Bacterial Wetwood and Slim Flux

Compiled by Ned Tisserat, Colorado State University:

Taxonomy

Domain	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Enterobacteriales
Family	Enterobacteriaceae
Genus	Enterobacter

Scientific Name

Enterobacter nimipressuralis

Scientific Name Synonyms

Erwinia nimipressuralis

Common Name

bacterial wetwood

Hosts

Bacterial wetwood is a disease most frequently seen in poplars (aspen and cottonwood), willow (especially globe), and elm. The disease also affects apple, ash, beech, birch, cherry, fir, honeylocust, linden, maple, mountain ash, mulberry, oak, sycamore, pines, and plum.

Diagnosis and Damage

The causal agents of bacterial wetwood have not been conclusively identified. However, several species of bacteria in the genera Enterobacter, Klebsiella, and Pseudomonas, which are often associated with wetwood, are thought to be directly involved. Research using affected elms suggests that an association of bacterial species, and possibly yeasts, act together to produce the complex of symptoms observed.

Wetwood typically develops in the center of the tree but may be found in the cambial area of stressed cottonwoods and globe willows. Wetwood is most easily recognized by the presence of a liquid that oozes from wounds, crotches, branch stubs, frost cracks, or other weak points in the wood or bark. As the liquid flows down the bark, vertical dark or light streaks remain. Oozing sap is initially colorless. After colonization by various bacteria and yeasts, the liquid becomes slimy and is often called slime flux. Foliage of severely affected trees sometimes wilts, and branches, sections of the trunk, or the entire tree may prematurely die. Symptoms of nutrient deficiency may appear due to poor water movement within affected trees.

Wetwood is important in forest trees harvested for lumber. Abnormal color and moisture in wetwood cause lumber to be devalued. Warping, increased drying time, and some cracking and splitting problems occur in boards cut form wetwood. The slime is toxic to cambial tissue so wounds can not close since the callus is killed. The wetwood also occurs in the cambium or just under the bark killing branches and stems of stressed cottonwoods and globe willows.

Biology and Disease Cycle

Very little is known about the bacteria and other microorganisms associated with wetwood and their transmission to healthy trees. Bacteria are assumed to enter trees through roots and thus it is hard to prevent. Insect vectors do not appear to be involved.

Conditions Favoring and Management of Disease

There are no chemical treatments effective for curing or preventing wetwood. Plant stress especially drought appears to favor disease development and appearance of symptoms. Adequate water during the summer months decreases wetwood symptoms. Recently transplanted trees may show symptoms of wetwood, especially in the cambial area, if not adequately established. In trees where oozing liquid has damaged the bark, the dead bark can be cut away to allow for drying and better wound closure. Use a smooth, continuous cut, to expose an elliptical area of bare wood. If this area exceeds 30 to 50 percent of the trunk circumference, the tree may not close the wound. The use of drain tubes is no longer recommended because the wet wood bacteria can move out into healthy tissue and decay fungi and insects may move into the tree.

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