What it is: Sooty mould is a black, soot-like coating on leaves — but here's the key: it's a symptom, not the real problem. It grows on the sugary honeydew that sap-sucking pests leave behind, so if you've got sooty mould, you've got a pest.
How to spot it
- A black, dusty or crusty film on leaves, stems and fruit that wipes off.
- Plants look grimy and get less light through the coating.
- Ants running up and down — they farm the pests for their honeydew.
Plants it affects
Growing any of these? See our guides to tomatoes and cucumbers.
Citrus is a classic, but any plant carrying sap-suckers can get it.
What causes it
The mould feeds on honeydew from aphids, whiteflies, scale and mealybugs. The mould itself doesn't infect the plant — but by blocking light it does slow it down.
How to fix it
- Deal with the pest and the mould stops coming back — that's the real fix.
- Hose or wipe the black film off once the pests are under control.
- Discourage the ants, which protect the pests from predators.
For the bigger picture, see our guide to managing pests and problems safely.
Plan your patch: our free planting calendar shows what to plant now where you live. Ready to grow? Browse our raised garden beds or build your own with the garden bed builder.
Image: L. R. Tehon (1920), no known restrictions, via Wikimedia Commons.
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