Why Does My Cherry Tree Have Canker Oozing from the Bark?

If you have noticed sunken, discoloured patches of bark on your cherry tree with sticky, amber-coloured gum weeping from the surface, the most likely diagnosis is bacterial canker — one of the most serious diseases affecting cherry trees in cool, wet climates. Unlike gummosis from physical damage, which typically produces cleaner gum from an otherwise healthy wound, bacterial canker involves dead or dying bark tissue beneath the ooze and often leads to branch death if not addressed promptly.

What is bacterial canker?

Bacterial canker is caused by Pseudomonas syringae pv. morsprunorum, a bacterium that infects cherry trees through wounds — most commonly pruning cuts, leaf scars and frost-damaged bark. The infection typically becomes established in autumn and winter when the bacterium's activity is highest and the tree's defences are lowest. The bacterium kills the cambium layer beneath the bark, creating an expanding dead zone that cuts off water and nutrient transport above it. As the tissue dies, amber gum is produced as part of the tree's defence response.

Identifying the symptoms

Look for one or more of the following: sunken, slightly darker areas of bark on branches or the trunk; sticky amber to orange-brown gum exuding from these areas; brown, dead wood visible when the bark is cut away; wilting and die-back of leaves and shoots beyond the canker; and in some cases, brown spots on leaves with shot-hole appearance (the same bacterium can cause a leaf disease). A canker that girdles a branch completely will kill everything above it.

Pruning out infected wood

The primary treatment for bacterial canker is surgical removal of all infected tissue. Cut back affected branches to at least 10 cm beyond the last visible signs of discolouration in the wood. Use sharp, clean tools and sterilise them between every cut with a garden disinfectant or 70% methylated spirits. Do not compost prunings — bag and bin them. If the canker is on the main trunk and has girdled more than half the circumference, the tree's long-term survival is in serious question.

Copper spray as preventive treatment

Copper-based bactericides (Bordeaux mixture or copper oxychloride) applied to the tree in autumn — at leaf fall — can significantly reduce new infections by killing bacteria before they enter through leaf scars. A second application in late winter before bud burst adds further protection. This is a preventive treatment, not a cure for existing infections, but it is one of the most important tools available against this disease.

Pruning at the right time

Prune cherry trees in dry summer weather (July to August) rather than in autumn or winter. The bacterium spreads most readily in cool, wet conditions and cannot infect quickly-healing wounds made in summer. Winter pruning, while normal practice for most fruit trees, carries a high infection risk for cherry trees with bacterial canker history.