Sooty Mold
Sooty mold is a black, soot-like fungal film that grows on the sticky honeydew excreted by sap-sucking insects.
Sooty Mold at a glance
| Also known as | Sooty mould, black mold |
|---|---|
| Affected parts | Leaf and bark surfaces |
| Most at risk | Any tree with a sap-sucking pest (aphids, scale, whitefly, mealybugs) |
| Main cause | Mold growing on honeydew excreted by sap-sucking insects |
| Severity | Cosmetic, but blocks light; the real issue is the pest causing it |
| Reversible? | Yes, clears once the pest is controlled |
What to look for
- Leaves: A dull black, sooty coating on upper leaf surfaces that wipes/rubs off (the leaf is green underneath); a sticky, shiny honeydew layer; reduced vigor from shaded leaves.
- Whole tree: Ants on the trunk; the sap-sucking pest itself somewhere above the mold.
Overview
Sooty mold is a black, soot-like fungal film that grows on the sticky honeydew excreted by sap-sucking insects. It doesn't infect the plant directly, but it coats leaves and bark, blocking light and looking alarming. It is best understood as a symptom: where there is sooty mold, there is (or was) an aphid, scale, mealybug, or whitefly infestation, usually above the blackened area.
Causes
Honeydew from aphids, soft scale, mealybugs, or whitefly feeding above; warm, still conditions; ants farming the pests.
Commonly confused with
It is itself a clue, not a primary disease. Don't confuse the wipe-off black film with leaf-spot fungus (which is in the leaf) or with dark mildew. If it wipes off and there's honeydew/insects above, it's sooty mold.
Which species are affected
Any species hosting a honeydew-producing pest; common on citrus-relatives, ficus, camellia, crepe myrtle, and indoor trees.
Treatment
Treat the underlying pest (see Aphids, Scale, Mealybugs, Whitefly), that stops new honeydew. Then wipe/wash off the mold with water and a little mild soap; horticultural oil both controls pests and loosens the film. Control ants. The mold won't return once the pest is gone.
Fix it
What the treatment above uses, from our shop:
Prevention
Keep sap-suckers in check (inspect, quarantine, treat early); control ants; maintain airflow; rinse dusty/honeydew-prone foliage.