Q: There is a big hackberry tree in my back yard that provides welcome shade to our deck; we tolerate the leaves it also drops. This fall the deck was covered with a sticky, saplike substance that turned a bright green umbrella black. It could also be seen on the leaves of nearby plants.

A: You likely have a population of aphids in the tree. Aphids suck sap from leaves and secrete a sticky liquid called honeydew. Honeydew coats everything below and makes great food for sooty mold fungus. It’s hard to control the aphids organically but in most years natural predators like lady beetles, yellowjackets, and wasps do a pretty good job. If the mold really bothers you, consider applying a systemic insecticidal drench to the tree root area next spring.