Wetwood disease and its associated product, slime flux, may be seen in landscape trees this summer. The sometimes foul-smelling and unsightly seepage from wounds in the bark or wood of various shade trees is known as slime flux. It occurs most commonly on bacterial wetwood-infected trees, such as elm, mulberry, poplar, oak, birch, and maple. Although slime flux development is seasonal, evidence of wetwood and slime flux-stained bark is visible anytime.
Symptoms and Cause.
Tree owners will first notice a vertical streak of wet bark on the trunk beginning at a crack or wound up on the trunk and extending all the way to the ground. This seepage is often accompanied by a discoloration of the bark in the area where the fluid flows. Wetwood seepage originates from infections of the heartwood and inner sapwood by common soil-inhabiting bacteria such as Enterobacter cloacae (Erwinia nimmipressuralis). There are many other bacterial species also associated with wetwood. Wetwood bacteria are capable of growing anaerobically (without oxygen) in the internal wood tissues of the tree and leaving the wood of trunks, limbs, and roots water-soaked. Methane and osmotic or metabolic liquids, two by-products of the bacterial activity, accumulate under pressure and are forced out of the tree through the nearest available opening, usually a trunk wound or branch stub. Pruning a branch or taking a core with an increment borer can sometimes release the materials under pressure, squirting the worker with foul-smelling liquid and gas.
Normally flowing to the wounded bark surface, the wetwood fluid is a clear watery liquid containing several nutrients. On the surface it soon changes into a brown, slimy ooze, as a result of feeding by fungi, yeasts, bacteria, and insects (Figure 6). This surface slime flux may kill injured cambium and bark surface organisms as well as grass growing near the base of the tree. Otherwise, wetwood disease does not appear to be directly harmful to the tree. However, as the internal tissues are infected, the tree may lose some of its stored carbohydrate reserves and have less energy for warding off other diseases or insects or the effects of drought or pruning. Once a tree is infected, the disease does not normally go away.
Wetwood-infected trees have an internal core of wood that is wet but not decayed. These infected branch, trunk, and root tissues also have a high pH. Wetwood-infected wood is resistant to decay by fungi. The extent of wetwood spread in the tree may be limited by tree defenses; however, wetwood can spread into new tissues as new injuries occur. Thus deep injection holes and pruning can expand wetwood infection. Tree workers must take care to avoid pruning live branches on infected trees.
Control. Thus far, no effective preventive or curative measure is known. If the bark is being stained it may be helpful to drain the slime flux away from the branch or trunk so that it drips on the ground. Drilling a hole into the tree and inserting a copper or semi-rigid plastic tube has helped in some cases; however, this results in additional wounding and the threat of expanded wetwood or decay should be considered. Loose dead bark should be carefully cut away so that the area can dry.
American elm tree showing slime flux, evidence of infection with bacterial wetwood. Photo by Mike Schomaker, Colorado State Forest Service, Bugwood.orgReprinted from "Bacterial Wetwood and Slime Flux of Landscape Trees" By John Hartman in the current edition of the Kentucky Pest News. For the article with associated pictures go to http://www.uky.edu/Ag/kpn/kpn_09/pn_090804.html
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