Diagnosing, Controlling and Preventing Tree Slime Flux (Wetwood)

Bacterial Wetwood Can be Managed

Advanced Slime Flux. Joseph OBrien, USDA Forest Service, Bugwood.org

Most everyone has seen these symptoms in a tree at some point: an oozing, weeping spot in the bark of the tree, often near a crotch or pruning scar, but sometimes just appearing randomly. The elm trees that line boulevards in many communities are a prime place to spot these wet, slimy weeping spots, but a number of other trees can also exhibit the symptoms. 

Bacterial Wetwood or Slime Flux

This familiar symptom is called bacterial wetwood, or slime flux disease.

It is a major cause of rot in trunks and branches of hardwood trees.  Slime flux is caused by bacterial infection in the inner sapwood and outer heartwood areas of the tree, and  is normally associated with wounding or environmental stress, or both.

In elm trees, bacteria Enterobactor cloacae is the cause of slime flux, but numerous other bacteria have been associated with this condition in other trees, such as willow, ash, maple, birch, hickory, beech, oak, sycamore, cherry, and yellow-poplar. These similar bacteria include species of Clostridium, Bacillus, Klebsiella, and Pseudomonas. These bacteria are feeding and growing inside the tree wound, and they use tree sap as the favorite source of nutrients.

Symptoms of Slime Flux

A tree with slime flux disease has water-soaked patches and "weeps" from visible wounds and sometimes even from healthy-looking bark. The actual "weeping" from the patch may be a good sign, as it is allowing for a slow, natural draining of an infection that needs a dark, damp environment.

In the same way that an infection in an animal or person is relieved when the wound drains, a bole (trunk) infection in a tree is helped when drainage occurs. A tree with this form of bole rot is trying its best to compartmentalize the damage.

The attacking bacteria in a slime flux infection alters wood cell walls, causing moisture content of the wood to increase to the point of injury.

 Slime flux is identified by dark liquid streaks running vertically below an injury and a foul smelling and slimy seepage running down the bark. Chemically, the weeping liquid is actually fermented sap, which is alcohol-based and toxic to new wood. 

Treatment for Slime Flux Disease

At one point, experts advised that holes drilled in a tree could allow gases and liquids to drain from an area of slime flux rot,  but more recently, several United States Forest Service reports advise against this practice, as it is thought to further spread the bacteria.  There is still some debate about this practice, but the consensus now is to refrain from drilling holes. 

In reality, there are no active measures to effectively treat the bole rot caused by slime flux disease. The best advice is to maintain the tree's overall health so that it can isolate the spot and grow good wood around the diseased portion, as determined by the late Dr. Alex Shigo's research. Affected trees will usually overcome the problem and seal off the damage.

Another common treatment that really has no benefit is the use of insecticides applied in the hopes of preventing the rot from spreading within the tree. The urge to try this treatment is because people often notice insects feeding on the rot, but it should be remembered that the insects have not caused the disease nor do they spread it.

There is even some opinion that by removing the decaying wood, insects may actually help the tree. Spraying for insects in an effort to cure slime flux is a waste of money. 

Preventing Slime Flux Disease

The basic control for slime flux disease is prevention. Avoid wounding the tree, and make sure to plant trees in locations where there are no stresses from urban soil compaction, such as walking and vehicle traffic. Trim away broken, torn branches promptly. 

And remember that a healthy tree will usually overcome slime flux. If you keep your trees healthy in other ways, they almost certainly will overcome a bout of slime flux disease.