Frequently, tree limbs and leaves are covered by an unsightly, black, sooty growth called sooty mold. It may occur on any tree or shrub but is most common on pine, elm, and linden (basswood). Sooty mold is caused by saprophytic fungi. It is not a disease and does not infect living plant tissue. Heavy growth by the fungus can reduce photosynthesis but does not harm the plant in any other way.
Photo: U of MN Plant Disease Clinic
Sooty mold is often found on plants infested with sap sucking insects such as aphids, white flies, or scales which produce a sugary secretion called honeydew. This honeydew drips down onto leaves and branches providing a food base on which the sooty mold fungi can grow. Sooty mold may also grow on sap or resin associated with wounds.
We do not recommend control of the mold itself. However, the presence of sooty mold is often an indication of insect activity which has the potential for causing damage. Proper identification of the insect is necessary to determine if chemical control is warranted. Light coverings of the mold will gradually disappear during dry weather when its nutrient source is eliminated. Sooty mold can be physically washed off small plants if desired.
Chad Behrendt, Crystal Floyd