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Sooty mould

Also known as Sooty mold, Cladosporium, Alternaria

DiseaseHarmful
Sooty mould on Schefflera arboricola leaves Photo: Amnon Shavit · CC BY-SA 3.0

In short

Sooty mould is a black coating of several kinds of fungi that grow on honeydew, the sugary liquid excreted by sap-sucking insects. The fungi do not infect the plant, but a heavy layer blocks light and slows growth. Clearing the insects that make the honeydew is the real cure.

How it looks

A black, soot-like film on the upper surfaces of leaves, twigs and fruit, usually lying over a sticky, shiny layer of honeydew. It sits on the surface and can often be wiped or peeled off, revealing green tissue underneath. It usually appears on leaves below colonies of aphids, scale insects, mealybugs or whiteflies.

Signs

Blackened, sticky leaves; in heavy cases yellowing, slowed growth and premature leaf shedding from reduced light. Ants running over the plant are a common clue to honeydew-producing insects.

Life cycle

Airborne fungal spores land on honeydew, germinate and grow dark strands over the sugary film. The coating gradually weathers away once the honeydew source stops.

What favours it

Any infestation of honeydew-producing insects: aphids, whiteflies, mealybugs, soft scale, leafhoppers and psyllids. Ants that tend these insects protect them and make the problem worse.

Prevention

  • Check leaf undersides and stems regularly for sap-sucking insects and act early.
  • Quarantine and inspect new plants for scale, mealybugs and aphids.
  • Encourage natural enemies such as ladybirds, lacewings and parasitoid wasps outdoors.
  • Discourage ants, which protect honeydew producers.

Treatment

Start with the first step and move down only if the problem persists.

  1. Control the insects producing the honeydew first; without them the mould stops growing.
  2. Wipe or wash affected leaves with tepid water and a little mild soap to remove honeydew and mould.
  3. Use least-toxic insect controls, such as insecticidal soap or horticultural oil, on the honeydew producers if natural enemies are not coping. Follow the product label.

Pros and cons

Cons

  • Reduces photosynthesis and can slow growth or cause leaf drop.
  • Unsightly, and a sign of an insect infestation that needs attention.

Look-alikes

Black spot or leaf spot
Leaf spot lesions are dead tissue within the leaf and cannot be wiped off.
Dirt or dust
Dust is dry and not sticky, and there are no insects above it.

Affected plants

Any plant carrying honeydew-producing insects, including citrus, gardenias, azaleas, camellias, crepe myrtles, scheffleras, mulberries and many indoor foliage plants.

Sources

  1. Honeydew and Sooty Mold — University of Maryland Extension
  2. Sooty Mold — Illinois Extension
  3. Sooty mold — Wikipedia

Written in our own words from these sources. Check a product label before using any treatment.