Slime flux

Mixed bacteria — Enterobacter, Klebsiella and others

also called Bacterial wetwood

A dark, sour-smelling ooze running down a tree trunk from a crack or a pruning wound. It looks appalling and is very rarely what kills a tree.

Bacterium Disfiguring Spread by contactSpread in water

What you'll see

A wet, dark streak running down the bark from a crack, a branch crotch or an old wound.
A sour, fermented, vinegary smell around the ooze, strongest in warm weather.
A pale gray or white crust dried onto the bark below the seep, where the liquid has evaporated.
Wasps, flies and butterflies gathering at the seep to feed on it.
Bark and any grass beneath the streak killed by the liquid, which is salty and alkaline.

Why it happens

Bacteria fermenting the sap inside the wood of the trunk, generating gas that builds pressure until the liquid is forced out through any weak point. The bacteria are common and get in through roots and wounds; the wetwood itself sits in the heartwood, which is already dead tissue, so the tree is largely unaffected by it.

Older trees, trees with old wounds or included bark in a tight fork, and species that are simply prone to it — elm, poplar, willow, maple and mulberry. The flow is heaviest in spring and summer when sap pressure is highest.

Treating it

Do nothing to the tree itself. There is no cure, and none is needed.
Hose the ooze off the bark occasionally to keep it from killing more bark below the seep.
Do not drill a drain hole into the trunk. It was standard advice for decades, it does not relieve the pressure, and it makes a permanent wound.
Do not cut into or gouge out the seeping area; the flux is inside the heartwood, not in what you can reach.
Have the tree checked if the flux is at a tight fork with included bark, which is a structural concern quite separate from the ooze.

Keeping it away

Avoid wounding trunks and major roots with mowers, strimmers and digging.
Prune correctly at the branch collar and in the dormant season.
Train out tight, included-bark forks on young trees.
Keep the tree well watered and mulched so it is not otherwise stressed.
Accept it on an old tree; a slime flux is a sign of age and injury rather than a disease taking hold.

Who gets it

Elm, poplar, willow, maple, mulberry, oak and birch above all — most often older or wounded trees.