Why does my peach tree have canker oozing gum?

Sunken, discoloured patches on the bark of a peach or nectarine tree — often oozing a transparent to amber sticky gum — are the hallmark of bacterial canker, caused by Pseudomonas syringae. It is one of the most widespread diseases of peach in temperate climates and can kill branches and eventually whole trees if not managed.

Symptoms of bacterial canker

In late winter and early spring, sunken brown patches appear on branches and the main scaffold. The bark in the affected area feels soft and waterlogged at first, then shrivels and darkens. Gum oozes from the edge of the lesion. In spring, buds above the canker may fail to open, or young shoots emerge and then collapse — a symptom called "blossom wilt and shoot dieback". On young trees, a canker encircling the trunk can girdle and kill the tree within a season.

The bacterium's entry points

Pseudomonas syringae enters the tree through leaf scars (where leaves detach in autumn), pruning wounds, frost cracks, and insect damage. It proliferates under cool, wet conditions and remains active on the bark surface through winter. The highest risk period for new infection is from October to March. This is why pruning in wet autumn weather is particularly damaging — fresh cuts are exposed during exactly the period when the pathogen is most active.

Cutting out cankers in summer

Remove cankers by cutting back affected branches to healthy wood in late summer — August to September — using sterilised sharp tools. Pare the margins of remaining wounds into clean, smooth ellipses to encourage rapid callus. Apply copper-based wound paint to all cut surfaces. Burn all removed material immediately. Confirm the cut has reached healthy wood by checking that the exposed inner bark is cream or white rather than brown.

Copper spray programme

Spray the whole tree with a copper-based bactericide at leaf fall in September, again in October, and once more in November. These autumn sprays protect the leaf scars as they form and reduce the bacterial load on the bark surface during the high-risk winter period. A further copper spray in late January before bud swell gives additional protection. Copper treatments do not cure existing cankers, but they substantially reduce new infections.

Supporting the tree against stress

Trees under drought stress, waterlogged, or growing on waterlogged or compacted soil are significantly more susceptible to bacterial canker. Ensure drainage is good, maintain consistent moisture through summer, and apply a balanced fertiliser in early spring. Avoid excessive nitrogen, which promotes soft growth that is more vulnerable to both the bacterium and frost.