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Disease name:  Wetwood

Pathogen name: Wetwood is caused by various species of bacteria.

Hosts:  Wetwood, also referred to 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, horse chestnut, mulberry and oak. Wetwood is normally not a serious disease. A tree with a chronic case of wetwood, however, may decline in general vigor over time.

Wetwood appears as a dark brown to black water-soaked area in the wood. A sour-smelling liquid called slime flux, 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 wet, the liquid can become frothy and very messy in appearance and can attract insects. 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 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. The oozing slime flux 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. While potentially damaging, this disease is not considered to be life threatening in most cases.

There is no satisfactory control for wetwood. Proper care of the tree, in terms of watering, and fertilizing, if needed, will help to maintain the overall health of the tree. To help prevent infection, avoid wounding trees. 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.

For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or