Many species of fungi decay the wood of living maples. Often, the decay is not noticed or appreciated until trees are so weak that they break in strong winds or under heavy snow. Most fungi that cause decay produce fruiting bodies (mushrooms, conks) on affected trees before the trees are structurally hazardous, and one should learn to recognize these signs of potential trouble.

When decay is detected in trees near houses, vehicles, or pedestrian traffic, it should be carefully monitored by a professional arborist. Seriously decayed trees may have to be pruned or removed.

Some common wood decay fungi on maples follow.

1) Ganoderma applanatum

This fungus produces large (up to 50 cm wide) platelike conks on living or dead trees. The conks are brown to gray on top and white underneath. The white bottom tissue is easily removed by scraping with a stick or other hard object, and the conks have thus become a favored medium of some artists.

2) Steccherinum septentrionale

This organism produces a massive, fleshy fruiting body on trees in which it is decaying wood. The fruiting body is white to light brown and has numerous layers. Each layer has many tiny spines projecting from its underside. S. septentrionale is most common on sugar and silver maples.

3) Dadaelea unicolor

D. unicolor is characterized by large numbers of thin, tough shelflike structures projecting from cankers on stems and branches of affected trees. Each fruiting body is slightly hairy when fresh and has multicolored concentric zones on top. Undersides are cream to light gray with many irregularly-shaped pores. Because this fungus is able to kill bark as well as to decay wood, it can be lethal to trees. Red, sugar, and Norway maples are its most common hosts in the genus Acer.

4) Oxyporus populinus

This fungus usually fruits in cankers and other wounds on living trees. The fleshy, shelflike fruiting bodies are 4-12 cm wide and project out from the tree 2-8 cm. They are pale yellow when young with coarsely hairy tops and pores beneath. As they age, the fruiting bodies turn white but algae growing on them gives a greenish tint.