Why Is My Tree Oozing Foamy, Smelly Sap?
A tree oozing sour-smelling, foamy liquid from a crack or old wound most likely has slime flux, also called bacterial wetwood. Purdue University's Forestry and Natural Resources extension explains that gases produced by bacteria and yeasts inside the trunk build enough internal pressure to force sap out through cracks in the bark, where it ferments and gives off a foul odor. The University of Massachusetts Amherst notes that the bacteria involved are not tree pathogens and that for most trees wetwood appears inconsequential to their overall health.
One old fix should stay in the past. UMass explains that drilling holes into the colonized heartwood breaks the tree's natural compartmentalization barrier and opens the trunk to wood decay fungi, and says the practice is no longer performed. Purdue says inserting a drain tube to relieve pressure was once an accepted treatment but is no longer recommended and may do more harm than good. This post covers what slime flux is, how to tell it from other oozing, when it matters, and what to do.
What Is Slime Flux?
Slime flux is a condition in which bacteria infuse water into a tree's heartwood, often creating an oxygen-poor environment, according to UMass. The wet, fermenting wood produces gas, and the pressure pushes liquid out through the path of least resistance.
Purdue says the organisms responsible are common surface-dwelling bacteria or yeasts that get into the trunk through wounds, including:
Improper pruning cuts
Broken stems
Injection sites
Cracks from freeze injury
Weak branch crotches
The University of California's Integrated Pest Management program adds that the bacteria can enter through sites where pesticides have been injected into trees.
What Does It Look Like, and Which Trees Get It?
Purdue describes dark sap oozing down the trunk, sometimes frothy and white where it exits, leaving dark streaks that turn light gray or white as they dry. The University of California describes a discolored, water-soaked area exuding sour or rancid, reddish or brown fluid from bark cracks or wounds. UMass notes the stained bark can look gray, pale brown or yellow, and the affected wood itself has a strong, pungent odor.
Many common yard trees can carry it. Purdue says slime flux is extremely common on mature elms, oaks, and mulberries and is seen less often on maples, paper birch, sycamore and walnut. UMass lists elms, poplars, true firs, maples, mulberries, sycamores, oaks, willows and hemlocks as susceptible.
How Is Slime Flux Different From Alcoholic Flux?
Not all oozing is the same, and the smell is the quickest clue. Mississippi State University Extension draws the distinction this way:
Bacterial wetwood, or slime flux: a foul or sour smell, beginning deep inside the tree and bacterial in origin.
Alcoholic flux: milky, very frothy or foamy, and smelling like alcohol, sometimes with a vinegar-like or sweet odor. It starts just under the bark, usually near a wound, most commonly a pruning wound, and the alcohol smell lasts only a short time in summer.
Mississippi State notes that alcoholic flux is not the result of disease and that the same cultural practices used for bacterial wetwood apply to it.
Is Slime Flux Harmful to the Tree?
Usually not. The University of California says wetwood rarely causes serious harm and that infected limbs may be as strong as healthy wood. Purdue calls slime flux not usually a serious problem.
Severe cases are the exception. UMass notes that leaf scorch and canopy dieback can occur in severe cases. If a tree with slime flux is also losing branches or leafing out poorly, the flux may be a sign of a larger problem worth checking, rather than the problem itself.
The ooze is mostly a cosmetic nuisance. It stains bark and smells, which is why homeowners notice it on trees near patios, decks and driveways.
Why Shouldn't You Drill or Drain the Tree?
Older advice suggested drilling into the trunk and installing a tube to let the liquid drain away from the bark. Current extension guidance says not to. UMass explains that drilling into colonized heartwood breaks the natural compartmentalization barrier and opens the trunk to wood decay fungi, and that the practice is no longer performed. Purdue says the same about drain tubes: no longer recommended, and they may do more harm than good.
The reason matters. The tree has already walled off the colonized wood inside its trunk, and a new hole punches through that barrier. Trading a cosmetic ooze for internal decay is a bad deal for the tree and for anyone standing under it later. Our post on signs a tree is rotting inside explains why internal decay is the bigger concern.
What Can You Do About It?
UMass says there is no preventive treatment or way to eliminate wetwood from an affected tree, so management focuses on the tree's overall condition:
Avoid new wounds. The University of California's first recommendation is to avoid injuries to bark and wood, since the bacteria enter through wounds.
Prune correctly. Purdue lists improper pruning among the entry points, so cuts should be made properly and only where needed.
Leave the ooze alone. Do not cut into the bark or drill the trunk to stop it.
Watch the canopy. Leaf scorch or dieback on a fluxing tree is the point to have it inspected.
Oozing from a fresh crack after a hard winter is also worth a look, since freeze cracks are one of the entry points Purdue lists. Our posts on what causes winter tree cracks near Pittsburgh homes and whether cracked trunks mean immediate removal cover how serious a crack is.
When Is Oozing a Sign of Something Worse?
Slime flux by itself rarely calls for removal. The tree needs a closer look when oozing shows up alongside:
A cavity, soft wood or a hollow sound when the trunk is tapped
Mushrooms or shelf fungi on the trunk or roots
A long vertical crack or a split between large stems
Dead branches or a thinning crown
Those signs point to structural problems that the flux may be riding along with. Our guide to telling whether a tree is hollow or unsafe walks through what to check.
FAQ
Will slime flux kill my tree?
Rarely. The University of California says wetwood rarely causes serious harm, and UMass calls it inconsequential to the overall health of most trees.
Why does the ooze smell so bad?
Purdue explains that the sap ferments as it leaks out, releasing a foul odor.
Should I wash the ooze off the bark?
You can rinse it for looks, but it will not stop the flux. Avoid scraping or cutting into the bark, since the University of California's main advice is to avoid injuring bark and wood.
My tree smells like alcohol. Is that slime flux?
Probably not. Mississippi State University Extension describes alcoholic flux as milky, very foamy, and alcohol-scented, starting just under the bark near a wound, and notes it is not the result of disease.
Can pruning cause slime flux?
Improper pruning is one of the wound types Purdue lists as an entry point for the bacteria and yeasts.
Is there a spray or injection that cures it?
No. UMass says there is no preventive treatment or way to eliminate wetwood from an affected tree.
Tree Inspections, Pruning and Removal From Brinko Tree Service
Brinko Tree Service, based at 1010 Sunset Blvd in Harrison City, brings nearly 20 years of experience to homeowners and businesses across Westmoreland and Allegheny Counties. When an oozing tree also shows cracks, decay or dead branches, the team provides pruning and trimming to remove dead or hazardous limbs and tree removal when a tree is no longer safe, followed by stump grinding and cleanup. Brinko is fully licensed and insured, follows OSHA and ANSI safety regulations with an OSHA safety site supervisor on every job site, and offers free estimates.
Visit Brinko Tree Service or call 412-373-7450 today. Contact us for more information.
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