Heartwood Rots of Trees

The sulphur shelf fungus (Laetiporus sulphureus) has been fruiting on the trunks of trees on ISU campus, and in the woods elsewhere. Their large bright yellow and orange overlapping fruiting structures mark them as one of the most colorful of the wood rotting shelf fungi that fruit on living or dead trees. "Well ok," you might say. "We are happy to have such a lovely fungus recycle the wood from a dead tree, but what does it mean to have a shelf fungus fruit on a living tree? Should we be concerned? Is there anything we can do for the tree? Is it likely to spread to other trees?" As for so many other questions, the answer is "It depends…."

The sulphur shelf fungus is one of many fungal species that attack the heartwood of trees, and manifest its presence in the form of bracket-like or shelf-like fruiting bodies on the tree's trunk. Many of these fungal species are somewhat particular about which tree species they can become established on. The sulphur shelf favors oak trees in the midwest, while Fomes fomentarius is partial to birch. If one tree has heartwood rot, it doesn't mean that other trees will get it, even if they are of the same species. This is because the trees are well protected by their bark. The fungus must be blown (as spores) or carried (e.g. by insects) in through a wound in order to become established. Once established, the fungus will slowly decompose the heartwood, the dead wood at the center of the tree (which is most of the tree). The key word here is "slowly". By the time that a fruiting body is produced on the trunk of a living tree, it usually means that the fungus has been there for years, and is too well established for us to do anything about it. This is bad news for the timber owner who had hoped to harvest sound wood. However, the age that each tree species should be harvested prior to rotting by a heart rot fungus is now well-known, and can be obtained easily from the state forester.

For the homeowner, the good news is that heartwood rots do not usually kill trees in a big hurry. Under normal circumstances, they can live with a tree for decades before the tree succumbs, usually to something else. The rotting of the interior wood can weaken the tree, however, making it more vulnerable to breakage, wind throw, insect damage, and other diseases. The concern about heartwood rot for the homeowner is the potential injury or damage caused by falling limbs, or the downing of the whole tree. One must take these things into consideration when making the excruciating and potentially expensive decision about removing a tree with heartwood rot.

If the tree is close to a habitation, removal may be a necessary precaution. If it is on an acreage, and not likely to do irreparable damage should it fall, it may be worth considering leaving it alone. Many wildlife, such as the red-headed woodpecker, depend on such rotten hearts to make their own habitations. Shelf fungi that cause sapwood rot, butt rot and root rot are of more serious concern, and will be considered in another HHPN.

Laetiporus sulphureus fungus fruiting on a downed log

Laetiporus sulphureus fungus fruiting on a downed log


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