Why Does My Apricot Tree Have Canker on Its Branches?

Bacterial canker caused by Pseudomonas syringae is one of the most serious diseases affecting apricot trees in the UK. The bacterium infects through wounds, pruning cuts, leaf scars and frost-damaged tissue, causing sunken, weeping lesions on branches and scaffolds. If left unchecked it kills branches and can eventually destroy the whole tree. Understanding when the bacterium is most active — and when to prune to avoid it — is central to managing the disease.

How to identify bacterial canker

Bacterial canker first appears as a sunken, darkened area of bark — usually in spring when the rest of the tree is breaking dormancy. The affected wood beneath the lesion is orange-brown and dead. Gum (amber-coloured resin) often exudes from the margins of the lesion. Shoots or branches beyond the lesion fail to break into growth in spring or produce a handful of leaves that wilt and die. On leaves, the same bacterium causes small brown spots with yellow haloes that later fall out, leaving ragged holes (shot hole symptom).

Why summer pruning is essential

Pseudomonas is most active and most infective during wet autumn and winter weather. Pruning in autumn or winter creates large, fresh wounds that the bacterium infects readily. Summer pruning in dry weather (June–July) allows wounds to callus rapidly and the tree is in its most resistant phase during summer. If you must remove a large branch in winter, do it in a spell of dry settled weather and apply wound paint immediately — though this is second-best compared to summer pruning.

Removing infected wood

Cut back to at least 15 cm below any visible discolouration in the wood — the bacterium tracks ahead of the visible lesion. Use clean, sharp tools and disinfect between every cut with methylated spirits or 10% bleach solution. Burn or bag all pruned material immediately. Do not leave cut wood lying under the tree. After pruning, apply a copper-based wound paint to all cut surfaces.

Copper sprays

Copper-based fungicide-bactericide (Bordeaux mixture or copper oxychloride) applied at leaf fall (October) and again at bud burst (late February–March) reduces the bacterial population on the bark surface. These sprays do not cure existing canker lesions but reduce the likelihood of new infections establishing during the high-risk autumn and early spring periods.