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Sooty mould

Sooty moulds are fungi that grow on the sugary honeydew dropped by aphids, scale, whitefly and mealybugs. They do not attack the plant directly but block light from the leaves. Deal with the insects and the mould fades.

Fungal diseases
Citrus leaves coated with black sooty mould
Citrus leaves coated with black sooty mould

What it looks like

It sits on the surface like soot.

  • Black powder or crust on upper leaf surfaces.
    Black film Black powder or crust on upper leaf surfaces.
  • The leaf is sticky under the mould.
    Sticky beneath The leaf is sticky under the mould.
  • Scale, aphids or whitefly on the leaves or stems above.
    Insects above Scale, aphids or whitefly on the leaves or stems above.
  • Dried mould flakes off in sheets.
    Peeling crust Dried mould flakes off in sheets.

How to confirm it

Wipe a leaf.

  1. Wipes off

    The black comes away, leaving green leaf.

  2. Look up

    Find the insects producing honeydew above.

  3. Ants

    Ants tending insects nearby.

What to do

Treat the insects.

  1. Control the insects

    Deal with the scale, aphids, whitefly or mealybugs.

  2. Wash leaves

    Wipe or hose leaves with water and a little soap.

  3. Stop ants

    Band trunks to keep ants from protecting insects.

  4. Let rain help

    Outdoors, rain washes old mould away once insects are gone.

Prevent it next time

  • Check for sap-sucking insects regularly.
  • Encourage predators.
  • Keep indoor citrus in good light.

What not to do

Do not spray fungicide; the mould disappears only when the insects do.

In the growing guides

Where the guides mention it, with plant-specific advice.

Sources

  1. Sooty moulds, Royal Horticultural Society