What is canker on a tree or shrub, and can I cut it out?

Canker is a shape rather than a single organism. Several fungi and bacteria produce it, and what they have in common is that they kill a patch of bark and the thin living layer beneath it, while the plant fights back by laying down new tissue around the edge. Reading that fight is what tells you whether to act.

Safety

Step by step

1. Identify what you are looking at.An area of bark that has died and sunk below the level of the healthy bark around it, often cracked, sometimes flaking to expose the wood. It may be small and roughly oval, or large and running along a branch.
2. Look for the callus roll, which is the key.A raised ridge of new tissue around the edge of the dead area. A canker completely surrounded by a strong callus roll is being walled off successfully and may remain that way for years. One with no callus, a ragged edge, and tissue that looks wet or discoloured is still spreading.
3. Check whether it has girdled the stem.The living layer just under the bark carries water up and food down, all the way round. Once a canker has widened to meet itself on the far side, everything beyond it dies. Wilting, sparse leaves or dieback beyond the canker confirms it.
4. Look for fruiting bodies.Small raised pustules on the dead bark — white, cream, orange, coral pink or black depending on the organism — release spores in wet weather. Their presence means the canker is producing inoculum for the rest of the plant and its neighbours.
5. Recognise the common named cases.Apple and pear canker produces flaky, ringed, sunken lesions and kills spurs and small branches. Bacterial canker on plums and cherries produces flattened sunken areas that ooze amber gum, with shot-holed leaves above. Coral spot appears as bright coral-pink pinhead cushions on dead and dying twigs, and moves into live wood on stressed plants. Bleeding canker on horse chestnut oozes dark liquid down the bark.
6. Cut well back into clean wood if it is a small branch you can reach.Not to the edge of the visible damage but several centimetres beyond it, cutting at a natural fork or just outside the branch collar. Do it in dry weather, because rain spreads the spores, and disinfect the blade between cuts.
7. Do not cut a canker on a trunk or a major limb.There is nowhere to cut back to, and removing bark there does more harm than the canker. This is where an arboriculturist assesses whether the tree is safe, which is a different question from whether it is healthy.
8. Bin or burn the prunings.Do not compost or chip them onto a garden bed. A cool heap preserves the fruiting bodies, and prunings left in a pile continue producing spores.
9. Address why the plant was susceptible.Canker organisms are largely opportunists that exploit stressed or wounded plants. Waterlogged or compacted ground, drought, poor planting, strimmer damage, hard pruning in the wrong season, and a plant on the wrong soil are what open the door. On apples, canker is much worse on heavy wet ground and on susceptible varieties.
10. Change what you grow if it recurs.Some apple varieties are notably canker-prone and some are resistant. On heavy wet ground, variety choice and improved drainage do more than any amount of cutting out.

Tips

• Prune plums, cherries and other stone fruit only in dry weather in summer. Winter pruning is how bacterial canker and silver leaf get in, and it is the commonest avoidable cause.
• Coral spot is a signal about the plant's health rather than an aggressive invader. If a shrub is covered in it, look at drainage, drought and damage rather than reaching for a treatment.
• There are no fungicides available to gardeners that cure a canker. Cutting out and improving conditions are the interventions that exist.
• A canker that has been present for years with a strong callus roll and no dieback beyond it is often best left entirely alone.

Common mistakes

• Cutting just beyond the visible edge — The organism is already in tissue that still looks normal, and the canker restarts at the cut.
• Pruning in the wet — Spores are dispersed by rain and enter fresh cuts. Dry weather pruning is a genuine control measure.
• Painting the wound with a sealant — Wound paints are no longer recommended. They seal moisture and spores in against the cut rather than letting it dry and callus.
• Treating a trunk canker as a DIY pruning job — It is a structural safety question. Get it assessed before anything is cut.

Questions people ask