Diagnosing, Controlling, and Preventing Tree Slime Flux (Wetwood)

Bacterial wetwood can be treated and managed

Bacterial Wetwood on Lime (Tilia sps) at Kingencleugh next to the River Ayr, East Ayrshire, Scotland.
Rosser1954/Wikimedia Commons/CC BY-SA 4.0

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 associated with bacterial wetwood or slime flux disease. This disease is a major cause of rot in the trunks and branches of hardwood trees. Slime flux is caused by a 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 Enterobacter cloacae are 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. In an infected tree, these bacteria feed and grow inside a tree wound and they use tree sap as their 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

For many years, experts advised that holes drilled in a tree could allow gases and liquids to drain from an area of slime flux rot. More recently, several United States Forest Service reports advise against this practice. It is now 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. As determined by Dr. Alex Shigo's research, the best current advice is to maintain the tree's overall health so that the tree can isolate the spot and grow good wood around the diseased portion. Affected trees will usually overcome the problem themselves and seal off the damage.

Avoid Insecticide Use

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 impetus to trying this treatment stems from people noticing insects feeding on the rot. It should be remembered, however, 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 and may actually perpetuate slime flux disease. 

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. 

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. 

Watch Now: What Are the Parts of a Tree?