One of the strangest garden questions that I get as a horticulturist goes like this, "My tree seems to have foam, similar to shaving cream or soap suds, coming out of cracks in the bark." This is not a typical plant disease symptom, and gardeners are often reluctant to even ask such a question because they fear someone will question their eyesight, their sanity or both.
Trees are not supposed to produce foam or soap suds, and rarely ever do. This unusual symptom, causing confusion and consternation to gardeners, is a disease called bacterial wetwood, or slime flux. Slime flux disease produces the previously mentioned symptom and another interesting side effect. Many insects are attracted to the sap, and this is why many gardeners initially notice the problem. The insects, be they worms, maggots, beetles, flies or wasps, are only incidental. They are never the cause of the problem and are not damaging the trees. They are looking for a free drink of the sweet fermented liquid that occurs when the bacteria ferment the sugary sap of the tree.Infections also result in wet, gray-brown areas on limbs and trunks. The slime from the infection runs down the bark, discolors the plant tissue, and results in a build-up of dry, unsightly scum. In addition to the unsightliness, the oozing material has a putrid smell and can damage surrounding plants. The ooze drips onto the lawn or other plants below, resulting in dead or discolored spots.
Bacteria, as plant disease organisms, are not aggressive. They primarily enter through wounds in the bark. Even very tiny wounds such as cat scratches can serve as infection sites, but the disease usually enters through larger wounds that result from improper pruning, boring insects, poor branch angles, lawn mowers or weed eaters. Healthy, intact bark is an excellent barrier, much as your skin is to you, but openings allow the bacteria to infect the plant.
There are actually two types of slime flux in Utah. The heartwood type and the bark cambial type affect trees differently, and the treatments are different. Heartwood bacterial wetwood can build internal pressures of 60 pounds per square inch. This can cause bursting of the infected tree, but pressure normally oozes out through cracks that extend from the inner heartwood portion of the tree to the bark's surface. The cracks commonly occur in large pruning wounds. The flux runs down the tree and may kill the bark tissue. This is a common disease on elms (particularly Siberian elm), cottonwoods and other popular species. There is no cure for the heartwood infections. Many old arboriculture books suggest treating the tree by drilling into the wound and inserting pipes. This has a limited benefit, as it prevents the unsightly slime from running down the trunk or allows the oozing to be piped to an area where it will not drip on the sidewalk or the turf, but it does not eliminate the infection.
Bark or cambial infections are much more serious and often result in the death of the tree within a short time. The bacteria invade between the outer bark and the wood. Because this infection is on the outer bark of the tree, inserting a tube does not relieve the problem. Trees that suffer from cambial slime flux can sometimes be saved by promptly cutting away diseased tissue. With smaller branches, it is best to prune out the branch where it joins another main branch or trunk. With larger branches or trunks, remove all the discolored bark down to the wood. Cut away the infected bark until you come to healthy, yellow bark. This sometimes involves removing a great deal of bark. If infections cover more than half of the tree it is probably easier to remove the tree and start over again with a less susceptible tree.
Shape the wound to allow it to heal properly and sterilize the knife with rubbing alcohol between the cuts to prevent spreading the bacteria. Finally, disinfect the wound with rubbing alcohol or a 10 percent bleach solution (one part household bleach and nine parts water). Continue to watch the treated area for evidence of the disease. If tissue on the edge of the wound begins to ooze or flux, removal of infected tissue was not complete and re-treatment may be necessary.
Tree species susceptible to heartwood infections include elm, cottonwood, poplar, boxelder, Russian olive and ash. Trees susceptible to bark or cambial infections include mountain ash, aspens, poplars, fruitless mulberry and willow, with the "Globe" or "Navajo" willow being the most commonly affected tree.
Next time you see your trees oozing or frothing at the bark, don't question your own sanity. It is a legitimate plant disease and one that causes considerable concern if your tree is the victim. Prompt treatment may save some trees, but the disease must often run its course, and the trees will either live or die, depending on the extent of the infection. Hopefully, yours will be spared this affliction.