Sooty mould is a black velvety coating that can encrust leaves and branches of entire shrubs. It affects deciduous plants and evergreens alike, and is not only unsightly but damaging, as it reduces photosynthesis and results in reduced colouring, smaller growth, and increased susceptibility to diseases. The culprits behind black sooty mould are garden pests like aphids, scale, and mealybugs; these insects produce a sticky substance called honeydew on the plant surface, which in turn produces black sooty mould. By controlling these insects and using some good gardening techniques, you can get rid of sooty mould on your shrubs.
Prune to provide increased sunlight and air circulation, both of which discourage sooty mould. Trees that are overhanging and blocking the sun of smaller shrubs can be trimmed, and the smaller shrubs themselves pruned from the inside to allow more light and air. Move container plantings to sunnier locations, and consider transplanting overcrowded shrubs.
Use a magnifying glass to examine affected shrubs for aphids, scale insects and mealybugs in order to determine exactly what pests are causing your sooty mould.
Clean all affected needles, leaves, stems and twigs with a spray made of one teaspoon of liquid dish detergent mixed with a gallon of water. This not only removes sooty black mould, but also helps to control the aphids and scale that can cause it. Spray on the undersides of leaves as well in order to target aphids where they live.
Rinse affected shrubs with forceful blasts of water from a garden hose to not only clean off residual soap solution but physically knock off aphids and mealybugs.
Control pests further by using yellow sticky cards under shrubs to trap the adult aphids, smothering scale insects--identified as round, waxy bumps and white specks on leaves--with horticultural oil, and swabbing mealybugs on leaves with a cotton ball dipped in rubbing alcohol.
Use a commercial pesticide containing carbaryl, acephate or malathion if your shrubs are still affected by insect pests and black sooty mould.
Monitor your shrubs--especially magnolia, rhododendron, azalea and viburnum, which are particularly susceptible--for insect pests, sticky honeydew, and black sooty mould, and treat at the first sign of infestation.
The presence of a large number of ants climbing on your shrubs is often a tip-off to the existence of honeydew-producing insects. For natural control of mealybugs, aphids and scale, release a few ladybirds, which feast on them.