Bacterial Wetwood (Slime Flux)
Enterobacter cloacae, Klebsiella pneumoniae, and other anaerobic bacteria
Overview
Bacterial wetwood, often called slime flux, is a condition of woody plants associated with mixed bacteria in the heartwood and sapwood. Elms, oaks, maples, mulberries, and many other trees ooze fermented sap from wounds, crotches, or pruning cuts. The sap may foam and smell sour as microbes ferment sugars. Wetwood itself is often more of a chronic stress and aesthetic issue than an acute killer, but the constant moisture can attract insects, stain bark, and indicate underlying trunk injury or poor compartmentalization.
Symptoms
Dark, water-soaked streaks run down the trunk from cracks, stubs, or branch crotches. A sticky, sometimes bubbling liquid seeps out and dries into a crusty stain. The flux may smell alcoholic or foul. Leaves on affected branches can show mild wilting or yellowing when pressure and toxins build, though many trees continue to leaf out normally. Horizontal cracks or frost cracks sometimes coincide with wetwood zones. Activity often peaks in warm weather when microbial gas production increases internal pressure.
Environmental Conditions
Variable
Prevention
Minimize trunk wounds from mowers, string trimmers, and construction. Prune young trees carefully to develop strong branch unions that resist splitting. Avoid topping and large stubs that never seal. Maintain soil health and moderate irrigation so trees are not chronically drought-stressed. Plant species suited to the site; chronically wet or compacted soils worsen trunk stress that can accompany wetwood symptoms.
Management
Do not install drain tubes into the trunk; older advice to drill and drain can enlarge wounds and invite decay. Keep the tree vigorous with deep watering in drought and a mulched root zone free of lawn competition. Gently wash slime from bark if it is attracting pests, without scraping live tissue. Prune only when necessary, making clean cuts at proper locations, and avoid large flush cuts. Remove hazardous limbs if decay accompanies chronic flux. Replace trees that are structurally failing rather than repeatedly wounding them.