Sooty Mould
Mostly cosmeticFungal disease · does not spread · also called black film
Sooty mould does not infect the plant at all. It grows on honeydew — the sugary excretion of aphids, scale, mealybugs or whiteflies feeding above. It is therefore a symptom of an insect problem rather than a disease, and treating the mould without finding the insects achieves nothing.
What it looks like
A black, dry, soot-like film coating the upper surfaces of leaves and stems. It wipes off with a damp cloth, leaving green leaf underneath, and it is always accompanied by stickiness.
How to confirm it
- Wipes off with a damp cloth, unlike any true leaf disease.
- Always accompanied by sticky honeydew.
- Sits on the upper surface, often heaviest directly under an infested branch.
- Look above the affected leaves for scale, aphids, mealybugs or whitefly.
What to do
- 1Find and treat the insects producing the honeydew. That is the actual problem.
- 2Wipe or wash the mould off once the insects are dealt with.
- 3Check overhanging branches and plants — the honeydew source is often above rather than on the affected plant.
What it gets confused with
One observation settles each of these.
Black spot is discrete fringed spots within the leaf tissue and does not wipe off. Sooty mould is a continuous film that does.
Preventing it next time
- Control sap-sucking insects early.
- Watch for ants, which farm and protect honeydew producers.
Plants affected
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.