Various sap feeding insects such as aphids, whiteflies, scale insects and leaf suckers excrete a sticky substance known as honeydew. Honeydew drips on to the upper surface of leaves or other plant parts below. It is quite likely that sooty mould can be found on plants that are pest free but which are growing beneath plants troubled with sap feeding insects. Heavy coatings of sooty mould can not only spoil a plants appearance but it can reduce the amount of light and air reaching the foliage.
Treat the cause, not the symptom. Identify the insect pest and treat with an appropriate insecticide for the affected plant/pest.
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Sooty deposits on pest free plants can be removed if appropriate with a damp cloth.