Why is there black sooty mould on my plant?

The mould is harmless and the insects above it are the real problem — how to find them, and why treating the black film alone never works.

Difficulty: beginner
Time: 30 min
Read: 4 min

Short answer

Sooty mould grows on honeydew, the sticky excretion of aphids, scale or whitefly. The mould itself does little harm. Find and treat the insects — usually on the plant above — and wash the leaves, or it comes straight back.

Black film on leaves looks like a disease attacking the plant. It is not. It is a fungus living on something sugary that an insect dripped there.

Step by step

Check that it wipes off.Sooty mould sits on the leaf surface and comes away on a damp cloth, leaving green leaf underneath. A spot that will not wipe off is a different problem such as blackspot.
Feel for stickiness.Honeydew makes leaves tacky, and it drips onto anything below — leaves, paths, furniture, car bonnets. That stickiness is the confirmation.
Look above the affected leaves.Honeydew falls. The insects are usually higher up, and frequently on a completely different plant or an overhanging tree, which is why people treat the wrong plant.
Identify which insect it is.Aphids cluster on soft new growth. Scale insects are small immobile brown or fawn bumps along stems and leaf veins. Whitefly rise in a cloud when the plant is disturbed. Mealybugs look like white fluff in the joints.
Treat the insects, not the mould.This is the whole job. Remove the honeydew source and the mould has nothing to live on, weathers away and does not return.
Start with water.A firm jet dislodges aphids and whitefly effectively. Repeat every few days — one pass never clears them.
Scrape or dab scale insects.They are protected by their shells, so sprays work poorly. A thumbnail, or a cotton bud dipped in dilute soapy water, removes them physically.
Wash the leaves.Warm water with a little washing-up liquid, and a soft cloth on both sides. It removes the mould and the honeydew that would let it regrow.
Encourage the predators outdoors.Ladybirds, lacewings, hoverflies and small birds control aphids effectively. Broad-spectrum insecticides kill them too and usually make the following year worse.
Check for ants.Ants farm aphids and scale for their honeydew and actively defend them from predators. A column of ants up a stem tells you the infestation is being protected.
Improve airflow.Crowded, still, humid conditions favour both the insects and the mould. Thinning growth and spacing pots helps on both counts.
Isolate an affected houseplant.Scale and mealybug spread readily between plants indoors, and are far easier to clear on one plant than on a shelf of them.
Keep checking for a few weeks.Eggs and unnoticed juveniles restart the cycle. Check weekly until new growth stays clean and unsticky.

Tips

The mould is a symptom. Look above the affected leaves for the insects producing the honeydew.
Sticky leaves are the giveaway. No stickiness means it is probably not sooty mould at all.
Ants running up the stem mean an infestation that is being actively protected.

Common mistakes

Spraying fungicide on the mould — The fungus is feeding on honeydew, not attacking the plant. Kill it and more grows within days because the sugar supply is untouched.
Treating only the plant with the black leaves — Honeydew drips downward, so the insects are frequently on a plant or branch above. Treating the visibly affected plant alone changes nothing.

If it doesn't work

Black film that wipes off, leaves sticky

Cause: Sooty mould on honeydew — Fix: Find and treat the insects above, then wash the leaves with mild soapy water.

Cleared then returned within weeks

Cause: Insect source not dealt with — Fix: Look higher up and on neighbouring plants, including overhanging trees.

Small brown bumps along stems

Cause: Scale insects — Fix: Remove physically with a thumbnail or a soapy cotton bud. Sprays are largely ineffective against the shells.

Sticky deposit on a car, path or furniture

Cause: Honeydew from a tree above, often lime or sycamore — Fix: Nothing practical treats a mature tree. Move what is underneath if you can.

Black marks that do not wipe off

Cause: Not sooty mould — likely blackspot or another leaf disease — Fix: Remove affected leaves, improve airflow, and treat as a fungal disease.

Questions people ask

Does sooty mould kill plants?

Rarely by itself. A heavy coating blocks light and weakens the plant over time, but the insects producing the honeydew do the real damage.

Can I just wash it off and leave it?

It will return within weeks unless the insects are dealt with. Washing is the finishing step, not the treatment.

Is it safe to eat fruit or vegetables with sooty mould?

Yes, once washed thoroughly. The mould is on the surface and is not toxic, though heavily affected plants often crop poorly anyway.

Why does my car get sticky under a tree?

The same honeydew, from aphids in the canopy. Lime, sycamore and maple are the usual culprits, and sooty mould often follows on the paintwork.