Q: I have a maple tree on which the trunk is turning black. Should I be worried?
A: Art Morris, Master Arborist for Bartlett Tree Experts says he sees this quite a bit. Dark staining of maple tree trunks is most commonly an indication that the trees are infested with gloomy scale. The maple trunk gradually turns black as sooty mold accumulates on the honeydew that the scales produce. Gloomy scales often go undetected for years due to their size. They’re about the size of a pin head (1/16 to 1/8 of an inch) and look like very small bumps on the bark of the tree. Although they blend into the bark very well, they’re easy to identify by gently scratching the bark with a fingernail, causing them to flake off.
If you decide your trees need treatment, hire an arborist to apply horticultural oil (click for sources) during the dormant season, followed by an insect growth regulator in the mid-summer.
See Gloomy Scale