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Figure 1. Wood decay fungi help recycle organic components of dead trees.
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One of the most important tree problems in the urban landscape is wood decay. Wood decay is the progressive deterioration of woody tissue in both living and dead trees. Of course, wood decay is essential in the recycling of organic components of dead trees. Unfortunately, wood decay in a living tree in the middle of a park is not good. Advanced decay can result in limb or trunk breakage during adverse weather. |
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Figure 2. Wood decay in silver maple. Note cylinder of decay in center of trunk is surrounded by healthy wood. |
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Many factors are involved in the dynamic process of wood decay. Wood decay fungi must first compete with other non-decay microbes at the wound site. Once established, these fungi must also breach physical and chemical defense barriers within the tree. Wood decay in trees primarily is caused by a small group of fungi capable of degrading the wood components of cellulose and lignin. These fungi often enter trees through wounds on branches or trunks caused by mowers, pruning saws, etc. Wood decay can alter both the chemical and physical properties of wood, resulting in a loss of wood strength. Therefore, it is important to identify and remove trees with extensive wood decay, especially in parks, along streets, or in other high hazard areas. Identifying trees with wood decay can, at times, be difficult. This is because most of the decay occurs in a cylinder in the center of the trunk. This decay may be hidden from view by a ring of healthy wood and bark. On some trees, there may be no external evidence of advanced root or trunk decay. Nevertheless, indicators of decay are usually present on trees. It is important to recognize these decay indicators before serious personal or property damage occurs because of branch or trunk breakage. Evaluate large trees periodically for overall tree health. Trees with branch dieback, limb breakage, or general unthriftiness may be symptoms of numerous maladies. However, they can also be associated with wood decay. When pruning out dead limbs, look at the cross sectional cut to see if the wood is punky or decayed. If it is, continue to check for decay further back on the branch. This is especially important if it appears that the decay continues into the main trunk. Any open, hollow areas in the trunk are indicators of poor tree health and potential wood decay. These sites may be partially covered by dead bark. Carefully peel away dead bark to determine the extent of the decay. |
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Figure 3. Open wounds at tree base often signal extensive wood decay. |
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The best indicator of decay is the presence of fungal fruiting structures on the trunk or roots. These may be soft, mushroom-shaped bodies or hard, hoof-shaped structures called conks. The underside of the fruiting structures may have a gill or pore-like surface. The shape, color and texture of the conks are important in species identification. |
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Figure 4. Soft, mushroom-shaped fruiting structure. |
Figure 5. Hard, hoofed-shaped conk on trunk. |
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Figure 6. Multiple conks on tree trunk indicate extensive decay. |
Figure 7. Conks may also be located on roots or near the base of the tree. |
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Conks are the "tip of the iceberg" and signify a column of decay inside the trunk. The expanse of the decay within tree can sometimes be determined by the type of conk present. However, this can vary with tree species and environmental conditions. Submit fungal fruiting structures to a diagnostic lab for identification. This may help determine the seriousness of the decay. Also, there are mushrooms that are associated with tree roots that are beneficial and do not cause decay. Make sure you have the fungus properly identified! |
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Figure 8. Not all mushrooms associated with trees indicate wood decay. Be sure to properly identify the fungus. |
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Several methods are available for determining the extent of the decay column within the tree. These include drilling small holes in the trunk to measure resistance, and using electrical resistance or ultrasound meters. It may be difficult to interpret results of these tests. Furthermore, some of procedures can result in tree injury. Therefore, only trained arborists should attempt them. Wood decay can be prevented by avoiding unnecessary wounds and proper pruning. Unfortunately, there is no cure for trees with advanced decay symptoms. Currently, fungicide applications are not effective at suppressing the decay process. Trees with advanced decay are a hazard. Do not take the risk of keeping any tree with evidence of internal decay even though that tree may still be structurally sound unless it has been thoroughly examined by an arborist. Any tree in a high traffic area (pedestrian or autos) and exhibiting conks or extensive symptoms of wood decay should be removed. |
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Web updated 9/01/06 |