Sooty moulds are fungal diseases caused by a number of different fungal species. They don't attack or damage the plant they are growing on directly, but their presence on the leaf blocks out sunlight and so reduces plant vigour.
Spreading dark brown or black, superficial marks appear on the top part of the leaves of numerous plants – but particularly glossy-leaved evergreens.
The presence of sooty moulds are most often a sign that the plant is being attacked by one or more sap-sucking insects – including scale insects, aphids, mealybugs and whitefly. These insects extract sap from the plant. Their excreta, which is very high in sugars – called honeydew – drops on the leaves below where they are feeding. The sooty moulds then grow on this honeydew.
Sometimes, but rarely, the sooty mould is growing on naturally occuring sugary exudates produced by the plant itself.
Numerous ants may also be present, feeding on the sooty mould or directly on the honeydew produced by the sap-sucking insects.
Treatment and Control
The first step is to to control the sap-sucking insects that are producing the honeydew. Scale insects and mealybug can be difficult to control, so treat them with a systemic insecticide.
Once the insects have been dealt with, the amount of sooty mould will start to diminish. To speed this up, either wipe the leaves with a damp, soft cloth or spray them with water from a hosepipe.