Those dark, dusty spots on your shrubs that look like someone emptied the ash bin on them aren't soot, but a fungus called sooty mould. Several different species of fungus can cause sooty mould, which grows in sticky excretions called honeydew, produced by plant-sucking insects. Though the mould itself doesn't harm the plant, it's unsightly and if allowed to flourish could block the sunlight the plant needs for photosynthesis.
Removing the Mold
Sooty mould may grow on leaves and the stems and twigs of plants. You can wash the mould off with the jet of a water hose. For stubborn deposits, try mild soap and water. You can scrub the leaves by hand, or use a wand designed for washing the car. Rinse off the soap with clean water.
Sooty mould may also spread to nearby structures such as lawn furniture, lawn ornaments or even the outside walls of your home. You should be able to clean these items with hot soapy water and a scrub brush. If you need a stronger cleaning solution, mix a quart of bleach with 2/3 cup trisodium phosphate and 1/3 cup detergent and 3 quarts of water and scrub with this.
The combination of sooty mould and honeydew can be sticky and tough to remove. In stubborn cases, you may have to let the mould weather away over time, once you've take steps to keep it from flourishing.
Controlling the Source
Since sooty mould grows in the secretions of insects, you'll need to get rid of the insects to prevent the mould from returning. Whiteflies, aphids, scale insects, mealybugs and psyllids all secrete the gooey honeydew, which provides the perfect habitat for sooty mould.
For light infestations, pruning away affected branches may be enough to get rid of both mould and insects. For more pervasive problems, spray the shrubs with neem oil or dormant oil, or a mild insecticide.
When treating the plants with insecticides, spray a light coat on a calm day. If the solution drips off the leaves, you're probably using too much. Excess chemical can run off into nearby water sources. Wear rubber gloves and avoid breathing in the mist from the sprayer.
You may be able to control the harmful insect population by targeting ants. Ants feed on the honeydew these insects secrete, so they'll protect them from predators. Eliminate the ants and you open the door for natural predators to move in and take care of the problem.