Bacterial Wetwood (Slime Flux): Causes, Effects, and Management

Understanding Bacterial Wetwood and Slime Flux in Mature Trees

Many mature trees (elms, oaks, tulip poplar, maple) exhibit large light/dark vertical trunk streaks from slimy liquid oozing from cracks/wounds—called "slime flux." If cut, heartwood is darker than surrounding wood ("wetwood"). Symptoms are unsightly but usually cause little damage unless trees are under severe stress (soil compaction), when bacteria can invade sapwood causing yellowing, wilting, and branch dieback.

Symptoms and Signs

• Light/dark bark streaks from cracks/wounds running vertically down trunk
• Slimy, sometimes foul-smelling liquid bubbling out and running down trunk
• Heartwood much darker than surrounding sapwood

Cause and Effects

Soil/water bacteria (Clostridium, Bacillus, Enterobacter, Klebsiella, Pseudomonas) reside in young trees or enter older trees through wounds. They grow using sap nutrients. As sap is used: oxygen depleted (anaerobic conditions), methane produced, sap pH increases (6 in healthy → 7-8 in wetwood), high pressure develops (60 psi vs. 5-10 psi normal). This environment inhibits interior rot fungi. Liquid kills grass/herbaceous plants at base. Affected wood has reduced lumber value (discoloration) and dries much slower.

Management

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

Historically, pressure relief was attempted by drilling holes and inserting pipes for liquid/gas escape. No longer recommended because: 1) affected trees generally survive well untreated, and 2) drilling creates more slime exit points.

Key References

Carter 1964; Sinclair & Lyon 2005; Stipes & Campana 1981.