Aphids and Scale Insects
Aphids and scale insects are the culprits to blame when a crepe myrtle tree contracts sooty mold. As these two common pest insects feed on the leaves of the crepe myrtle tree, they secrete a sweet, sticky substance known as honeydew that accumulates on the surface of the tree. This honeydew builds up on leaves, bark, branches and even the trunk of the tree, and it provides a habitat for sooty mold to begin growing.
Fungi from the Capnodium species produce sooty mold, which appears as a black film covering the surface of the crepe myrtle tree. The fungal spores are sometimes carried by the pest insects that create the growing situation that allows for sooty mold. Sooty mold is frequently no more than a cosmetic problem that leaves unattractive black patches on leaves or the bark of the tree. Severe cases of sooty mold lead to problems and require some treatment.
Treating Sooty Mold
Heavy infections with sooty mold have the potential to stop leaves from photosynthesizing, which stunts the growth of the crepe myrtle and makes the plant more susceptible to other diseases and pest problems. Sooty mold comes off the tree if it is washed with water, although insecticidal soaps have the added benefit of both washing off sooty mold and controlling the aphids and scale insects that bring it to the tree. Fungicides are also effective at killing the sooty mold.
Controlling the presence of aphids and scale insects on a crepe myrtle tree is one way to avoid the problem of having black residue appear later in the growing season. Applying insecticidal soaps to the tree or spraying the tree with a mild pesticide on a regular schedule throughout the spring and summer limits the damage that aphids, scale insects and sooty mold can do to a crepe myrtle tree. Follow all directions when using chemicals.