Bacterial canker

What you are looking at

A patch of bark on a branch or the trunk goes flat, sunken or discoloured, and thick amber gum wells out along its edge and runs down the wood. Above that point the branch struggles. Buds fail to open in spring, or open and collapse, and the leaves that do appear are small and pale while the rest of the tree looks normal.

By midsummer the affected limb may die outright, with brown leaves hanging on it into the autumn.

The leaves give a second, earlier sign. Small round brown spots appear on foliage in summer, the dead tissue drops out, and the leaf is left punched with clean holes. Shot hole on a cherry or plum, with any gumming anywhere on the tree, points at this.

Gum on its own means very little

Every Prunus gums. A pruning cut, a knock from a mower, a canker, a boring insect, drought or waterlogging will all produce it, because gumming is how the whole family responds to injury of any kind. Gardeners routinely diagnose bacterial canker on gum alone and are often wrong.

What makes it this disease is gum together with dying bark and a branch failing above the point where the gum is. Pare the bark back at the edge of the patch with a sharp knife. Healthy tissue underneath is bright green and moist. Dead brown tissue running under apparently sound bark, spreading well beyond the sunken patch, is the canker.

Why the pruning month decides everything

Pseudomonas syringae lives on the leaf surfaces of healthy trees all summer doing nothing visible. Populations build through late summer, and the bacteria get into the tree in autumn and winter, through leaf scars as the leaves fall, and through any wound in the bark.

A cherry or plum pruned in December is being offered a fresh wound during the months when infection pressure is at its highest and the tree is dormant and cannot heal. Prune the same tree in June and the wound closes within weeks while the bacterial numbers are low.

That is the reason plums and cherries are pruned in summer, and it is the same reason silver leaf gives. Two serious diseases of the same family, both entering through winter wounds, both prevented by the same habit.

What to do about it

Prune only between June and August, and never in winter.

Cut affected branches out in summer, taking them at least 30 cm below the lowest sign of damage into clean wood, and disinfect the saw between cuts.

Burn or bin the prunings.

Keep the tree unstressed. Mulch, water in a dry summer, feed in spring, and keep grass off the root zone.

Check tree ties every year and keep the strimmer away from the trunk.

Take out a tree that is cankered around the trunk, because it will not recover and it is infecting anything else in the garden.

Copper sprays were once applied in autumn for this and Bordeaux mixture is no longer approved for amateur use in Great Britain, so the pruning calendar is now the whole of the prevention.

What it gets mistaken for

Silver leaf turns the leaves on an affected branch a dull metallic silver, and a cut across a dead branch more than 2.5 cm thick shows a purple-brown stain in the wood. There is no gumming and no sunken bark.

Honey fungus kills from the roots up, with a sheet of white fungal growth under the bark at ground level and black bootlaces in the soil around the base.

Blossom wilt kills the flowers and the short spur behind them in spring, leaving dead flowers attached, and it does not gum or sink the bark of a main branch.

Physical damage gums freely with no dying bark and no branch failure. Look at whether anything above the gum is struggling.

Peach leaf curl and shot hole on peaches both mark the leaves without touching the bark. Shot holes plus gumming on the branch is canker; shot holes alone usually are not.