This is the Garden Column for the week of June 15, 2007.
By Christine Engelbrecht
Iowa State University
Ever seen a tree that looks like it’s sprung a leak? Wondered why your favorite elm is suddenly oozing stinky liquid from its trunk? A condition called bacterial wetwood (also called slime flux) is likely to blame. Bacterial wetwood occurs most frequently on elms, maples, poplars, oaks and birches, although it can occur on other trees as well. Affected trees may leak copious amounts of liquid out of their trunks or branches, discoloring the bark and dripping onto the surrounding ground.
Bacterial wetwood occurs as after bacteria infect the wood of a tree. Bacteria can enter the wood through any wound in a trunk, limb or root. Once inside the tree, certain bacteria (called anaerobes) thrive in the low-oxygen environment there. Usually many diverse kinds of anaerobic bacteria are present in an infected tree, living together in a wet mess. They may multiply within the tree for several years unnoticed, and can slowly spread several feet from the initial entry point. As they reproduce, the bacteria produce slimy ooze and methane gas, which builds pressure inside the tree.
Pressure builds slowly, and eventually the bacteria are forced out of the tree through the weakest point available, usually near a wound or trunk crotch. The clear to brown bacterial ooze may seep from the tree continually through the growing season, leaving a yellow to brown stain on the bark when it dries.
Because the ooze seeping from a tree is full of bacteria, it may smell awful. Other bacteria, fungi, and insects may feed on the ooze once it is outside the tree, contributing to the stench. Often the liquid is toxic to plants, killing grass where it drips. Sometimes the ooze kills the bark where it seeps out, and elms with wetwood often develop yellowed leaves and branch dieback as a result of the toxic liquid. On many infected trees, though, the foliage appears healthy, and wetwood often does not cause much damage to the rest of the tree.
What can be done for a leaky tree? Unfortunately, there is no way to cure a tree of bacterial wetwood. However, an infected tree may survive for many years after infection with few problems other than the presence of the ooze. An infected tree may produce ooze every summer for many years.
Minimizing other stresses can help to prolong the life of an infected tree. For example, avoid wounding the tree, and water it deeply if the weather is especially dry. Drought stress can make symptoms worse. Older recommendations suggested inserting a spout in the tree to drain out the liquid and relieve pressure. This treatment is now known to not be helpful, and can actually make the problem worse since it involves wounding the tree.
Maintaining overall vigor is the best way to manage a leaky tree.