Bacterial Wetwood Or Slime Flux

Many mature trees, including elms, oak, tulip poplar, and maple, exhibit large light or dark vertical streaks on their trunks.
Bacterial Wetwood Or Slime Flux - Articles


Wetwood affected oak.

These vertical streaks result from a slimy liquid oozing out of cracks or wounds and running down the bark called 'slime flux'. Sometimes this liquid is very abundant and foul smelling. If an affected tree is cut down, the heartwood is darker in color than surrounding wood, thus the name 'wetwood'. Although the symptoms are unsightly, little damage is done to the tree in most situations. However, if affected trees are under severe stress from other factors such as soil compaction, wetwood bacteria can move into the sapwood and cause leaf yellowing, wilting, and a branch dieback.

Symptoms and Signs

  • Light or dark streaks on the bark originate at a crack or wound and run vertically down the trunk
  • Slimy, sometimes foul smelling liquid bubbles out of the tree and runs down the trunk
  • Heartwood deep in the interior of the tree is much darker than surrounding sapwood.

Cause and Effects

Bacteria, commonly found in soil and water, take up residence in young trees or gain entrance to older trees through wounds. The bacteria, including species of Clostridium, Bacillus, Enterobacter, Klebsiella, and Pseudomonas, grow within the tree using the sap as a nutrient source. As the sap is used, oxygen in the heartwood is depleted (creating anaerobic conditions), methane is produced, the pH of the sap is increased (pH 6 in healthy trees to pH 7 to 8 in wetwood), and a high pressure develops in the wood (60 psi in affected trees vs. 5-10 psi in wetwood-free trees). The resulting environment greatly inhibits the growth of fungi that can cause interior rots. The liquid kills grass and other herbaceous plants that it contacts at the base of the tree. The wood of affected trees has greatly reduced value as lumber because of the unsightly discoloration. Affected wood dries much more slowly than wood taken from wetwood-free trees.


  • Avoid wounding the tree.
  • Protect the tree from other stresses, especially soil compaction from vehicles or pedestrians.

Many years ago, it was thought that the pressure within the tree should be relieved. Holes were drilled into the trunks of affected trees and pipes were inserted to allow the liquid and gases to escape. This is no longer recommended because 1) affected trees generally survive well without any treatment and 2) drilling holes in the tree creates yet another place where slime oozes out.

References Used in Preparing This Fact Sheet

  • Carter, C. J. 1964. The wetwood disease of elm. Illinois Natural History Survey Circular 50. 19 pp.
  • Sinclair, W. A. and H. H. Lyon. 2005. Diseases of trees and shrubs. Cornell University Press, Ithaca, NY. 660 pp.
  • Stipes, R. J. and Campana, R. J. (eds.) 1981. Compendium of Elm Diseases. APS Press, St. Paul, MN.

Prepared by Gary W. Moorman, Professor of Plant Pathology