Even if you don’t know much about mushrooms, you can identify a shelf mushroom: it’s the wide, flat fungus growing out of a tree trunk. The mushrooms can be brown, gray, green, white, or even orange. Shelf mushrooms do good by aiding in the decomposition of wood. It is said that if there were no fungi of this type, there would be no forests at all, because the forests would become choked with branches and stumps that didn’t decompose. These mushrooms are also diagnostic—they signal that a tree has internal rot.
Damage
If shelf mushrooms have appeared on the trunk of your tree—often at the base of the tree or on the root flare—it’s a sign that your tree is rotting inside. Unfortunately, by the time the shelf mushrooms appear, the rot is quite advanced, and there’s nothing you can do about it. That said, some trees will stand for years even though they are suffering from heart rot, root rot, or butt rot. There’s no way to predict when the tree will fall, but you can be sure that it is losing strength.
Control
Removing the mushrooms won’t make the rot go away. The best strategy is to keep your tree healthy so that rot doesn’t begin. The pathogens that cause rot in trees enter through wounds in the bark. In a garden, injuries are commonly caused by weed trimmers and lawnmowers. But animals, insects, wind, and lightning can also cause damage that opens a path for infection. A circle of mulch around your tree will keep the lawnmower at a safe distance. Try not to dig near your tree. Most of a tree’s roots are in the upper 18 inches of soil, so they are easily damaged. Pruning small limbs is safer than pruning large ones, as the injury will heal faster.
—photo courtesy of the Kemper Center for Home Gardening at the Missouri Botanical Garden.