Sooty mould

Sooty mould.

Sooty mould grows on plants such as azaleas and camellias but also objects like fences and stones.
It is caused by fungi belonging to at least half a dozen Ascomycota genera.

A plant is susceptible to infection if it is infested by sap sucking pests such as aphids, scale insects,
    mealy bugs or whiteflies that produce honeydew.
Occasionally a plant itself produces a sugary exudate.

Sooty mould appears as black or dark brown areas of fungal growth usually on the upper surface of leaves.
It may be a thin dusting or a thick layer that cracks and peels off.

Spores of all species are dark.

Sooty moulds cause little harm to the plant and is mainly a cosmetic problem.
If there is a thick layer of fungus it may reduce photosynthesis due to lack of sunlight.