Why is my plum or cherry tree oozing gum from the bark?

Telling ordinary gummosis, which is a stress response, from bacterial canker, which kills branches — and the summer cutting and vigour that give a tree its best chance.

Difficulty: intermediate
Time: 45 min
Read: 4 min

Short answer

Amber gum alone is usually gummosis: a stress reaction to drought, waterlogging or injury, not a disease. Gum with sunken dead bark, a flattened branch, dieback and shot holes in the leaves is bacterial canker. Cut affected branches out in summer, well below the damage, keep the tree vigorous, and expect to replace a badly affected young tree.

Stone fruit produces gum readily. It is a general wound response, so gum on its own tells you the tree has been stressed or injured, not what by. Bacterial canker is the serious version: a bacterium that infects through leaf scars in autumn and through wounds, kills patches of bark, and can girdle and kill a branch or a whole young tree. It also causes the small round holes in the leaves people notice separately and call shot hole.

Step by step

Look at the bark, not the gum.Healthy bark oozing gum from a point of damage is gummosis. Bacterial canker shows a flattened or sunken area of dead bark, often elongated along the branch, with gum coming from its margins and the wood beneath discoloured.
Look at the leaves in early summer.Bacterial canker produces small brown spots that dry and drop out, leaving neat round holes — shot hole. On its own it can also be caused by other things, but with cankered bark it is confirmatory.
Look at what the branch is doing.A canker that has girdled a branch kills everything beyond it: buds fail to open in spring, or leaves emerge, yellow and wither in early summer while still attached.
Rule out the alternatives.Silver leaf silvers the leaves and stains the heartwood. Brown rot mummifies fruit and produces buff pustules. Simple gummosis has healthy bark around it.
Cut out affected branches in summer, between May and July.Cut well below the canker into clean wood — check the cut face for staining and go further if you find it. Summer is when the tree seals fastest and bacterial numbers are lowest.
Destroy the prunings.Off site or burned where permitted. Do not leave them nearby.
Disinfect tools between cuts.Bacterial canker is spread readily on blades, particularly between cuts on the same tree.
Keep the tree growing strongly.Vigour is the best defence available. Feed in late winter, mulch, water in drought, keep grass off the base, and stake young trees so the stem is not rubbing or rocking.
Do not prune stone fruit in autumn or winter.Leaf scars in autumn are the main infection route, and this is exactly when the tree is least able to seal a wound. It matters as much for canker as for silver leaf.
Replace a young tree that is badly affected.A canker on the main stem of a young plum or cherry generally kills it. Replace with a variety with some resistance — Warwickshire Drooper and Marjorie's Seedling among plums, and for cherries choose a tree on Colt or Gisela rootstock from a reputable nursery.

Tips

Morello cherries and many plums are susceptible; sweet cherries vary a great deal by variety. If canker has killed one tree in a garden, it is worth choosing more resistant replacements.
Gum around a stake tie, a strimmer scar or a rubbing branch is mechanical damage and the fix is to remove the cause.
Waterlogged ground causes gummosis reliably. If gum appears every wet winter and the bark stays healthy, drainage is the problem to solve.
Bacterial canker is worse in wet, windy areas and on cold heavy soils.

Common mistakes

Treating any gum as bacterial canker — Gum is a general stress response. Panicking and cutting out healthy branches makes more wounds, which is exactly how the real disease gets in.
Cutting out cankers in winter — The tree cannot seal the wound, and autumn and winter are when infection pressure is highest. Stone fruit is cut in summer, without exception.
Painting cankers with sealant — It traps bacteria and moisture in the wound. Clean, open cuts made in summer are the correct treatment.

If it doesn't work

Amber gum from a point on otherwise healthy bark

Cause: Gummosis from drought, waterlogging or mechanical injury — Fix: Find and remove the cause — tie, strimmer damage, poor drainage — and improve watering and mulching. Not a disease.

Sunken flattened dead bark with gum at the edges

Cause: Bacterial canker — Fix: Cut the branch out in summer well below the damage, destroy it, disinfect tools, and improve the tree's vigour.

Small round holes in the leaves

Cause: Shot hole, often bacterial canker — Fix: Check the bark for cankers. If none, feed and water — shot hole alone is often a minor problem.

Buds fail to open on one branch in spring

Cause: A canker has girdled the branch — Fix: Remove the branch in late spring or summer, cutting into clean wood well below the canker.

Canker on the main trunk of a young tree

Cause: Bacterial canker established in the stem — Fix: The tree will not recover. Replace it, ideally with a more resistant variety and not in exactly the same spot.

Questions people ask

Why is my plum tree oozing sap?

Usually gummosis, a general stress response to drought, waterlogging or physical injury. It is only bacterial canker if the surrounding bark is sunken and dead and the branch is dying back.

Can bacterial canker be cured?

There is no chemical cure available to gardeners. Cutting out affected wood in summer and keeping the tree vigorous is the whole approach, and it works on some trees and not others.

Is the fruit still edible?

Yes. The disease affects bark, wood and leaves rather than the fruit.

When can I cut out a canker?

Between late spring and midsummer, on a dry day. Never in autumn or winter, when leaf scars and cold wet conditions make infection much more likely.