Wetwood is a non-decaying, bacterial disease occurring in trunks, roots, and branches of many tree species, particularly elms. In severe cases, wetwood may disrupt the vascular flow causing branch dieback. However, this is rare and wetwood is mainly considered a cosmetic problem of the bark.

Bacteria associated with wetwood are commonly found in soil and enter through wounds. The presence of bacteria causes discoloration of the inner wood as well as increased moisture content, alkalinity, and gas pressure. Some of these changes give the infected wood resistance to wood decay organisms, but do not seem to change the strength of the infected wood.

During warm weather, bacterial metabolic activity increases, resulting in increased gas pressure which forces a liquid out of cracks, wounds, or branch crotches. The most noticeable symptom of bacterial wetwood is this liquid, called slime flux, which exudes from openings in the tree bark. Slime flux is tan or colorless, turns dark on exposure to air, and may be foamy or have an odor. As it flows down the trunk it dries a white-gray color, which is often seen as light discolored strips running down elm tress. The flux is toxic enough to kill grass at the base of the tree and prevent callus formation which is necessary for wound healing.

Wetwood on cottonwood tree Photo: U of MN Plant Disease Clinic