Canker on a tree trunk or branch: what it is, what to cut

What a sunken, cracked patch of dead bark means, which cankers kill chestnuts, butternuts, walnuts and stressed oaks, and when to prune or plant another tree.

A canker is a patch of bark killed by a fungus or a bacterium, which often gets in through a wound. The bark sinks, darkens or splits from the living bark around it, and on an oak with Hypoxylon canker it falls away. Most canker fungi need a tree already stressed by drought, a wound or a poor site, though chestnut blight and butternut canker kill vigorous trees too. No spray cures a canker. A cankered branch is cut out in dry weather, and a canker that circles a trunk kills everything above it.

An American beech trunk blotched with dark dead bark and salmon-orange crusts of fungus, old carved marks above — Beech bark disease on an American beech on June 25, 2023. Where a canker kills the bark, the inner bark turns black as it dies and splits from the healthy bark. Photo: Blue Whale Trees

What a canker does to a branch or trunk

Drought, transplanting, winter injury, flooding, packed soil, wounds, borers and root rot all open a tree to canker fungi. Under the dead bark the fungus damages or kills the cambium, the thin layer that makes wood, and blocks the flow of water and nutrients. Its fruiting bodies, black or colored bumps the size of a pinhead, spread spores in wet weather. A vigorous tree forms callus around a canker and may seal it off.

The newest leaves on a cankered branch usually show it first, small and pale, yellow or brown. A canker can take months or years to circle a large branch or a trunk, killing everything beyond it. Meanwhile the cankered branch breaks more easily in a storm.

A canker or something else

Four other marks on bark get taken for a canker:

Frost crack. A lengthwise split from a sudden deep cold that seals each summer and often reopens, building a raised rib, as frost cracks and sunscald explains.
Sunscald. Dead bark on the south or southwest side of a young, thin-barked trunk, killed by winter sun and a fast freeze.
Burl. A hard, rounded swelling with living bark over it, which burls on trees tells from a canker.
Lichen. A gray-green crust or leafy growth on living bark that takes nothing from the tree, as lichen and moss on bark explains.

A canker is bark dead in place, sunken, cracked, swollen or discolored, though some are flat and easy to miss on rough, dark bark. Frost cracks and sunscald can themselves become cankers, since canker fungi move into bark hurt that way.

A tree trunk on a lawn with a long vertical seam down its bark, closed by rolled ridges of new wood — An old frost crack, the wound healing, on October 26, 2026. Canker fungi usually enter a tree through an injury, so a frost crack can become a canker. Photo: Blue Whale Trees

The cankers that matter on native trees

Chestnut blight, butternut canker and thousand cankers disease are the killers here, and the others mostly take weakened trees. Beech bark disease, in which a scale insect opens the bark of American beech to canker fungi, is covered in beech leaf disease, and bleeding canker in a tree dripping sap or foam.

Chestnut blight

Chestnut blight, a fungus from Asia, enters through bark wounds and girdles the stem, which dies above the canker. The roots live on and sprout, and the blight usually kills the sprouts before they bear much. Wild American chestnut has no resistance, while Chinese chestnut walls its cankers off and grows on, as chinquapin or chestnut sets out.

Butternut canker

Butternut canker kills butternut trees of every age, and it now reaches the whole range. Young cankers are elongated, sunken patches that commonly start at leaf scars and buds, often with an inky black center and a whitish edge. Older ones are often ringed by layer after layer of callus, and the cankers join until they girdle the tree. Rain splashes the spores, and nothing practical prevents an infection.

Gray butternut trunk with a long, sunken canker of blackened bark, ridges of callus rolled along its edges, snow behind — Butternut canker on a butternut trunk in Vermont on March 8, 2023. Cankers ringed by callus join until they girdle the tree, and nothing practical prevents the disease. Photo: Bruce Cook, CC0 (source)

Thousand cankers disease

The walnut twig beetle carries a fungus into the limbs of black walnut, and the small cankers around its tunnels join to girdle twigs and branches, with little to see outside. The leaves yellow and the upper crown thins first. In the Rocky Mountain states black walnuts have died within three years of the first signs, and in the East the disease has spread from Tennessee in 2010 to six more states by 2025.

Hypoxylon canker on oaks

The Hypoxylon fungus may live on any oak and spreads on the wind. It infects only oaks already injured or stressed, most often by drought, heat, wounds or chemicals. Its common hosts include post oak, blackjack oak and southern red oak. Leaves yellow and wilt and branches die. Then the bark sloughs off to bare the fungus’s crust, dusty brown, silver, white or black by its age. By then the tree is decaying, and no treatment saves it:

“There is no treatment for hypoxylon. The best option an owner has at their disposal is to always keep trees healthy.” (Mississippi State University Extension, Caterpillar, fungus affects drought-stressed trees)

Take an infected oak down, since its spores can infect the oaks around it. Water the others through droughts.

Silver-gray fungal crust speckled with black where the bark has fallen away, rough bark edges above and below it — Hypoxylon canker in Missouri on February 27, 2016, the bark fallen away to bare the fungus's crust. It infects only oaks already injured or stressed. Photo: weed lady (Sylvia), Mushroom Observer, CC BY-SA 3.0 (source)

Nectria and Cytospora cankers

Nectria canker invades hardwoods damaged by freezing, hail, animals or insects. The bark sinks slightly around the wound, and in spring small salmon, pink or cream cushions of the fungus form over it. Sweet birch gets it more readily than most trees, and it can scar the trunk of yellow birch. A vigorous tree usually walls it off within a year. Cytospora canker girdles the branches of spruces stressed by drought or winter injury, coating each canker in bluish-white resin. It rarely kills a spruce but can badly deform one.

What pruning and care can and cannot do

No chemical cures a canker, so cut out what can be cut:

Cut a cankered branch back to a branch collar several inches below the canker, leaving no stub.
Prune in dry weather, never on wet bark or in a wet fall.
Sterilize the tools between cuts.
Expect to prune a spruce with Cytospora canker two seasons running.

A canker on the trunk is another matter:

“Once a trunk canker develops, the tree may begin to seal off the area by forming a callus around the canker. Avoid cutting into such cankers because it may renew fungal activity and increase damage.” (The Morton Arboretum, Canker Diseases)

An old wound on a trunk where a branch died, its decayed center ringed by a thick rolled lip of woundwood — Woundwood, or callus, sealing off an old dead branch on June 29, 2024. A tree may seal off a trunk canker the same way, by forming a callus around it. Photo: Blue Whale Trees

Water through droughts, as drought and an established tree explains, and keep from wounding the trunk, packing the soil or drifting herbicide over the roots. Since canker fungi need a weakened host, a tree suited to its ground is the best defense. The planting-site check grades a species against your zone, sun and soil moisture.

When to plant something else

A tree girdled at the trunk is dead above the canker, and where a lethal canker takes a species everywhere, plant it with that in mind. The blight is present nearly everywhere in the American chestnut’s range, so plant one as a living link to the old forest. For nuts, Chinese chestnut is the reliable chestnut in the East, and Allegheny chinquapin keeps bearing as its root crown resprouts after the stems canker. Both are among our chestnuts and chinquapins.

A young American chestnut stem with sunken, cracked bands of dead bark where blight cankers circle it — Chestnut blight cankers on a young American chestnut on May 23, 2024. A canker can take months or years to grow large enough to girdle a big branch or a trunk. Photo: Blue Whale Trees

Sources disagree on butternut: the Morton Arboretum does not recommend planting it anywhere in its native range, while Purdue’s conservation guide counts new plantings among the ways to keep it. Where thousand cankers disease is present, planting more black walnut is not recommended. Replanting where a tree came down is the subject of planting where a tree was removed.

A lab names the fungus from a sample that takes in the canker’s edge, where dark dead wood meets healthy green wood. A county extension office can make the diagnosis or send the sample on, and the National Plant Diagnostic Network’s Laboratory Directory lists the labs by state. Blue Whale Trees takes questions about a tree you already have on its support page. Photos of the whole tree and a close-up of the canker can follow by email once we reply.

Common questions

What does a canker look like on a tree?

A patch of dead bark, sunken, cracked, swollen or darkened, often oval and ringed by callus. Fungal cankers carry pinhead fruiting bodies, and the twigs beyond one may wilt and die.

How do you treat a canker on a tree?

No spray cures one: cut a cankered branch off several inches below the canker in dry weather, sterilizing the tools between cuts. Leave a trunk canker alone and water the tree through droughts.

Can a tree recover from a canker?

Often, when the canker is on a branch and a vigorous tree seals it off with callus. A canker that circles the trunk kills everything above it, as chestnut blight does to American chestnut stems.

Is Hypoxylon canker contagious to other oaks?

Its spores ride the wind and rain, but they infect only oaks already injured or stressed by drought, heat, wounds or chemicals. Take an infected oak down and water the others through droughts.

Canker Diseases: The Morton Arboretum: canker fungi and bacteria entering through wounds, weak organisms that need a weakened host, the stresses that open the way, the sunken bark that splits from the living bark, pinhead fruiting bodies and wet-weather spores, the leaves that show it first, months or years to girdle, branches weakened for storms, the cuts several inches below at the branch collar, no pruning on wet bark, tools sterilized between cuts, the quotation on trunk cankers, no chemical treatment, and trees suited to the site.
Canker Sampling of Trees & Woody Ornamentals (PDF): University of Kentucky Cooperative Extension, PPFS-OR-W-27 (Kim Leonberger, Stacy Borden and Nicole Gauthier, 2019): the cambium damaged or killed and water and nutrients blocked, cankers cracked, sunken, swollen or discolored and some flat and easy to miss, samples taken at the margin where dark wood meets healthy green wood, and the county extension office that diagnoses or sends a sample on.
Nectria Canker: Penn State Extension (Gary W. Moorman, updated July 7, 2025): Nectria in wood damaged by freezing, hail, animals and insects, slightly sunken bark around the wound, a canker hard to see on rough, dark bark, the salmon, pink or cream cushions of spring, vigorous trees that wall it off within a year, and pruning in dry weather and not in a wet fall.
Hypoxylon Canker on Oaks: Oklahoma State University Extension: blackjack, post and southern red oak among the common hosts, stress from drought, heat, wounds or chemical injury, yellowing leaves and dying branches, bark that sloughs off to bare the stroma in its colors, removal to spare other trees, and spores spread by rain and wind.
Caterpillar, fungus affects drought-stressed trees: Mississippi State University Extension (July 12, 2024): Brady Self on Hypoxylon canker, which may live on any oak, spreads on the wind and infects only injured or stressed trees, a tree already decaying by the time the bark sloughs, the quotation, and watering and avoiding soil compaction, injury and herbicides.
Conservation and Management of Butternut Trees (PDF): Purdue Extension, FNR-421-W (Lenny Farlee, Keith Woeste, Michael Ostry, James McKenna and Sally Weeks): young cankers at leaf scars and buds with inky black centers and whitish margins, older ones bordered by callus, cankers that join and girdle the tree, rain-splashed spores, no practical prevention, a disease fatal even to vigorous trees, and new plantings among the ways to conserve butternut.
Butternut: The Morton Arboretum: butternut not recommended for planting anywhere in its native range, since butternut canker is found throughout it.
Thousand Cankers Disease of Walnut FAQ: Colorado State University Extension (Whitney Cranshaw and Ned Tisserat): the walnut twig beetle and its fungus, cankers that join to girdle twigs and branches and show little outside, yellowing leaves and a thinning upper crown, deaths within three years in the Rocky Mountain states, the eastern finds from Tennessee in 2010 to six more states by 2025, and no further planting of black walnut where the disease is present.
Cytospora canker: University of Minnesota Extension (Michelle Grabowski and Cynthia Ash Kanner): Cytospora canker on spruces stressed by drought or winter injury, branches girdled over several years, bluish-white resin over the canker, trees deformed but rarely killed, and two seasons of pruning.
Laboratory Directory: National Plant Diagnostic Network: the directory of the network’s plant diagnostic labs, searched by state.

Cankers on a trunk or branch

What a canker does to a branch or trunk

A canker or something else