Apple canker

What you are looking at

An oval patch on the bark where the surface has died and sunk in. Around it the bark cracks into rings, one inside the next, each stepping down towards the centre, so the lesion looks like a shallow crater with terraced sides. The wood in the middle is often exposed and bare.

Along the rim the tree fights back. It lays down new tissue at the edge of the wound, which swells and distorts the branch, and every winter the fungus kills that new growth and the tree makes more. Those alternating rings are how a canker records the years.

Where the lesion has gone right round the branch, everything beyond it dies. The leaves brown in summer and hang on rather than falling, which makes a cankered tree easy to spot from a distance in July.

The fungus fruits on the dead bark twice a year and both stages are diagnostic. In summer it produces cream or white cushions the size of a pinhead. From autumn through winter it produces tiny spheres of a bright red that nothing else on an apple branch matches, clustered along the edge of the lesion.

How it gets in

Through wounds, always. The fungus cannot get through sound bark.

The largest source of wounds on an apple tree is leaf fall. Every leaf that drops leaves a scar that takes a few weeks to seal, and leaf fall in Britain coincides with the start of the wet season. Spores are washed and splashed onto the tree in rain, land on those open scars and germinate. Most of the infections a tree picks up in a year happen in October and November.

The other entry points are all things that can be managed. Pruning cuts, picking scars where a stalk tore, frost cracks, scab lesions on young shoots, woolly aphid feeding sites and damage from a strimmer or a tie all let it in.

The infection then sits quietly. A canker that started at a leaf scar in November is often not visible until the following summer, which is why the disease seems to appear from nowhere and why the tree in front of you is showing last year’s weather.

Why some orchards have it and others do not

Wet ground, more than anything else. Canker is a disease of heavy, cold, badly-drained and acid soils, and the same cultivar on free-draining loam a mile away may carry almost none. Anyone planting on clay should take that seriously at the planting stage, because it cannot be undone afterwards.

Cultivar comes second and it is a large second. ‘Cox’s Orange Pippin’ is the classic sufferer, along with ‘Spartan’, ‘James Grieve’ and ‘Worcester Pearmain’. ‘Bramley’s Seedling’, ‘Newton Wonder’ and ‘Grenadier’ carry noticeably less on the same ground.

Age works the opposite way round to how it usually does. A mature tree tolerates a good deal of canker and goes on cropping around it for decades. A young tree with a lesion on its single stem is finished, because there is nothing to cut back to.

The sprays that used to be recommended

Bordeaux mixture and other copper products were the standard advice for canker for most of the twentieth century, applied at leaf fall to protect the scars. Amateur approval for Bordeaux mixture has been withdrawn and it is no longer available to gardeners in Great Britain. A good deal of the advice still online for this disease recommends it.

That leaves the knife, and the knife is enough for a garden tree. Cut the cankers out in dry summer weather, burn the prunings, keep the tree’s other wounds to a minimum, and a cankered apple can go on cropping for a very long time.

What to do about it

Cut cankers out between June and August, on a dry day, taking at least 15 cm of clean wood below a branch lesion.

Pare lesions on a trunk back to sound bark and tissue with a sharp knife.

Burn or remove every piece cut out, and do not leave it in a heap on site.

Go over the bare tree in winter, mark the cankered spurs, and take them and the mummified fruit off.

Wipe the secateurs with disinfectant between trees.

Prune in dry weather and avoid pruning in autumn.

Grub out a young tree that has been girdled, and replant elsewhere with a different cultivar.

When planting, put apples on well-drained ground and avoid the cultivars known to suffer.

What it gets mistaken for

Coral spot makes bright salmon-pink cushions all over dead wood, usually following something that had already died back, and its colour is pink rather than the scarlet of canker’s winter fruiting bodies.

Bacterial canker attacks plums and cherries, oozes amber gum, and produces shot-holed leaves in summer.

Fireblight blackens shoots from the tip back in a matter of weeks in early summer, with the dead leaves clinging on and a shepherd’s crook at the shoot tip.

Silver leaf silvers the foliage and stains the heartwood, and it enters through winter pruning cuts on plums.

Woolly aphid covers bark in white fluff that smears between the fingers and sits over soft lumpy galls rather than dead sunken tissue.

Honey fungus kills from the base upwards and leaves white sheets under the bark at ground level with black bootlaces running through the soil.