Sooty mold is a charcoal black fungus that appears as a black coating on the surface of leaves, fruits, twigs, and branches of many deciduous and evergreen shrubs and trees.
It is not a fungus that causes disease so it is not directly harmful to the plant. However, heavy coatings of sooty mold may affect the plant indirectly by blocking sunlight and air from reaching the leaf surface and thus it can interfere with the process of photosynthesis.
This is the more important question because the reason it grows is the real problem for the plant.
The growth of sooty mold usually indicates that some type of piercing, sucking insect such as whitefly, scale, mealybugs, or aphids has attacked the plant and these guys can do real damage if they are not controlled. When these insects suck the juices from the plant, they excrete a sticky substance called "honeydew" that drips down and coats the leaves and stems of the plant.
Fungal spores carried by the wind or rain become trapped on the sticky honeydew and germinate there. The fungus then grows on the leaf surface using the honeydew as food and eventually covers the leaves with a black, sooty coating.
In order to solve a sooty mold problem, you first need to get rid of the insects that secrete the honeydew which allows the mold to grow.
For an organic control, both Bonide All Seasons Oil and Pure Spray Green are listed for control of aphids, spider mites, mealybugs, scale, and whitefly. These horticultural oils work by smothering the different stages of the insects.
When you spray, it is very important that you coat the stems and especially the undersides of the leaves because these areas are where most of the insects are found. Be sure to read the label and apply accordingly.
Sometimes a combination of selective pruning and spraying with horticultural oil works well.
Insects and diseases often attack plants that are weakened or stressed due to drought, competition with weeds, or poor nutrition. Keep your plants healthy by fertilizing them, keeping them weeded, and watering them properly.
Unfortunately, sooty mold will remain on the leaves for a while even after the insects are gone but eventually it will wear off. If the mold is particularly heavy on the leaves, you might try washing it off with a blast of water or wiping the worst of it off with a soft cloth.
Remember -- look for insects not only on the affected plants, but on overstory plants as well for infestation when sooty mold appears.