What causes fungus on tree trunks? How do I get rid of it?

The black fungus you see on crape myrtle and maple trunks is causes by sap sucking insects. On crape myrtles it is the crape myrtle bark scale and, on the maple, it is an aphid. The insects secrete a sugary substance called honeydew which falls on the trunks or lower leaves. Sooty mold then grows on the honeydew. There are several ways to get rid of the scale and aphids. The fungus will just come off naturally over time.

Related Videos:
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Scale
Systemic Insecticides

Related Resources:
Crape Myrtle Bark Scale Identification and Control
Maple Aphids