What is the sunken cracked bark on my apple tree branches?

Canker enters through a wound: a pruning cut, a leaf scar, a scab lesion, frost damage, woolly aphid feeding, a broken twig. Once in, it kills the bark outward from that point, and where it rings a branch, everything beyond it dies. It is worst on heavy, wet, poorly drained soils and on susceptible varieties, and a badly cankered tree in wet ground will keep producing new cankers however carefully you cut.

Safety

What you'll need

• Sharp pruning saw and secateurs
• A sharp knife for paring
• Disinfectant for tools
• Bags for infected prunings

Step by step

1. Confirm what you are looking at.An elliptical sunken area of dead bark, often with concentric ridges and flaking, cracked edges. Small white or cream pustules appear in summer and red-brown ones in winter. The shoot beyond a ringing canker dies back and its leaves wither but hang on.
2. Separate it from other bark problems.Woolly aphid produces white fluffy wax with lumpy swellings; lichen is a flat grey-green crust that harms nothing; bacterial canker on plums and cherries oozes amber gum, which apple canker does not.
3. Prune out affected small branches entirely.Cut at least 15cm below the visible damage, back into clean healthy wood, and preferably to a junction. Anything ringed by canker is dead beyond it anyway.
4. Pare cankers on larger limbs back to clean tissue.Where the branch is too important to remove, cut away all diseased bark and any stained wood with a sharp knife until you reach clean, healthy, pale tissue on all sides. Leave the wound open to dry.
5. Do not paint the wounds.Wound paints trap moisture and do not stop infection. A clean, dry, open cut is what the tree needs to compartmentalise the damage itself.
6. Disinfect tools between cuts and between trees.Canker is spread on blades. Wipe or dip between cuts, particularly when moving from an infected tree to a healthy one.
7. Remove every piece of infected prunings from the garden.Do not leave them in a pile or on a log stack — they release spores. Burn where permitted, or bag them for green waste.
8. Fix the underlying wet soil.This is what makes canker persistent. Improve drainage, avoid planting susceptible varieties in low wet ground, and where planting new trees on heavy soil, plant on a slight mound and mulch rather than digging a sump.
9. Reduce the wounds canker enters through.Control woolly aphid, manage scab, prune cleanly with sharp tools, avoid pruning in wet weather, and do not leave torn or broken branches.
10. Replace hopeless trees rather than fighting them.A young tree with canker on the main stem near the graft will not recover. Take it out, improve the ground and plant a resistant variety.

Tips

• Cox's Orange Pippin, Spartan, James Grieve and Worcester Pearmain are notably susceptible. Bramley, Newton Wonder and Lane's Prince Albert are considerably tougher.
• Autumn is a high-risk time: leaf scars are fresh wounds, and it is wet. Pruning in a dry spell in late winter is safer than pruning in a wet November.
• Canker on a young tree is much more serious than on an old one, because the branches are small enough to be ringed and killed quickly.
• Cankered wood is also a scab reservoir. Cutting it out helps both problems at once.

Common mistakes

• Cutting just to the edge of the visible canker — The fungus extends into apparently healthy tissue beyond what you can see. Cut at least 15cm past it, into clean wood.
• Leaving prunings on a log pile or in a corner — They keep producing spores for a long time. Infected wood has to leave the garden.
• Sealing pared wounds with wound paint — It seals moisture and fungus in. Open, dry wounds heal better.
• Replanting the same susceptible variety in the same wet spot — The site is what caused it. Fix drainage or choose a resistant variety, and preferably both.

Questions people ask