Discovering Bacterial Wetwood

Bacterial Wetwood, also called Slime Flux, is a disease commonly affecting the central core and bark of shade and ornamental trees. Several bacteria species inhabiting the soil can infect trees through wounds to multiply in root or trunk heartwood. Wetwood establishment degrades and soaks affected cell walls, raising wood pH from around 6 to 7 or 8. The alkaline brew may soak, stain, and bleach the bark it erupts onto. Wetwood, though chronic, is normally not a serious disease. However, if the tree is under severe stress, the bacteria-laden ooze can clog conductive tissue to cause wilting, leaf yellowing, branch dieback, and bark decay.

Hosts

Trees most susceptible to wetwood include: elms (Ulmus), apple and crabapple (Malus), redbud (Cercis), aspen and cottonwood (Populus), dogwood (Cornus), magnolia (Magnolia), Russian olive (Elaeagnus), beech (Fagus), fir (Abies), maple and box-elder (Acer), blackgum (Nyssa), birches (Betula), hemlock (Tsuga), mountain ash (Sorbus), sycamore and London planetree (Platanus), hickory (Carya), mulberry (Morus), sweetgum (Liquidambar), butternut and walnut (Juglans), buckeye and horsechestnut (Aesculus), oaks (Quercus), tuliptree (Liriodendron), linden (Tilia), pines (Pinus), black locust (Robinia), and willow (Salix).

Symptoms and Diagnosis

We often discover wetwood when bacterial fermentation builds up gases like methane and nitrogen, forcing out a sour-smelling liquid. Bubbling or seepage can occur at defective tree crotches or wounds from natural causes or pruning. The liquid is colorless or pale while inside of the tree, but it darkens when exposed to air. As the seepage dries, it leaves a pale gray to white crust on the bark.

Many insects, including flies and beetles, are attracted to bacterial wetwood and they feed off the slime. Larvae found in the mass would have hatched from eggs laid there for them to feed on. Insects simply feeding or stepping in the flux are not known to cause damage or transmit the bacteria. However, some wood boring insects such as beetles may do so, as the exudate clings to their bodies.

Staining of bark on an elm tree resulting from bacterial wetwood liquid draining from an old pruning wound. Credit: Penn State Department of Plant Pathology and Environmental Microbiology Archives, Penn State University, Bugwood.org.

Management Strategies

Wetwood bacteria, including Clostridium, Bacillus, Enterobacter, Klebsiella, and Pseudomona are not tree pathogens. The alkaline bacterial infection inhibits wood-rotting fungi like Armillaria. There is no satisfactory control, and preventative care is the best way to avoid establishment. Prune only when needed and follow proper pruning techniques. Protect the tree from environmental stress such as soil compaction, especially during construction. Water and fertilize only as needed to avoid creating more stress. If pruning an infected tree, sterilize pruning tools between cuts.

Boring a hole into the trunk to relieve gas pressure and direct the flow of the fluid is no longer recommended. Drying the area in this manner may cause more limb breakage as it removes lubrication of wetwood-degraded fibers.

Though wetwood infection may be unattractive, it need not cause alarm. Without other concerns, your infected tree may continue to function fully in your landscape. Managing the condition may call attention to ways you can reduce plant stresses. Best practices include pruning properly and keeping root space undisturbed. Fertilize only according to soil test recommendations, and manage water needs practically for best results.

References

Brazee, Nicholas. Wetwood and Slime Flux. University of Massachusetts at Amherst. March, 2018.

Missouri Botanical Garden. Slime Flux.

Moorman, Gary W. Bacterial Wetwood or Slime Flux. Penn State Extension.

Morton Arboretum. Wetwood.

Texas A&M Forest Service. Tree Health Issues: Bacterial Wetwood.