Slime flux and wetwood on trees: oozing sap, as extension services describe it

This page describes what the named sources say. It does not diagnose a tree, say whether any tree is safe or unsafe, or tell anyone to cut, keep or treat one. Each cause, tree list and figure belongs to the source that gives it.

What the sources say it looks like

The University of Maryland Extension (updated February 28, 2023) calls the most obvious symptom sap flowing from bark cracks or other wounds in the trunk or branches. It says the flow may be heavier at certain times of year, or may stop and then restart, and that it leaves dark streaks on the bark that turn gray or white when dry. It adds that fresh liquid may smell fermented or alcoholic. Utah State University Extension (Integrated Pest Management) says that in willow the disease can show as a white to gray foam bubbling from under the bark, while in elm and cottonwood it shows as wet gray to brown areas on limbs and trunk, with slime running down the bark and leaving a dry scum. It says the liquid also drips to the ground, where it kills grass or other plants under the tree.

South Dakota State University Extension, in its Tree Pest Alert of September 30, 2026, shows white slime and dark liquid oozing from a crack in a young cottonwood and calls it wetwood. It says the disease is common on elm and cottonwood in South Dakota and also found on mulberries and willows.

The cause the sources name

All three sources point to bacteria or yeasts working in the wood and sap. Maryland says common surface-inhabiting bacteria or yeast fungi invade wounds and live off the nutrients in the sap, and that gas produced by these organisms builds pressure that forces the sap out. Utah State describes a bacterial fermentation of tree tissue, which it links to the disagreeable odor, and says the bacteria enter through open wounds in the bark, some as small as cat scratches. It names improper pruning, boring insects, poor branch angles, tree houses and lawn mowers as common routes. South Dakota says the gas builds in the trunk through fermentation, that the pressure reaches 60 psi and pushes liquid out through cracks and wounds, and that the wetwood did not cause the wound but used the opening to infect.

Two types in Utah

Utah State separates a heartwood type from a bark and cambial type. For the heartwood type it says internal pressure of up to 60 pounds per square inch can burst the tree, but more often forces ooze through cracks that run from the heartwood to the bark surface, and that trees are rarely killed by it. It lists elm, cottonwood and other poplars among them. For the bark and cambial type, it says infections frequently result in the death of the tree within one to two years, and lists willow, mountain ash, aspen, boxelder, Russian olive, fruitless mulberry and ash. Maryland does not split the condition this way and says trees may be injured for several years before symptoms of slime flux show.

Trees each source names

Source	Trees it names
University of Maryland Extension	Elm, oak and mulberry as the common ones; maple, birch, butternut, redbud, sycamore and walnut to a lesser extent
Utah State University Extension	Elm, cottonwood, poplar, aspen, willow, boxelder, mountain ash, Russian olive, fruitless mulberry and ash, split by type
South Dakota State University Extension	Elm and cottonwood as common; mulberry and willow also found

What the sources say about treatment

Maryland and South Dakota both say there is no treatment that cures it; South Dakota writes that there are no effective treatments. Maryland says the installation of drain tubes is generally not recommended, because drilling can cross natural barriers within the tree and may spread the microorganisms through larger areas. It says formerly water-soaked wood may also be open to attack by wood decay fungi, and that unless the tree is in decline the symptoms can be monitored along with cultural practices that minimize stress. Utah State differs: for the heartwood type it says drilling a hole and inserting a plastic pipe can relieve the internal pressure and stop the slime running down the trunk, but does not eliminate the infection. Utah State also sets out a surgical procedure for bark and cambial infections, in full on its own page listed under sources. On the pesticide side, no source here names a product, and any chemical question belongs with the licensing office, see the pesticide licences guide.

Utah State Extension Forestry also answered a 2020 question on a Siberian elm with slime flux. It said that slime flux, while not visually appealing, typically is not harmful to trees, and that it occurs in response to a previous wound.

The other growth forms are on the odd tree growth page, and cracks that open a trunk to infection are covered in frost cracks on trees.