Sooty mold is a general name for a collection of different species of fungi that grow on the honeydew secretions found on plants. The existence of such sugary substances on plants is often related to the presence of sap-sucking insects and could point to a possible pest infestation. Aphids, mealybugs, whiteflies, cicadas, and scales are just a few examples, but tend to be the most well-known ones.
Those insects feed from the plant using their mouths to suck up sugary fluids. They can’t utilize all of the nutrients inside the fluid; therefore, they assimilate what they need and excrete the rest as the byproduct we call “honeydew.” Wherever there is honeydew, let it be leaves, twigs, fruits, or concrete, sooty molds can develop.
Mycelium is black and can give leaves or other plant parts the appearance of being covered with a layer of soot (hence the name.)
Sooty molds can damage the plant by coating the leaves to the point where reduction in sunlight is critical. Without enough sunlight, photosynthesis is reduced, thus leading to slower plant growth. In addition, coated leaves might prematurely age causing premature leaf drop.