If you notice black dust that looks like soot covering a plant’s leaves, stems and twigs, this is probably Black Sooty Mould (not poor housekeeping!).  This black substance is a fungus that grows on the sweet honeydew secreted by insects such as aphids, scale, mealybugs and white flies. It can appear on any plant indoors or in the garden.

The mould does not feed on the plant, however the coverage on the leaf blocks light, reduces photosynthesis and restricts the plant’s growth.

Treatment: 

• Wipe the leaf surface: The mould is cosmetic and shouldn’t cause significant damage to healthy plants. Usually, it can be wiped off with a damp cloth or rubbed off with a diluted mixture of soap and water.
• Identify the pest feeding the mould: Eliminating the sap sucking insect will remove the source of the honeydew and dry up the sooty mould. It will eventually fall or wash off the foliage. Once the sooty mould starts to dry out and flake, hosing the plant can help to remove the flaky bits more quickly. Inspect the plant closely to try to identify the problem pest. Aphids are usually clearly visible. Scale can be good at camouflage, especially on the underside of the leaf, and mites are very small and difficult to spot. Ants are also attracted to the honeydew. If there are ants crawling over the plant, go looking for the insect producing the honeydew which attracts the ants and feeds the sooty mould. Once the problem pest is identified, apply the appropriate insecticide.
• Introduce Beneficial Bugs: Attracting beneficial bugs to the garden that feed on sap sucking insects will help to prevent the appearance of sooty mould. Live beneficial bugs can be bought online and introduced to the garden to feed on the pest causing the outbreak of sooty mould.
• Plant Flowering Perennials: Use plants such as Marigolds, Alyssum, Dill, Coriander, Queen Anne’s Lace, and Baby’s Breath to attract a range of beneficial insects that love to feed on those sap suckers.
• Organic Insecticides: Natural control methods can take time to eliminate the problem pest. For more immediate control of a severe outbreak of pests and mould, use an insecticide/miticide recommended for a range of sap sucking insects.
  Yates Nature’s Way is an organic Natrasoap Pest Spray for the control of Aphids, Mites and Thrips, Eco-Oil is a botanical oil that controls various sap sucking pests and Eco-Neem is a botanical insecticide that controls various sucking and chewing insects.
  As the honeydew gradually runs out, the sooty mould will disappear.
• Keep the garden healthy: Strong and healthy plants are more resistant to pests and diseases. Regular applications of a quality fertiliser and plant tonic, plus making sure plants have the right trace elements available to take up nutrients will help them cope better with any attacks.

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Please note that this is intended as general information only. Please ask one of our qualified horticulturists for more specific advice for your situation.