Reports on Plant Diseases

Wetwood wilt and dieback of elms are most common in the Midwest and Great Plains. It is associated with the death of branches in large cottonwoods and with the dieback and premature death of Lombardy poplars, some as young as two years. Most trees die back slowly over a period of several years or more, however, small trees may wilt and die suddenly.

Symptoms vary with location suggesting that environment influences disease development. Trees in the western United States show more variability in internal disease development and much less bleeding and symptom expression than trees in the East and Midwest.

Wetwood is normally not serious in most trees but as a chronic disease it can contribute to a general decline in tree vigor, especially of older trees growing under adverse conditions. Wetwood in landscape trees is usually unimportant except for the disfiguring appearance of light or dark streaks where liquid seeps out of cracks and wounds and flows down the bark (Figure 1). As the bleeding occurs, the liquid or flux flows down the trunk, wetting and soaking large areas of bark. As it dries, a light gray to white incrustation is left (Figure 1). If the exudation continues over a period of time, the bark has a two-colored vertical column of brown with strips of white at the margins. This liquid is toxic and commonly causes localized death of the cambium at the base of a pruning cut (Figure 2) and around trunk cracks. The liquid on the bark surface becomes contaminated with mixed populations of many different kinds of airborne bacteria, yeasts, and filamentous fungi that give it a slimy texture and often a fetid odor. The gray to brown, foamlike foul liquid is called slime flux or wetwood slime. The slime flux may prevent the healing of wounds by retarding or preventing callus formation. Fluxing occurs in Illinois from April to December but is most conspicuous during the summer, ceasing during the winter.

Figure 2. Fluxing sap of wetwood-affected tree - sufficiently toxic to prevent callus formation and kill bark at base of pruning wound (IL Natural History Survey photo).

Extreme gas pressures, up to 60 pounds per square inch (4.22 kg/cm2) have been detected in discolored wetwood in elms, while gas pressures up to 0.7 kg/cm2 are common. Pressures fluctuate seasonably with the highest readings from May through September. The high gas pressures frequently force liquid to seep from wounds and flow down the bark (Figures 1). Gas is produced in wetwood-affected trees from the metabolic activity of bacteria. The gas typically consists of 45 to 60 percent methane, 0 to 7 percent oxygen, 6 to 16 percent carbon dioxide, 23 to 34 percent nitrogen, and 1 percent hydrogen. The air in normal wood contains about 19 percent oxygen, 75 percent nitrogen, and up to 5 percent carbon dioxide. Methane and hydrogen are produced by certain bacteria under anaerobic conditions and do not occur in normal wood. Enterobacter cloacae, and probably other bacteria, ferment carbohydrates, producing carbon dioxide which is converted to methane by further bacterial action. The nearly anaerobic condition of wetwood prevents decay by wood-rotting fungi.

The first external sign of wetwood is usually the bubbling and seepage ("bleeding" or fluxing) from wounded tissue in V-shaped branch crotches, wounds made by removal of branches, injection holes (for fungicides, insecticides, or nutrient uptake), and trunk cracks, ribs, or beaks (Figures 1 and 3). Internal gas pressure commonly reopens old wounds. The sour liquid is colorless to tan as it oozes out and may contain up to 10 billion bacteria per milliliter.

Figure 5. Brown streaks produced by wetwood in young sapwood of elm branches - can be confused with similar streaking caused by other wilt diseases of elm (IL Natural History Survey photo).

The water content of affected trees varies from near normal (in white fir) to twice normal (in elms and cottonwoods). These differences, together with a raised carbonate content, are sufficient to account for the raised pH and for movement of water along an osmotic gradient from normal wood into a wetwood zone.

Foliage in the tops of trees severely affected by bacterial wetwood sometimes curl, scorch, droop, turn yellow, and defoliate from mid to late summer; scattered branches die back or the entire crown gradually declines over a period of years. Foliar wilting in elms is generally believed to occur when quantities of wetwood liquid are forced under pressure into the xylem vessels and thus are carried to the crown. The leaves first curl upward along their margins, the petioles wilt and the leaves drop off while still green or turn a dull green-brown or bronze before they fall. Slowly wilting leaves turn yellow or brown before dropping. In some cases, the leaves turn yellow or a dull green-brown between the veins and along the margins of leaves. Wilting is much more common in young trees; older trees are most likely to develop a general decline in vigor or a branch dieback in the upper crown (Figure 4).

In elms and poplars with wilting twigs and branches, wetwood appears as gray-brown streaks or bands in the current-season wood and often resembles wood stained by vascular wilt fungi such as Dutch elm disease or Verticillium wilt. The streaks extend from the trunk into small branches and twigs and merge to form solid brown rings in the wood of one or more seasons (Figure 5).

Figure 6. (Top) FLuxing can be stopped at pruning wounds, (center) at cracks in branch crotches, (bottom) through cracks in bark of trunk by boring holes into diseased wood where toxic sap and gas have accumulated. (Center and bottom left). Hole should be slanted upward so toxic sap will flow out through opening. Short piece of threaded iron or plastic tubing inserted into hole far enough to be firm., will carry toxic sap away from tree (Lenore Gray drawing).

Awareness of bacterial wetwood is important to the lumber industry. Abnormal color and moisture loss cause the lumber to be devalued. The affected wood of oak and hemlock commonly cracks along or perpendicular to the growth rings. The crushing strength and toughness of poplar wetwood is inferior to normal wood. Such weakness is believed to be due to enzymatic degradation by wetwood bacteria of part of the binding substance(s) between cells. In standing trees this probably promotes cracks caused by bending action, differential expansion along temperature gradients, as well as the freezing and expansion of water within the degraded part of the cell wall.

When frozen, the wet zones in wood are more rigid than normal wood. A log with wetwood often shows no defect until the lumber sawn from it shrinks and/or cracks during kiln drying. Even when no shrinkage or cracks develop, wetwood dries only about one-third to one-half as rapidly as normal wood and thus requires twice as much energy.