Bacterial Wetwood and Slime Flux

Bacterial wetwood and slime flux refer to bacterial “ooze” that runs down trunks of trees.  Oozing fluids are the result of a bacterial infection known as bacterial wetwood (Figure 1). Advanced development is called slime flux, in which a foul-smelling, unattractive slime leaks from wounds in the bark or wood of infected trees. These fluids can flow at any time throughout the growing season. Wetwood and slime flux are usually unsightly but cause minimal damage to trees.

Not all leakage is cause by bacterial infections. It is common for sap to “bleed” from pruning cuts, especially in early spring with tree species such as maples and birches.

Bacterial Wetwood & Slime Flux Facts

  • Bacterial wetwood can be caused by numerous types of bacteria. These bacteria colonize heartwood and use plant sap as a source for nutrients.
  • Wetwood bacteria produce enzymes that degrade internal wood, causing slight weakening of the wood. These bacteria are not pathogenic and do not cause wood decay.
  • Initial infections occur through wounds or natural plant openings. Stressed and wounded trees are at the greatest risk for colonization by wetwood bacteria.
  • Bacterial cells and resulting methane build up cause pressure within plant tissues (approximately 60 psi).  Fluids accumulate under pressure and are forced out through the nearest available opening, usually a trunk wound or branch stub.
  • This fluid changes to a brown, slimy, sometimes bubbly ooze once on the exterior of trees (Figure 1); this is known as slime flux.
  • Bacteria, fungi, and yeasts on the outside of the tree colonize the slimy ooze, further darkening and thickening the fluid. These fluids stain tree bark (Figure 2).
  • Wetwood and slime flux liquids can be toxic to actively growing plant tissues (cambium), internal rot fungi, and even grass and herbaceous plants underneath the tree.
  • Trees such as ash, birch, elm, horse chestnut, maple, oak, poplar, and willow are most commonly infected by bacterial wetwood.

Figure 1: Slime flux flowing from a tree infected with bacterial wetwood. (Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

Figure 2: Slime flux resulting in bark staining on an American elm. (Photo: Joseph Obrien, USDA Forest Service,

Disease Management

There are no effective management strategies known for bacterial wetwood or slime flux; thus, prevention is important. Proper planting and maintenance practices may help to reduce plant stress, colonization by the bacteria, and thereby the incidence of bacterial wetwood. Proper timing of pruning cuts for rapid wound closure is also beneficial.  Avoid drilling holes or damaging trees to release pressure.  Trees can usually survive for many years while colonized by wetwood bacteria.

Additional Information

  • Department of Plant Pathology Extension Publications (Link)


By Nicole Gauthier, Plant Pathology Extension Specialist and Kim Leonberger, Plant Pathology Extension Associate

[Adapted from the Bacterial Wetwood and Slime Flux is Different form Winter Pruning Sap Flow article by John Hartman, UK (KPN)]



Posted in Landscape Trees & Shrubs
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