Locust Leaf Drop and Sooty Mold on Understory Plants

Two Linked Problems

Premature yellowing and leaf drop on your locust, plus a black coating on dahlias and hydrangeas below, share a common source: insect honeydew.

Diagnose the Black Coating

Rub a wet finger on the blackened leaves. If it smears off, it’s sooty mold—a fungus growing on the sugary excrement (honeydew) of aphids, scale, or other sap-feeders in the locust canopy.

Management Plan

• Water the locust deeply during hot, dry spells to reduce stress-induced leaf drop.
• Inspect the locust for insects (check leaf undersides, twigs). Have them identified at a nursery.
• Treat the honeydew producers with the appropriate insecticide (systemic or contact) to cut off the sooty mold’s food supply.

Once the insects are controlled, the sooty mold will weather away.