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Wetwood, also known as slime flux, is a very common bacterial disease that occurs in many kinds of trees. Nearly all elm and poplar species are affected, as are numerous other trees including crabapple, beech, birch, maple, dogwood, horsechestnut, linden, oak, pine, redbud, sycamore, and tuliptree. Wetwood is normally not a serious disease. However, a tree with a chronic case of wetwood may decline in general vigor.

Wetwood appears as a dark brown to black water-soaked area in the wood. A sour-smelling liquid seeps out of the tree, frequently at tree crotches, cracks in the bark, or pruning wounds. The liquid is colorless or pale while inside of the tree, but darkens when exposed to air. When the liquid dries, it leaves a pale gray to white crust on the bark.

Wetwood is caused by a number of species of bacteria that enter the tree through wounds. Since bacteria associated with wetwood are common in soil, root wounds are probably a major point of entry. Infection is usually confined to the inner sapwood and heartwood. Gases and liquid are produced in infected trees from the fermentation action of the bacteria. The gases within the trunk cause a buildup of pressure that forces the liquid out of the tree. The build-up of gases fluctuates during the year; the highest pressure is usually found in summer when the bacteria are most active. This oozing material is toxic to plants and could transfer the disease to a new stem or branch. If this liquid is transported internally through the vascular system up to the branches, wilting and/or defoliation may occur. Occasionally, branch dieback also occurs. However, in most healthy trees, this disease is not considered to be life threatening.

There is no satisfactory control for wetwood. To help prevent infection, avoid wounding trees. Fertilizing trees to stimulate growth is reported to lessen wetwood severity, however, vigorous trees may be more susceptible after fertilization than trees that are stressed. Remove dead and weak branches. If pruning an infected tree, sterilize pruning tools between cuts. The once-common practice of boring a hole into the trunk and inserting a short length of pipe to relieve gas pressure and direct the flow of the fluid is no longer considered to be a useful procedure. On the contrary, today it is believed to be more harmful to the tree than beneficial. There are some extreme cases where wetwood may cause leaf wilting, premature leaf drop, branch dieback or death of bark tissue, which may warrant the installation of a drainpipe.