the permit guide before you cut.

Worried about a tree on your property?

Tree King can assess your trees and tell you what they need. Need an arborist in Ottawa? Call 613-627-2017 or get a free quote.

Sources

• Tree Protection By-law No. 2020-340 (City of Ottawa)
• Canadian Food Inspection Agency, emerald ash borer information
• Ontario Ministry of Natural Resources and Forestry, forest health information on Dutch elm disease