UTAH PESTS Staff
Utah Plant Pest Diagnostic Lab
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Bacterial Wetwood on Cottonwoods
Over the past two years, we have had reports of cottonwoods of all ages dying suddenly or in decline in parts of Utah (Cache, Iron, and Salt Lake counties). After several unsuccessful attempts by plant pathologists at Oregon State University to isolate plant pathogens from the wood, we came to the conclusion that the problem was simply, bacterial wetwood, which is difficult to pinpoint to one causal organism. If you are familiar with bacterial wetwood, the symptoms we have seen are not as clear-cut as you might think.
Bacterial wetwood is a disease that affects many hardwoods and some conifers. In general, it is an asymptomatic condition in trees, but in the case of cottonwoods (and globe willows and sometimes elms), it is lethal. Naturally occurring soil and water bacteria enter the tree through root wounds or are transmitted from tree to tree through natural root grafting or by wood-boring insects. The bacteria multiply and occupy the xylem, killing and staining the wood and causing a water-soaked condition. In the absence of oxygen, these bacteria degrade pectin, starches, and sugars via fermentation to produce methane and carbon dioxide. High gas pressure and moisture content forces fermentation by-products to outer limbs, and in the case of cottonwoods and globe willow, to the cambium, causing localized death. Fluid that seeps out of wounds or branch stubs is colonized by airborne bacteria and yeasts, resulting in the foul-smelling and toxic “slime flux.”
Affected cottonwoods in Utah do not express the classic slime flux, but have several small (quarter-sized) areas of tar-like ooze on the bark. Removal of the bark over these oozing areas reveals a brown discolored cambium and fermented odor. The xylem beneath the dead cambium is streaked red. In the case of the dying cottonwoods in Utah, the symptoms indicated multiple entry points through roots and by spread of flatheaded borers. Trees contained both basal and upper stem cambial wetwood cankers that, in advanced stages, coalesced into a single massive formation. Some trees developed cracks in the upper limbs due to the intense pressure within the tree. Symptoms of disease expression started with yellowing, wilting, and scorching foliage in parts of the canopy, branch dieback, and failure to leaf out in spring. Death eventually occurred when the cambium was girdled at the base of the tree.
Most of the cottonwoods were growing in environments that favors spread, including monoculture plantings that allow for root to root spread, compacted soil, surface roots exposed to wounding, and in some instances, drought conditions or excessive water. Most trees also had a high incidence of cytospora canker that probably contributed to tree death. Dead trees ranged in age from 20-60 years.
Although isolating bacteria from wood is very difficult, there have been reports of over 13 genera of bacteria associated with wetwood tissue. Species from two genera, Clostridium and Enterobacter, are most prevalent, but cannot be assumed to be the causal agents of wetwood because they have never been tested for ability to induce wetwood in laboratory conditions.
There are no chemical treatments to prevent or cure trees with bacterial wetwood. Trees may have become affected by wetwood at a very young age (less than 10 years), and not show symptoms for many years. Areas that have important cottonwoods should be managed for prevention of disease. The most important factor is to prevent damage and stress to roots. Trees should be given optimal watering and fertilization.
Shink, Bernard, J.C. Ward, and J.G. Zeikus. 1981. Microbiology of wetwood: role of anaerobic bacterial populations in living trees. Journal of General Microbiology. Vol 123: 313-322.
Johnson, D.W., and J.W. Riffle. 1986. Wetwood in elm, cottonwood, and mulberry. In: Diseases of Trees in the Great Plains. USFS General Technical Report, RM-129, pp 64-65.
Scott, Eileen. 1984. Populations of bacteria in poplar stems. Eur. J. For. Path. Vol 14: 103-112.
Featured Picture of the Quarter
The ascus (“skin bag”) shown at left is a cell produced by a group of fungi called ascomycetes that contains spores called ascospores usually eight spores (in this case, you can see six of the eight spores). In powdery mildew fungi, the number of asci range from one to a few dozen within a chasmothecium depending on the powdery mildew species. Chasmothecia typically form in the fall to survive the winter, and look like ground pepper flakes scattered on infected plant tissue.
-Image by Claudia Nischwitz