A Field Guide to Tree Cutting Ppe Essentials
Learn how to identify, prevent, and treat bacterial wetwood, a common but often misunderstood condition that causes oozing slime and foul odors in shade trees.
- Bacterial wetwood is a chronic bacterial fermentation in heartwood, not lethal decay. Fermentation gases force sour-smelling slime out through trunk cracks or wounds. Elms, maples, and oaks over 25 cm diameter are most commonly affected. Never drill drainage holes; they worsen infection and create new entry points. Control odor by surface cleaning with 10% bleach and installing a simple drip diverter.
You noticed the dark stain on the bark first, then the sour, vinegar-like smell. A thick, slimy liquid is seeping from a crack in the trunk of your mature elm, streaking the bark and killing the grass underneath. Neighbors mention “slime flux” and you wonder if the tree is dying. As Michael Hartman, ISA Board Certified Master Arborist, I evaluate these exact symptoms weekly across Texas. By the end of this article you will know why bacterial wetwood develops, which species are most susceptible, how to distinguish harmless oozing from serious decay, and the practical steps that actually reduce odor and staining without harming the tree.
What Bacterial Wetwood Is—and Is Not
Bacterial wetwood is a chronic, internal infection of the central heartwood and sapwood by anaerobic bacteria that ferment the tree’s own sap. The fermentation produces methane, carbon dioxide, and acetic acid, raising internal pressure up to 60 psi and forcing a foul-smelling liquor out through any crack or old wound. The condition is sometimes called slime flux, but that term really describes the symptom (the oozing slime) rather than the disease itself.
Term: Wetwood refers to the water-soaked, discolored wood created when bacterial fermentation displaces normal gases in the xylem with liquid and microbial by-products.
Importantly, wetwood is not a true decay fungus and it rarely kills the tree. Instead, it acts like a chronic low-grade infection that stresses the vascular system and can predispose the tree to secondary problems such as wood decay fungi or borer attack. The bacteria do not produce the enzymes needed to break down cellulose or lignin, so the wood remains structurally sound for many years despite the unsightly discharge.
How to Recognize the Symptoms in the Field
The first sign is usually a vertical streak of dark, wet bark on the lower trunk or at an old pruning scar. The liquid ranges from clear amber to opaque tan and typically has a distinctly sour or fermented odor. During warm weather the flow can exceed a liter per day, attracting flies, wasps, and yellow jackets. Beneath the flow, bark tissue may die and grass or understory plants can yellow from the alcohol and acid content of the exudate.
Warning Signs to Watch For
- Dark vertical staining: Usually 1–3 cm wide, starting at a wound or branch crotch, extending 1–2 m down the trunk
- Foul odor: Sour, vinegar-like smell noticeable within 3 m of the tree on warm days
- Excessive wetness:
- Secondary insect activity: Flies, bees, or beetles congregating at the oozing site
Bark remains wet to touch even after 48 hours without rain
Confirm wetwood by inserting a clean pocketknife 5 mm into the bark near the stain; the underlying wood will feel saturated and smell strongly acidic. If instead you find soft, punky wood or mushroom-like fruiting bodies, the tree has moved beyond wetwood into active decay and requires a different management plan.
Which Trees Get Wetwood Most Often
Elms, maples, and oaks top the list in Texas landscapes, but the condition also appears regularly in sycamore, cottonwood, willow, and honeylocust. Within a genus, older, faster-growing cultivars with naturally thin bark seem most prone. American elm (Ulmus americana) and red maple (Acer rubrum) send me more calls than any other species.
The common thread is high sap flow combined with a tendency to develop internal heartwood cracks. Trees that compartmentalize wounds slowly allow bacteria to colonize the water columns, while species that seal off injuries quickly rarely develop chronic wetwood. Age matters: almost all symptomatic trees I inspect are greater than 25 cm trunk diameter, suggesting it takes years of bacterial buildup before external signs appear.
Why Drilling or Tapping Makes the Problem Worse
Old extension bulletins sometimes recommended drilling a downward-slanting hole into the flux and inserting a pipe to “relieve pressure.” In practice this converts a sealed internal infection into an open wound, introducing oxygen and fresh bacterial inoculum while destroying the tree’s natural barrier zones. Within two growing seasons the drilled hole almost always begins to exude more heavily than the original crack.
Pro Tip: Never drill drainage holes or insert spiles. Instead, let the tree wall off the infection naturally and manage only the external symptoms to reduce odor and staining.
Research from the University of Georgia shows that trees left untapped produce fewer new flux sites over a five-year period compared with tapped controls. The takeaway: the tree’s own compartmentalization process is more effective than any mechanical drainage we can provide.
Practical Steps to Reduce Odor and Staining
While you cannot cure bacterial wetwood, you can suppress the nuisance factors. Start by removing any loose bark around the flux site with a blunt chisel to eliminate pockets where liquid pools. Next, mix one part household bleach with nine parts water and gently scrub the stained bark using a soft brush; bleach oxidizes the odor-causing organic acids without penetrating the cambium. Rinse thoroughly after two minutes.
Surface Treatment Protocol
- Wear eye protection and nitrile gloves
- Loosen and remove bark flakes with a wooden stick, not a saw
- Apply 10% bleach solution; keep it on bark only
- Rinse with plain water from a low-pressure hose
- Allow trunk to dry, then sprinkle a 2 cm layer of coarse wood chips under the drip line to absorb future drips
Install a simple drip diverter if the flux runs onto pavement: wrap a 5 cm-wide strip of aluminum flashing loosely around the trunk just below the flow point, slant the lower edge outward, and direct the liquid into a hidden mulch bed. Check the flashing quarterly to be sure it is not girdling the trunk.
When to Call a Certified Arborist
Schedule a professional evaluation if the staining expands more than 30 cm in a single season, if the exudate turns pink or blood-red (indicating possible Phytophthora infection), or if fruiting bodies of wood decay fungi appear nearby. Likewise, call immediately if the tree develops structural defects—cankers, cavities, or conks—because wetwood can accelerate decay when combined with other pathogens.
An ISA certified arborist can perform a resistograph or tomograph to quantify internal sound wood and determine whether the tree poses a hazard. If risk is low, the arborist can install a passive drain sleeve: a sterile, breathable fabric placed over the flux site that wicks liquid into an absorptive pad, reducing odor without wounding the trunk further.
Long-Term Tree Care to Minimize Recurrence
Stress reduction is the only sustainable strategy. Maintain a 5 cm-deep mulch ring out to the drip line, but keep mulch 10 cm away from the trunk to prevent chronic moisture buildup. Irrigate deeply yet infrequently—one slow soak every 7–10 days during summer drought—because fluctuating soil moisture increases xylem pressure and can force more flux.
Avoid flush cuts and unnecessary pruning during the growing season; each fresh wound is a potential entry point for new bacteria. If you must prune elms or maples, do it in late fall after leaf drop when sap flow is minimal and insect vectors are least active. Finally, fertilize only on the basis of soil test results; excess nitrogen stimulates succulent growth that bacteria exploit.
Conclusion: Manage the Symptoms, Support the Tree
Bacterial wetwood looks alarming and smells worse, but it is seldom a death sentence. Recognize the characteristic sour slime, rule out more serious decay, and resist the urge to drill. Clean the bark, divert the drip, and focus on overall tree vigor. With basic surface hygiene and sound cultural care, most infected elms, maples, and oaks continue to provide shade and beauty for decades. If in doubt, have an ISA certified arborist perform a hazard assessment; peace of mind is cheaper than emergency removal. For those weighing DIY vs professional approaches, tree cutting guidance can help decide when expert help is warranted.
