Why Is My Plum Tree Oozing Sticky Amber Gum?

Gum oozing from plum bark is one of the most common concerns raised by plum growers, and it causes considerable worry because it can look alarming — especially when the quantities are large. The important thing to understand is that gum production, called gummosis, is not a disease in itself. It is a symptom — the plum tree's response to stress, wounding, or disease. Reading the gum correctly tells you what you are really dealing with.

What gummosis actually is

Gummosis is the production and exudation of amber-coloured, sticky, water-soluble gum (primarily polysaccharides) from the bark of stone fruit trees. It is a response to cell damage of any kind — physical wounds, insect boring, frost, or bacterial infection. The gum forms as the tree attempts to seal off the damaged area, similar in concept to the blood clotting response in animals. On its own, gum is not damaging; it is what is happening beneath the gum-producing site that matters.

Stress gummosis — benign reaction to damage

Gum oozing from a point of physical injury — a pruning cut, a frost crack, a place where a stake or tie rubbed, or simply a natural growth split — and surrounded by healthy, firmly attached bark is benign stress gummosis. The gum dries to an amber resin over a few weeks and falls away. The bark around it looks normal. No treatment is needed — in fact, applying wound paint can trap moisture and delay healing. Allow it to dry naturally.

Gummosis from bacterial canker

When gum is associated with sunken, wet-looking, discoloured bark — especially on branches in the framework rather than at wound sites — bacterial canker (Pseudomonas syringae) is likely. The bark around the gum site looks brown and water-soaked, and the wood beneath is orange-brown rather than clean white. This type of gummosis requires action: prune the affected branch back to clean wood in summer, or pare the canker edge to healthy tissue on the main trunk.

Gummosis associated with insects

Shothole borers and other bark-boring insects can cause localised gum exudation at entry or exit holes. Entry holes are tiny and often visible on close inspection. The gum from insect damage is typically localised to a small point rather than spreading over a larger area.

Reducing stress gummosis

Gummosis from general stress can be reduced by maintaining good tree health: avoid waterlogging, do not wound the bark unnecessarily, prune only in summer, and feed appropriately to prevent nutrient stress. A consistently healthy tree produces less stress gummosis than one under multiple pressures.