Wetwood
Wetwood, also known as slime flux, is a condition in which bacteria infuse water into the heartwood of trees, often creating an anaerobic environment. Wetwood often supports large populations of several different bacterial genera, none of which are known to possess any host specificity (Sinclair and Lyon 2005). Wood harboring these bacteria often has a strong, pungent odor and can range in color from pinkish, yellow, olive-green, to dark brown. Over time, these bacteria may expand outward from the heartwood into the sapwood.
Hosts
Wetwood occurs in nearly all species of elm (Ulmus) and poplar (Populus). In addition, true fir (Abies), maple (Acer), mulberry (Morus), sycamore (Platanus), oak (Quercus), willow (Salix), and hemlock (Tsuga) can harbor wetwood bacteria (Sinclair and Lyon 2005).
Symptoms & Signs
Wetwood bacteria are not tree pathogens although they have been treated as disease agents. However, there are cases where wetwood-induced bacterial growth appears to harm trees. Yet, for most affected trees the presence of wetwood appears inconsequential to their overall health. Studies have indicated that wetwood bacteria may provide some level of protection against wood-rotting fungi (van der Kamp et al. 1979; Worrall and Parmeter 1983; Coleman et al. 1985). The lack of available oxygen in the saturated wood can prevent wood-rotting pathogens, like Armillaria for example, from establishing in the heartwood. Wood tissues colonized by wetwood bacteria tend to exhibit higher decay resistance compared to uncolonized wood (Sinclair and Lyon 2005).
Wetwood bacteria are both free living and common in soil and water (Sinclair and Lyon 2005). They must invade trees to establish but there is no evidence that this process causes necrosis to any live tissues in the roots, main trunk or branches. One study did identify an association between the presence of wetwood and fungal decay in western hemlock (Shaw et al. 1995). However, there is no evidence that the establishment of wetwood bacteria facilitates infection by wood-decaying pathogens. Wounds are the most common source of entry for wetwood bacteria, and these may simply be utilized by other organisms as well. Wetwood often develops in the roots or in the lower trunk of the tree, but over time it may be present high in the trunk or in major canopy branches (Murdoch and Campana 1981). Once established, wetwood bacteria colonize the heartwood tissues where they may persist for decades without ever causing any harm to the tree. Wetwood bacteria do produce enzymes that can degrade primary cell walls and other intercellular material, causing some weakening of the wood. But they lack the ability to cause decay that would result in reductions in wood density. At times, it has been shown that wetwood fluids under pressure expand radially into functional sapwood tissues, clogging xylem vessels and inducing water starvation (Hepting 1971; Murdoch and Campana 1981). These symptoms would manifest as leaf scorch, wilt and a general canopy dieback. In elms, the symptoms may resemble Dutch elm disease, complete with vascular staining.
Within colonized heartwood tissues, wetwood bacteria help to infuse water into the wood, thriving in the anaerobic environment. Their metabolic actions lead to increases in gas pressure, primarily from methane, increased cation (Na, K, Ca, and Mg) concentrations, and the formation of acetate, ethanol, and isopropanol, among other fermentation products (Schink et al. 1981; Murdoch et al. 1987; Xu et al. 2001). Depending on the host and bacterial community, pH can be higher or lower compared to uncolonized wood tissues (Murdoch et al. 1987; Jeremic et al. 2004). When accumulated gas pressure is released, through a branch crotch, seam, or pruned branch, large volumes of fluid can be released. The fluid is a mixture of bacterial and yeast cells and water. Free-living bacteria, yeasts and filamentous fungi on the surface of the bark are stimulated and grow within the fluid as well. This thick, slime-like fluid is often dark in color as it streams down branches or the main trunk. This is how the term “slime flux” became popularized, especially when the fluid was forcibly sprayed out of pruning cuts. Wetwood slime stains the bark and when dry it appears gray, pale brown to yellow in color. Stained bark from wetwood fluids is commonly observed on mature elms in the landscape.
Management
There is no preventive treatment or way to eliminate wetwood from an affected tree. Because wetwood is not a disease, there is no need for preventative treatment. At one time, the installation of drainpipes in the lower trunk was performed to drain fluid from the heartwood (Hamilton 1980). However, drilling holes into the colonized heartwood where wetwood bacteria are present breaks the natural compartmentalization barrier and opens it to the spread of wood decay fungi within the trunk. As a result, this practice is no longer performed to avoid needless wounding.
Literature Cited
Coleman JS, Murdoch CW, Campana RJ, and Smith WH. 1985. Decay resistance of elm wetwood. Canadian Journal of Plant Pathology 7(2): 151–154. https://doi.org/10.1080/07060668509501492
Hamilton WD. 1980. Wetwood and slime flux in landscape trees. Arboriculture & Urban Forestry 6(9): 247–249. https://doi.org/10.48044/jauf.1980.058
Hepting GH. 1971. Diseases of Forest and Shade Trees of the United States. Washington, DC: USDA Agricultural Handbook No. 386.
Jeremic D, Cooper P, and Srinivasan U. 2004. Comparative analysis of balsam fir wetwood, heartwood, and sapwood properties. Canadian Journal of Forest Research 34(6): 1241–1250. https://doi.org/10.1139/x03-287
Murdoch CW and Campana RW. 1981. Bacterial Wetwood. In: Stipes, R.J.; Campana, R.J., eds. Compendium of Elm Diseases. APS Press, St. Paul, Minnesota. 31–34.
Shaw DC, Edmonds RL, Littke WR, Browning JE, and Russel KW. 1995. Incidence of wetwood and decay in precommercially thinned western hemlock stands. Canadian Journal of Forest Research 25(8): 1269–1277. https://doi.org/10.1139/x95-140
Schink B, Ward JC, and Zeikus JG. 1981. Microbiology of wetwood: role of anaerobic bacterial populations in living trees. Microbiology 123(2): 313–322. https://doi.org/10.1099/00221287-123-2-313
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
van der Kamp B, Gokhale AA, and Smith RS. 1979. Decay resistance owing to near-anaerobic conditions in black cottonwood wetwood. Canadian Journal of Forest Research 9(1): 39–44. https://doi.org/10.1139/x79-007
Worrall JJ and Parmeter JR. 1983. Inhibition of wood-decay fungi by wetwood of white fir. Phytopathology 73(8): 1140–1145. https://doi.org/10.1094/Phyto-73-1140
Xu Z, Leininger TD, Lee AWC, Tainter FH. 2001. Chemical properties associated with bacterial wetwood in red oaks. Wood and Fiber Science 33(1): 76–83. https://wfs.swst.org/index.php/wfs/article/view/1885