Sooty mould: what it really means and how to fix it

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What is it?

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 do I 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.

Which plants does it affect?

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 do I 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 garden 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: Jochen Kreiselmaier, CC BY 4.0, via Wikimedia Commons.