Tree problems · Disease

Cankers: signs and what to do

Cankers is a tree disease, often seen on maples, oaks, ash and elm. Look for: sunken, flattened or swollen patches on a branch or trunk where the bark looks different.

General information, not a diagnosis. A certified arborist or your county extension office can confirm.

Disease
Maples
Oaks
Ash and elm
Poplars, cottonwood and aspen
Cherry and pear
Spruce and cypress

Signs to look for

Tick what you see. More ticks make it more likely, but only an on-site look can confirm.

Sunken, flattened or swollen patches on a branch or trunk where the bark looks different
Bark that cracks, peels or falls off in the patch, leaving dead wood underneath
Oozing sap, gum or dark liquid coming from the patch
A branch with wilted, yellow or brown leaves above the patch while the rest of the tree looks fine
Dead twigs or branches (dieback) that start at the tips and move toward the trunk
Small black or orange dots or tiny bumps in the dead bark
A dead patch that slowly grows larger each year

0 of 7 signs ticked. Ticks stay in this browser only.

Season

Cankers can be seen all year. Many are easiest to spot in winter when leaves are gone, and new damage often shows when the tree is stressed by drought, frost, sunburn or wounds.

How it spreads

Many canker fungi enter through wounds such as pruning cuts, mower or trimmer damage, frost cracks and storm breaks. Spores move by rain splash, wind, insects and dirty tools. Stressed trees are hit hardest.

Where it is found

Canker diseases occur on trees in every part of the US, and different fungi favor different tree species. Check your state official map; ranges change.

Official sources

What to do

1
Do not guess. Cankers have many causes, so have a certified arborist or your local extension office look at the tree.
2
Prune dead or cankered branches back to healthy wood, cutting below the patch, during dry weather.
3
Clean pruning tools with rubbing alcohol or a bleach solution between cuts so you do not carry the fungus to a new spot.
4
Do not paint over wounds or seal them. It does not cure a canker and can trap moisture.
5
Water deeply during dry spells and mulch the root zone so the tree can wall off the canker.
6
If a canker is on the main trunk and covers much of the circumference, ask a pro whether the tree is still safe to keep.

Safety first: do not climb, do not cut near power lines and do not use a chainsaw on storm-loaded wood. Call the utility for downed lines and 911 if anyone is in danger.

Prevention

Keep trees healthy with deep watering in dry weather, mulch (kept off the trunk) and the right tree in the right spot. Avoid wounding the bark with mowers and trimmers. Prune in dry weather with clean tools and make proper cuts.

When to call a pro

Call a certified arborist if a canker is on the trunk, if a limb above it looks weak or the tree leans, or if the tree is close to a house, car or play area. A trunk canker can weaken the wood, so have it checked before storm season.

Not sure who to call? Read how to hire a tree service or check is my tree dangerous?

Trees it affects

Usually seen on: Maples, Oaks, Ash and elm, Poplars, cottonwood and aspen, Cherry and pear, Spruce and cypress, Walnut, Dogwood and redbud.

See all species in our tree library, or pick a tougher tree with the tree picker.

For tree companies

How to explain cankers to a customer

Tell the customer that canker is a name for a dead patch of bark, not one single disease, and that the cause decides the fix. Explain what you can see, what you cannot tell without a lab or extension check, and when the patch is a safety issue. Offer pruning and a follow-up visit rather than a spray promise.

Send this page to the customer after your visit so they can read the signs in plain words. See also selling plant health care and the job photo checklist.

Questions

What is a canker on a tree?

A canker is a dead, sunken or swollen patch on bark. It is caused by fungi or bacteria that kill the bark and the thin growing layer under it, usually after the tree is wounded or stressed.

Will a tree with a canker die?

Not always. Many trees live for years with small cankers on branches. A canker that grows around most of a trunk or main limb can kill everything above it.

Can I cut a canker out of the trunk?

Cutting into a trunk usually makes a bigger wound and is not a good fix. Branch cankers are removed by pruning back to healthy wood. Ask a certified arborist about trunk cankers.

Should I spray a fungicide on a canker?

Sprays rarely cure cankers on established trees. Healthy care and good pruning work better. Ask your extension office before buying anything.

Can cankers spread to my other trees?

Some can, especially if tools or spores carry them to fresh wounds. Cleaning tools and avoiding wounds lowers the risk.

When should I prune cankered branches?

Remove them as soon as you see them, in dry weather. Some trees are best pruned at certain times of year, so ask a local pro.

Can Lirevon diagnose cankers on my tree?

No. Lirevon is a remote marketing team for tree service companies and does not do tree work. This page is general information, not a diagnosis. A certified arborist or your county extension office can confirm what is wrong with your tree.

How do I find a good local pro for this?

Search Google Maps for certified arborists or tree services near you, read their reviews and ask for proof of insurance. Get two or three written quotes. Our guide on how to hire a tree service lists the questions to ask.

When customers spot cankers, they call

Make sure someone answers. Missed-call text-back, lead follow-up, booking and review requests, set up for you. See our services and pricing.

See how we help Get my free audit