Slime Flux (Wetwood): Why Your Tree Trunk Is Oozing

By Linda Marsh, Pests & Diseases. Last updated: July 9, 2026.

If you are staring at a wet, discolored streak on your tree, this is what usually prompts the panic:

• A dark, wet stain running down the bark from a wound, crack, or branch crotch
• A sour, fermented, or vinegar-like smell near the ooze
• Bleached, gray, or white crust where the liquid has dried on the bark
• Dead grass or damaged groundcover under the drip line

What is slime flux, and why is your trunk oozing?

Slime flux is the liquid you see. Bacterial wetwood is the condition behind it. Bacteria colonize the inner wood (the heartwood and older sapwood) and ferment the sap, producing gas as a by-product. That gas builds pressure inside the trunk, and the pressure pushes fermented sap out through any weak point: an old pruning cut, a bark crack, a branch union, or a mower wound.

The pressure involved is real, not trivial. Penn State plant pathologist Gary W. Moorman notes that wood in affected trees can reach about 60 psi, compared with the 5 to 10 psi found in wetwood-free trees. That is why the ooze can run for weeks and why it seeps from points well away from the original wound. The fermenting bacteria also shift the wood chemistry: the same Penn State source reports the pH of infected wood rises from about 6 in healthy trees to 7 or 8 in affected wood.

The gas itself is mostly what you would expect from fermentation. A classic analysis of wetwood in eastern elm, published in the ISA's Journal of Arboriculture, found the flux gas was roughly 46% methane and 34% nitrogen, with smaller amounts of carbon dioxide, oxygen, and hydrogen. The sour smell many homeowners report is that fermentation, plus yeasts and other microbes that move in once the sap is running down the bark.

Is bacterial wetwood dangerous to your tree?

In most cases, no. Wetwood is a chronic condition that trees live with for years, and drilling into it does not cure it. As W. Douglas Hamilton wrote in the Journal of Arboriculture, "Wetwood is a chronic type disease that may contribute to the general decline of trees, especially old trees and trees of low vitality." Read that carefully: it contributes to decline in already-weak trees. It is rarely the primary killer of an otherwise healthy tree.

There are two real downsides. First, the ooze itself is toxic to living tissue: it can kill patches of bark it runs across, slow the callus that would otherwise close a wound, and scorch the lawn or plants it drips onto. Second, and more important, wetwood often shows up on trees that already have internal decay or structural defects, because the same wounds and cavities that let bacteria in are the ones that weaken a trunk. So the stain is worth taking seriously less for what it is and more for what it can be pointing at. If the ooze is coming from a long vertical split, our guide on tree bark splitting and cracking in Rochester covers when a crack is cosmetic and when it signals a structural problem.

What causes bacterial wetwood?

Wetwood starts with a way in. Bacteria enter through wounds and injured wood: improper pruning cuts, storm breaks, boring insects, poor branch angles with included bark, and the everyday damage from mowers and string trimmers at the base. Once inside, they set up in the water-saturated inner wood and stay there.

Certain trees are far more prone to it. Elm, maple, oak, and poplar are the usual local suspects, which matters in a region planted heavily with silver and Norway maple. Older, larger, and stressed trees carry it more often, partly because they have accumulated more wounds and more internal decay over their lives. That overlap with decay is the reason wetwood deserves attention: bacteria and wood-rot fungi both exploit the same openings. When you also see mushrooms or conks at the base of the tree, that is a sign decay fungi are active in the trunk or roots, and the combination is worth a professional look rather than a wait-and-see.

Should you drill drain holes to relieve the pressure?

No. This is the single most important thing to get right, because a lot of old advice, and a lot of well-meaning neighbors, still say the opposite. For decades, the standard fix was to drill into the trunk and insert a pipe to drain the fluid and relieve the pressure. That practice has been abandoned.

Gary W. Moorman, Professor Emeritus of Plant Pathology at Penn State, is blunt about why: draining "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." Purdue's plant-diagnostic and arborist authors, Tom Creswell and Lindsey Purcell, reach the same conclusion, noting that inserting a drain tube "is no longer recommended and may do more harm than good" because boring holes spreads the bacteria internally and can open the door to wood-decay fungi. Colorado State University Extension is equally direct that drain tubes are not recommended, since the holes let bacteria spread outward, introduce oxygen, and can invite decay.

So skip the drill. Skip the tube. And do not seal or paint the oozing wound either: wound dressings trap moisture, do not stop the flux, and can slow the tree's own compartmentalization.

How do you actually manage slime flux?

Since there is no cure and no product that eliminates the bacteria, management is about keeping the tree vigorous so wetwood stays a nuisance rather than a contributing cause of decline. Practical steps that authorities agree on:

• Reduce stress. Water deeply during Upstate New York's dry summer stretches and droughts, since a well-hydrated tree tolerates chronic wetwood far better than a stressed one.
• Protect the roots. Avoid soil compaction, grade changes, and trenching near the root zone, and replace turf under the canopy with a proper mulch ring to keep mowers and trimmers away from the trunk.
• Prune correctly. Use clean, correct cuts that close quickly, and avoid unnecessary wounding. Poor cuts are how the bacteria got in to begin with.
• Rinse the ooze. Hosing off the fluid periodically keeps it from killing more bark and lawn, though it does not treat the underlying condition.
• Do not fertilize on reflex. Feed only if a soil test shows a real deficiency, not as a cure for the flux.

Timing is worth noting locally. Colorado State University Extension reports that in elms the highest gas pressure occurs from May through August, so the heaviest oozing you notice in a Rochester summer is normal seasonal behavior, not a sudden crisis.

When should you call a certified arborist?

Call when the ooze is not traveling alone. Slime flux on a sound trunk is mostly cosmetic, but wetwood that shows up with any of the following deserves a professional risk evaluation: large open wounds or cavities, a long vertical crack, mushrooms or conks at the base, thinning or dieback in the canopy, or a lean that is new. Those are the situations where the fermenting stain is a symptom of a tree that may be structurally compromised.

A certified arborist can tell the difference between a tree that is simply weeping and a tree with internal decay that threatens a house, driveway, or walkway. That is a formal risk call, and our overview of how tree risk assessment works in Rochester explains what a qualified evaluation looks at and why the credential behind it matters. The point of the visit is not to treat the flux, which cannot be cured anyway, but to answer the only question that actually matters: is this tree safe to keep as-is?

FAQ

Will slime flux kill my tree?

Usually not on its own. Bacterial wetwood is a chronic condition that trees tolerate for years, and affected trees generally survive well without any treatment. It can contribute to decline in old or already-stressed trees, which is why keeping the tree vigorous matters more than treating the ooze.

Does the sour smell mean the wood is rotting?

The smell is fermentation, not rot. The odor comes from bacteria and yeasts fermenting the sap and producing gases like methane. Actual wood decay is caused by fungi, which is a separate problem. The two can occur together, so persistent oozing plus visible mushrooms or soft, punky wood is worth a professional look.

Should I seal the wound or paint over the ooze?

No. Wound dressings and sealants do not stop slime flux, they trap moisture, and they can slow the tree's natural ability to wall off the wound. Rinsing the fluid off with water is fine to protect the bark and lawn, but leave the wound itself open.

Is slime flux contagious to my other trees?

It does not spread tree-to-tree the way a leaf fungus does. The bacteria enter through individual wounds, so the way to protect your other trees is to avoid wounding them: prune correctly, keep mowers and trimmers away from trunks, and reduce stress rather than worrying about contagion.

Sources

• Monster Tree Service of Rochester
• Penn State Extension: Bacterial Wetwood or Slime Flux (Gary W. Moorman)
• ISA Arboriculture & Urban Forestry: Wetwood and Slime Flux in Landscape Trees (Hamilton)
• Purdue University FNR Extension: Slime Flux of Trees
• Colorado State University Extension: Bacterial Wetwood