Trees can take a lot of adverse conditions. Imagine you are a big leaf maple, planted between the sidewalk and the street. Not only is there limited area in which to grow your roots, but there is reflected heat off the street paving, sidewalks pushing against your roots, damage from trucks that didn’t quite make it under your lower branches, your upper branches are pruned to keep them away from the power lines, and to top it all, you probably don’t get much water during the heat of the summer.

All these insults will eventually accumulate over your life, and if conditions don’t improve, there may come a time your defenses just aren’t as good as they used to be and when that lawn mower whacked into your trunk the last time the grass was mowed, you just didn’t have the ability to heal properly. This is the perfect opening for a visit by the decay fungus Ganoderma.

Because Ganoderma likes openings: it enters through wounds. The fungus moves through the air by way of spores. Spores are microscopically small reproductive units of the fungus, somewhat similar to seeds of plants. The spores come from a Ganoderma fruiting body, called a conk.  Conks have pores on their undersides, where spores are produced by the millions. So many spores are produced that anything beneath a conk shedding spores will be coated with what looks like fine cocoa.

The spores move through the air, or perhaps on the bodies of insects that feed on or around a conk. Most of the spores don’t land on a suitable surface, but if they find a nice, open area on a tree where the bark is missing, and especially if other microorganisms have also make the opening their home, the spores can germinate and grow into the wood. Alternately, the fungus may move from one infected tree to one in contact with its infected roots. The fungus then slowly, and year by year, begins to break down the cellular components that make the wood cohesive and strong, gaining its nutrients from the wood. If this continues over enough years, the tree will become unstable, and is at risk to fall over. The effects of a tree weighing several tons falling on a structure is not something one wants to experience first-hand.

Poplar smashes bench

A metal lawn chair was crushed by this poplar tree when it fell after a strong wind.

Poplar and Ganoderma

This is the base of poplar tree, where the extent of the decay is obvious. Note the conk on the trunk in the upper left of the image.

Ganoderma is primarily a root rotting fungus, but it can and does move up the roots into the lower trunk of living trees such as apple, cottonwood, elms, oak, sycamore, and willow, as well as a wide assortment of other broadleaf trees. Other species occur on conifers, sometimes only on dead trees. There are several species of Ganoderma, and they all cause wood rot.

A gallery of Ganoderma growing on a variety of live trees and causing wood rot. © Photos from iNaturalist.

Some trees show no obvious symptoms before they fail. Other infected trees can sometimes give clues that all is not well by having reduced branch growth, fewer leaves than in previous years, yellowing leaves, and leaves that drop early in the year. Another clue is the emergence of one or more conks near the soil, which have a flat dark brown surface above with a white margin, and a light colored, pore filled surface on the underside. If conks are present, that means the tree likely has extensive decay.

maple and Ganoderma

The conk at the base of this bigleaf maple is visible only because of the white band at its edge. Conks are perennial, and continue to grow from year to year. The tree was planted in the space between the street and sidewalk.

Old trees, trees that are repeatedly wounded by mowers, and trees that are both old and with many old wounds, especially large trees near houses or other structures, could be at risk of failure. If you are concerned about the possibility of root rot in one of your trees, it is important to have someone trained in assessment of hazard trees come out to evaluate the situation.