Sooty mold is a black fungal growth that looks like a layer of soot covering the leaves of a plant or a sidewalk. The aptly named disease is common in gardens and landscapes, appearing wherever a large infestation of plant-sucking insects are found. Sooty mold grows on honeydew, a sticky substance excreted by plant-sucking insects.
While sooty mold doesn't actually damage plants or other surfaces, a thick growth of the fungus can block light to plant leaves, reducing photosynthesis. This can lead to stunted growth and premature leaf drop.
The key to reducing sooty mold is management of honeydew-producing insects, and ants. For specific tips on how to do achieve this, see the newly revised Pest Notes: Sooty Mold updated by Karey Windbiel-Rojas and Belinda Messenger-Sikes of the UC Statewide IPM Program.