This disease can be quite ugly in mature shade trees. It also slowly contributes to rotting of the interior of the tree. We do not consider wetwood and slime flux to be an immediate threat to the tree. Still, it is important to understand how this bacterial infection affects tree decline.
Wetwood and slime flux is a condition caused by bacteria that enter wounds in a tree. Enterobacter cloacae (formerly known as Erwinia nimipressuralis) and other bacteria are associated with this disease. This condition in trees is very noticeable because infected trees often have seepage coming from a major crotch or wound in the trunk. In some cases, the liquid emitted from the wounds has a foul odor because secondary microorganisms colonize it.
Wetwood causes a water-soaked condition of wood in the trunk, branches, and roots of many shade and ornamental trees, especially old street trees. Elms, poplars, cottonwoods, oaks, and maples seem most commonly affected in Illinois; but many other tree species are susceptible. This chronic but rarely serious disease of trees can contribute to general decline in tree vitality but is not known to cause tree death.
Wetwood is most visible externally as a bubbling seepage of bacteria and toxins from wounded tissue in V-shaped branch crotches, pruning wounds, injection holes, and trunk cracks. The liquid often runs down the trunk, leaving a white stain. You cannot always see the wound, but you can see the liquid. Bacteria in the inner sapwood and heartwood of the tree ferment, causing internal gas pressure. This pressure commonly reopens old wounds, and the sour liquid flows down the bark. As it dries, a light gray to white encrustation called slime flux remains. The liquid commonly causes localized death of the cambium. Fluxing occurs from April to December, but it is most conspicuous in the summer, especially now.
There is no cure for this condition, but the following may be helpful. Fertilize stressed trees in the spring or fall to stimulate vigorous growth. Some people like to install perforated plastic or iron drain tubes in the tree to relieve the gas pressure and to allow continual drainage away from the trunk. The idea is to keep the liquid off the trunk so the cambium is not killed. Be aware that drain tubes often make the problem worse internally. Trees have the ability to compartmentalize injuries or diseased wood. They may wall off the wetwood areas. Because drain tubes create a deep wound, they may also break the compartment that the tree has made to encompass the wetwood, allowing the internal discoloration and any future decay to spread outside the contained area.
Removing dead or weak branches, plus promptly pruning and shaping bark wounds, is helpful. These measures encourage rapid callousing of wounds. The sap flow that results from pruned branches is normal and is not the same as wetwood flow. The liquid we see with wetwood may flow year-round and is often followed by slime flux. Consult Report on Plant Disease (RPD), no. 656, "Bacterial Wetwood and Slime Flux of Landscape Trees," for more on this condition. RPDs are available in Illinois Extension offices or on Extension's VISTA Web site.