Sooty moulds are black fungal growths on the surface of leaves. Unlike other fungal diseases, which occur inside the leaf, these are on the surface and can be washed or scraped off.
They can vary from a fine powdery or soot-like deposit in superficial attacks or in its early stages, to a thick sheet of growth. This may even crack or peel away from the leaf, especially during dry conditions.
They rarely appear of their own accord. More often than not, they are a sign that the plant is under attack from sap-sucking insects, especially aphids (blackfly and greenfly), mealybug, scale insects and whitefly.
Not only do sooty moulds look unsightly, but because they prevent light reaching the leaf, preventing it from making food for the plant, affected plants can become weak and stunted.