Are the leaves of your lemon tree, oleander or Ficus covered in a black film, like soot, that partly rubs off under your finger? That’s sooty mould. It isn’t a disease attacking the plant from within, but fungi growing on the surface of the leaves on a sugary deposit: honeydew.
Where it comes from
Honeydew is excreted by sap-sucking insects: aphids, mealybugs and scale, whitefly. It sticks to the leaves, and the sooty mould fungi feed on it. In other words: no sooty mould without insects. Treating it without looking for the insects means seeing it come back within three weeks.
Why deal with it
The black film blocks the light: the leaf photosynthesises less, the plant weakens, and a weak plant attracts more pests. On citrus, the fruit gets dirty too. Above all, the deposit is a sign that insects have moved in.
Cleaning it and treating the cause
- Find the insects: turn the leaves over, look at the stems and leaf joints. Small shells or cottony clumps: mealybugs and scale. Colonies on the shoots: aphids. A white cloud flying up: whitefly.
- Spray ready-to-use black soap for plants over all the foliage. It softens the deposit and acts on contact against the insects.
- Wipe the leaves with cleaning gloves or a soft damp cloth, supporting each leaf with your other hand. Sooty mould comes off without hard rubbing.
- Repeat every five to seven days for three to four weeks, long enough to deal with the insects.
- Rinse small plants in the shower, protecting the soil.
Stopping them coming back
- Clean the foliage once a month with black soap: honeydew doesn’t have time to build up.
- Watch for ants climbing tree trunks: they often signal aphids or scale insects.
- Keep plants vigorous: good light, the right watering, and gentle feeding with our compost tea root feed during the growing season.
Frequently asked questions
Is sooty mould contagious?
It follows the insects: they’re the ones that move from plant to plant.
Should I cut off blackened leaves?
No: once cleaned, they do their job again. Only remove those that are damaged.
Need an opinion?
Send a photo of your plant to our Plant Doctor: the whole plant, a close-up of a leaf (top and underside) and the surface of the soil. The diagnosis is free. You can also drop by the studio at 6 Rue Zemmour, Aviation district, Rabat.
