Sooty mould
A black, powdery fungal coating that grows on the sticky honeydew left by sap-sucking insects, blocking light from leaves.
Also known as: Black mould, Sooty blotch
Symptoms and identification
A black or dark grey film on leaves, stems and fruit that can be wiped or peeled off. Leaves underneath stay green but receive less light, so growth slows. The coating is almost always accompanied by sticky honeydew and insects such as aphids, scale, mealybugs or whiteflies.
Causes
Sooty mould fungi do not infect the plant directly; they feed on honeydew excreted by sap-sucking insects. Trees and shrubs with heavy aphid, scale, mealybug or whitefly infestations, and plants growing under infested trees, are affected.
Organic treatment
First deal with the insects using neem oil or insecticidal soap. Then wash the mould off with a soft cloth or a spray of water with a few drops of liquid soap. Rain gradually washes it away once the honeydew stops.
Prevention
Control the insects that produce honeydew and the ants that protect them. Regularly check fruit trees and shrubs for aphids, scale and whiteflies, and prune for good airflow.
Chemical treatment (use with care)
Fungicides are not needed. Treat the underlying insect problem with a product labelled for that pest if organic methods fail.