A lot of people ask me what the black stuff is on the leaves, branches and fruit of their plants. It's caused by a range of different fungi (including Alternaria and Cladosporium) that are growing on the exudates of insects that are feeding on your plants further up the plant. The insect exudate is called honeydew and is very sweet and ideal for these fungi to grow upon. So to get rid of the sooty mould you have to get rid of the insects first.
Whilst this mould is unsightly it isn't feeding on the plant itself so is benign in that respect, however if the mould is sufficiently serious it can block out the sunlight and reduce photosynthesis in the leaves which may then weaken the plant, making it more susceptible to other more serious diseases. It's often seen on azaleas, gardenias, camellias, crepe myrtles, laurels and citrus.
The insects that are producing this honeydew are all plant suckers. That is, they all possess a siphon-like mouthpart that they plunge into the trees or leaves in order to feed on the sap. What they are after is the protein in the sap so that they can produce their live young or eggs. These insects include whitefly, scale, mealybug and aphids. However plant sap contains mainly sugars with very little protein, so for them to get sufficient for their needs they have to excrete all the surplus sugars which rains down onto your understory plants, leaves and fruit. It is here that the fungi take over and turn your plants black.
I'm told that it can be washed off with a mild detergent spray, ensuring all the plant is covered including the underside of the leaves. Only do this once as you don't want to strip off the waxy layer of your plants that is there to protect it. Don't scrub them either as you will damage them. You can then either hose it off once the detergent has done its work, after a short while, or let mother nature's rain wash it off. You can also not bother as once the insects have been controlled any new leaves will be OK. However if branches are severely coated with mould it may pay to get this off as it could debilitate the plant. Any damaged material should be trimmed off.
To control these insects use a light mineral oil spray, such as Conqueror oil or similar, from your garden supply nursery which is safe on plants. It is VITAL that you cover all of the plant, including the underside of leaves, as this product is a CONTACT spray. In other words it has to actually hit the insect itself. The oil will smother the insects so that they are unable to breath and kill them. Scale insects will be controlled but as they stick on your plants, like limpets, you may think that the spray hasn't worked, however it has but they will remain on the plant like a dried husk.