What is the black substance on Maple trees?

February 15, 2022 Ask the Expert

Question:

I have many maple trees with various amounts of a black substance on their trunks. I am assuming it is a fungus, but am not sure. On trees with just a little, it appears circular and is located near the base of the tree. On trees with a lot of fungus, it almost fully covers the trunk and extends upward towards the tree canopy. The leaves of the trees did have some black spots on them in the summer, but it was very minimal. The other thing noticed was that when one of these trees was cut, a white growth appeared on the cut edges of the wood and the outside bark. I have many trees over approximately one acre that have some degree of this fungus. Is it hurting the trees? Is there any organic remedy that I could use to reduce/eliminate this fungus?

Answer:

Lynne M. Holland, Horticulture and Social Media Professional

First, the wood on the ground has mold and fungi that are growing as the logs decay. This is normal and is not related to the issue on the live maples. Second, the black spots on the leaves in summer are likely tar spot. The amount of tar spot that develops usually depends on whether or not a summer is particularly wet or dry and rarely affects the health of the tree overall.

Finally, we believe you have sooty mold on the bark of the live trees perhaps due to minor splits in the bark from insects or the sun. Sap leaks out allowing mold to grow. There is no remedy and no need to scrap it off; however, if the canopy on the trees starts to die back or you want to harvest the wood, we suggest that you have an arborist look at the trees. Otherwise, in a natural setting like this it is fine to let nature take its course. Here are two sites that will offer further information sooty mold 1 and sooty mold 2.