Why Does My Plum Tree Have Canker Oozing Gum?

Sunken, wet-looking patches on plum bark that weep amber or brownish gum are the characteristic signs of bacterial canker — one of the most significant diseases of plum, cherry and other Prunus species in temperate climates. The bacteria responsible, Pseudomonas syringae pv. morsprunorum, are widespread in garden environments and become most active in cool, wet autumn and winter conditions. Understanding how the disease works is essential for managing it effectively.

How bacterial canker spreads and infects

Pseudomonas syringae bacteria live on leaf surfaces without causing disease during summer. In autumn, as leaves fall, they infect the leaf scars left behind on the bark — these are the primary entry points. In wet autumns the infection can be extensive. By winter the bacteria are established in the bark, where they kill patches of tissue and cause the characteristic sunken, water-soaked cankers that ooze amber gum. Pruning wounds made in autumn or winter are also major infection sites.

Identifying bacterial canker on plum

Cankers appear as sunken, oval or elongated patches on branches and the main trunk. Infected bark turns brown and the cambium beneath is discoloured. Amber or reddish-brown gum oozes from the margins of the canker, distinguishing it from dry, cracking bark. On shoots, bacterial canker causes a "blossom wilt and spur dieback" pattern in spring — shoot tips die back and flower clusters collapse before or just as they open. Small, round, brown spots with yellow halos on leaves (shot hole) are a leaf symptom of the same pathogen.

Copper fungicide spray programme

The most effective control is a three-spray copper-based fungicide programme: first application in late August, second in September, third in October. These sprays prevent bacteria from entering the leaf fall scars as leaves drop. A further spray in late January or early February provides additional protection to pruning wounds. Copper hydroxide or copper oxychloride products are available for use on organic and conventional gardens.

Pruning out cankers

On branches, cut back to clean, healthy wood at least 10 cm beyond the edge of visible discolouration, cutting in summer when bacterial activity is lowest and wounds heal quickly. On the main trunk, pare away the dead bark back to the margin where healthy bark meets the canker with a clean knife, and allow the wound to dry and callus in summer air. Destroy all pruned material.

Tree nutrition and resistance

Trees stressed by waterlogging, poor drainage, over-cropping or nutrient deficiency are more susceptible to bacterial canker. Maintaining good tree health through balanced feeding and appropriate drainage is a long-term resistance measure.