Michele Grabowski, University of Minnesota Extension Educator, has conducted several classes for the Chisago County Master Gardeners. This is because she has such a variety of expertise and is good at all of them. In one of Michele’s articles, she reminds us that when there are frequent rains, puffballs form on the lawns, and a mushroom type fungus grows on some tree trunks. These fungi show up in a number of shapes, sizes, and colors. The mushroom type fungus could come from the trunk, from the root flare of the tree or from the roots. In all cases, fungi growing directly on a live tree tell the tale of heart rot within.
Michelle said that in order to understand heart rot, you must understand a little bit about the wood within a tree trunk. Trees have several different kinds of wood within their trunk. Sapwood is composed of living cells that move the sap through the tree, store extra energy, close off wounds, and actively fight invading microorganisms. Other trees form a second type of wood known as heartwood at the core of their trunk. Heartwood cells are dead cells that serve primarily to add structure support to the tree.
They contain a number of toxic chemicals that protect the heartwood from wood decay fungi. How well chemicals protect heartwood varies from tree to tree. Trees like cedar and redwoods are so effective at defending against wood rotting fungi that their wood is highly valued for use in wood products like lumber.
Heart rot can cause decay in both heartwood and sapwood. Many different fungi can cause heart rot and are often seen on rotting logs, on dead trees, as well as on living trees. The mushroom, shelf fungi, and other interesting fungal structures that emerge in wet weather are spore-producing structures of the fungus, and are generally known as basidiocarps. If they are observed on a live tree, it indicates that there is rot within the tree.
Heart rot fungi are not aggressive and are unable to infect a tree through intact bark. The fungi take advantage of wounds from lawn mowers, weed whips, fire scars, deer rubbing, rodent chewing, frost cracks, broken branches and other injuries invading the sapwood and heartwood.
Trees often survive many years with heart rot and can go unnoticed because the tree can continue to grow with no symptoms of disease or decline in the canopy. The decay caused by many heart rot fungi progresses very slowly, often at a rate of about three inches a year.
A tree with signs of heart rot does not need to be removed immediately, but should be examined to determine how structurally sound it is.