Most cankers announce themselves the same way. A patch of bark goes flat and sunken, darker than the bark around it, sometimes cracked down the middle. A season later the patch is bigger, bark is flaking away, and the limb above it leafs out thin. That is not a surface problem. The living layer under the bark is dead there, and it is not growing back.
Canker is a description, not one disease. Several fungi and bacteria make cankers on North Texas trees, and they share one habit: they get in through damage and feed on trees already short on water. This guide covers the ones local trees get, and the line between pruning one out and taking the tree down.
What a canker actually is
Just under the bark sits the cambium, a thin living layer that builds new wood and moves water and food. Kill a patch of it and that spot stops growing while the wood around it keeps thickening. That is why cankers sink: the dead area stays flat, the living edges swell, and the bark over it dries, cracks, and falls off.
The tree fights back one way: it grows a roll of callus around the dead patch, trying to wall the infection off. Read the edge and you can tell who is winning. A thick callus roll that closes a little each year means the tree is containing it. Fresh cracking and new dead bark past the roll mean it is still moving. Our diagnosis guide puts this trunk check in its full order.
Entry is almost always a wound: mower and trimmer hits at the base, a flush pruning cut, cracks opened by sunscald, freeze splits, hail tears, and holes left by borer beetles. Well watered trees seal most wounds before anything takes hold. Stressed trees seal slowly, and slow is all a canker needs.
The cankers North Texas trees actually get
Botryosphaeria canker is the workhorse here. It hits stressed oaks, elms, and hackberries, starting with twig dieback, then sunken dark patches with tiny black fruiting dots in the dead bark. It turns aggressive the summer a tree runs dry.
Hypoxylon canker is the oak specialist: bark pops off in sheets over a tan to black fungal mat. Read that guide if your oak is shedding bark. That one rarely ends in a save.
On peach and plum, Cytospora and bacterial cankers work the trunk and scaffold limbs, often with amber gum oozing in spring. The practical answer there is often a young replacement planted while the old tree finishes.
How to read one: size and position
A canker on a small limb is a pruning problem. A canker on the trunk is a math problem. Estimate how much of the trunk the patch wraps. A quarter of the circumference on a full, vigorous tree is a watch item. Half or more is girdling: the canopy above thins and decay starts in the dead wood.
Position matters too. A canker low on the trunk or at a main fork carries the weight of everything above it, and dead wood does not carry load like live wood. Note the ooze: fresh gum or sour wet spots each spring mean the infection is active, not an old scar.
Take a dated photo with a coin in frame for scale and measure again in three months. No movement in a full growing season, with callus rolling in, means the tree is managing it.
What to do: prune limbs, leave trunks alone
Limb cankers come out with the limb. Cut back to a healthy branch union well below the dead patch, in dry weather, because rain splash spreads spores to fresh wounds. Get the wood out of the yard; infected wood stacked under the tree keeps making spores. Wipe the blade with rubbing alcohol between trees so the tool does not carry it across the yard.
Trunk cankers do not get surgery. Carving out the patch and painting it cuts the callus roll, seals in moisture, and makes the patch bigger. Our piece on wound dressing and pruning paint covers why trees seal their own wounds better. Trim only bark that is already hanging free, and stay out of the living edge.
What actually helps the tree
Every canker program that works is a stress program. Water deeply through dry spells; the method is in our drought watering guide. Lay a flat mulch ring two to four inches deep, pulled back off the trunk. That ring also keeps mowers and trimmers off the base, which removes the most common wound in a suburban yard.
Skip the extras. Fertilizer does not kill a fungus, and fungicide sprays do not reach cambium under bark. If a pitch promises to cure a trunk canker with injections or drenches, ask what it does to the dead patch itself. The dead layer stays dead either way.
When a canker means removal
Call it when the canker girdles more than half the trunk, sits on a main leader over a roof or driveway, or the wood behind loose bark sounds hollow to a mallet tap. A third of the canopy dead on top of an active trunk canker says the same thing. Get a risk assessment before a storm makes the call for you.
Plenty of cankered trees are not there yet. A walled-off patch on a full-canopied oak can ride for years with watering and a yearly photo check. Measure, do not guess: trees containing a canker keep their canopy. When the count turns, plan the removal in a calm month.
The bottom line
A canker is dead cambium with a cause behind it, usually a wound on a thirsty tree. Prune limb cankers out clean, leave trunk callus alone, water well, and measure the patch twice a year. If it wraps the trunk or the canopy goes with it, believe the tree.
Common Questions
What is a canker on a tree?
A canker is a patch where the bark and the living cambium under it have died, usually after a fungus or bacterium got in through a wound or a pruning cut. The patch sinks, the bark cracks or falls away, and the tree builds a roll of callus around the edge to wall it off. If the callus keeps up, the canker stalls. If the infection spreads faster, it grows.
Can a tree recover from a canker?
A small canker on a limb, yes: prune the limb off well below the dead patch and a healthy tree is done with it. A trunk canker is harder. Vigorous trees sometimes wall one off and carry it for years, but the dead wood never comes back, and a canker that keeps expanding rarely reverses. Fixing the stress behind it, usually drought, is what gives the callus a chance.
Should I cut a canker out or spray it?
Do not carve at a trunk canker with a knife. Cutting into the callus roll breaks the one barrier the tree has built, and home surgery usually makes the patch bigger. Sprays and paints do not cure dead cambium either. Limb cankers get pruned out in dry weather. Trunk cankers get watched, measured, and supported with water and mulch.
When does a canker mean the tree has to come down?
When it girdles the trunk. A canker wrapping more than half the trunk cuts off water and food to everything above it. Removal is also the call when it sits on a main leader over a roof, the wood behind the bark sounds hollow, or a third of the canopy is already dead.