Many callers have expressed concern over slimy, frothing liquid oozing from an otherwise healthy tree in the landscape. This is the time of year for appearance of wet wood and slime flux. This chronic, rarely serious disease can contribute to general decline in vitality of trees but is not known to cause tree death. It is probably most common on elm in Illinois, but we see it on many other trees, including oak, poplar, cottonwood, maple, redbud, sycamore, and other species. The causal organism is a bacterium called Enterobacter cloacae (Erwinia nimipressuralis). The bacterium gains entry into the tree, usually through wounds, where it ferments and causes internal pressure. Moisture containing the bacterium flows from cracks, wounds, or weak areas in the tree. The usual place is in crotches of the tree. The smell that sometimes develops is usually due to secondary rotting organisms. Although this problem cannot be cured, it is comforting to know that the wet regions are not decayed. Decay fungi do not thrive in this water-soaked wood.
You cannot always see the wound, but you can see the liquid from this disease. 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 remains--slime flux. The liquid commonly causes localized death of the cambium. Although fluxing occurs from April to December, 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 that 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 beyond the contained area.
Removing dead or weak branches, plus promptly pruning and shaping bark wounds is helpful. Proper pruning techniques encourages 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 the foul-smelling slime flux described. 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.