[From the August 2016 issue of the UC IPM Green Bulletin]

Two “sap flux” diseases observed in landscape trees—bacterial wetwood (or slime flux) and alcoholic (“foamy”) flux—often trigger demands that a landscaper “do something.” Yet the most appropriate action may be to provide cultural care and to monitor for any additional problems rather than to apply chemicals or undergo drastic “tree surgery.”

Symptoms

A single wound or bark crack located on the trunk or a large branch may be observed that is actively oozing or bleeding. The oozing liquid is usually clear and may be sour-smelling in the case of slime flux, or frothy (Figure 1) and “fermented-smelling” in the case of alcoholic flux.

The bleeding is often vigorous, and the liquid stains the bark below dark brown or black, whereas the bark above remains completely unaffected (the stain often resembles a comet with a long “tail” extending below the bleeding wound) (Figure 2).

Differential diagnosis

Flux problems can affect any tree species, but appear most commonly on mulberry, elm, and oak. The flux point occurs singly, may be isolated on an otherwise-healthy trunk, branch, or stub, is often associated with pruning wounds, and is usually not too close to the ground (usually above one meter in height or so).

When multiple flux points are present (seldom more than two or three), they originate from obvious wounds or bark cracks, and each remains isolated—they do not coalesce to form a large diffuse bleeding canker as seen with some Phytophthora infections. Removing the bark around the fluxing wound reveals healthy tissue, in contrast to Phytophthora infections like Sudden Oak Death, where discolored cankers are present below the bark.

In addition, the volume of the fluxing liquid is usually substantial, wetting the bark for some distance below (in contrast to canker diseases that produce only a few droplets of liquid or none at all).

Importantly, there will be no entry holes or other evidence of insect infestation, thus differentiating the fluxes from beetle-vectored infections such as foamy bark canker of oaks or the Fusarium infection carried by the polyphagous shothole borer (these two insects also invade trees en masse, creating multiple weeping points).

Causes

The two flux problems are thought to have different causes. Slime flux is associated with bacterial wetwood (Figure 3), a condition in which the heartwood and parts of sapwood become soaked with liquid containing high levels of bacteria. The bacteria ferment the liquid, increasing its pressure until it oozes out through a bark crack or wound.

Alcoholic flux (or foamy flux) is thought to be caused by bacteria in the bark and cambium—shallow components of the tree, not extending into the wood—and here, too, the fermenting liquid is forced out of a wound or crack.

The differences in bacteria and the location of infection (wood vs. cambium) likely cause the two liquids to differ in smell and appearance.

Management

Despite their prominence, both types of flux are thought to be minor problems in landscape trees (in contrast to within some fruit or nut trees, where they can be more serious). Neither disease affects tree structure, and slime flux only occasionally causes branch dieback.

Foamy flux is typically also benign, as it usually dries up with the onset of cool weather in late fall. Importantly, no treatment has been shown to consistently result in tree improvement, and chemical treatments are ineffective.

Slime flux has occasionally been treated with scribing (excision) of the margins of the bleeding canker, but this is supported only by anecdotal evidence. On the other hand, such “surgery” may risk interfering with the tree's own process for compartmentalizing and sealing-over the damaged area. Because of this concern, installation of drainage tubes is also no longer recommended.

Instead, provide appropriate cultural care—which may mean providing water—and avoid wounding the tree. The bark staining can often be washed off with water.

Additionally, continue monitoring the tree, as other problems (such as Phytophthora or other canker diseases like Armillaria) could occur on the same tree and should not be allowed to go unnoticed as they can resemble the flux diseases, but are distinguishable upon closer examination.

Visit the UC IPM Website for more information about bacterial wetwood (or slime flux) and alcoholic (“foamy”) flux.