Have you seen something recently on your plants that looks like a black mold covering the leaves? This black mold could be something we call sooty mold. Sooty mold is a fungus that can cause plants to appear dark and sooty or almost uniformly charcoal gray. It may form a continuous thin sheet on the leaf surface that eventually peels away.
Sooty mold is interesting in that the black coating on the leaves is caused by insects and not by a disease as many may think. Aphids, scales, mealybugs, and other sucking insect pests are to blame. These insects suck sap from the plants to get needed nutrients. Much of the water and sugars in the sap that pass through the insect are excreted. These sap-sucking insects excrete a sweet, sticky liquid referred to as honeydew. This sticky honeydew clings to leaf surfaces and as spores of the sooty molds are blown to the leaves the honeydew causes the mold to stick to the leaf surface.
Sooty mold on leaves of trees and shrubs lessens the aesthetic value of the plant and lowers the plant vigor by blocking sunlight which the plant needs to make its food and grow. Most plants will tolerate a small insect population and light amounts of sooty mold but the combination of feeding by many insects and the heavy coating of sooty mold may reduce the vigor and beauty of ornamental plants. Sooty mold can indirectly damage the plant by coating the leaves to the point that sunlight penetration is reduced or inhibited. Without adequate sunlight, the plant's ability to carry on photosynthesis is reduced which may stunt plant growth.
The first step in controlling sooty mold is to suppress the aphids, scales, or other insects that are excreting the honeydew which sooty mold sticks to. Homeowners and gardeners can use horticulture oils to control the sap-sucking insects. Horticultural oils are sold by many companies and can be found in garden centers, hardware stores, and similar establishments. The rates of application vary with the time of year. Avoid applying horticultural oils to tender, new growth to prevent damage to the plant. Follow the label instructions to apply the correct amount. When applying, be sure to get good coverage on the plant because these insects are often hidden on the underside of leaves.
There are several other insecticides available for home gardeners that can help control aphids, scales, or other insects. When selecting an insecticide, be sure to check the label to see if the label states that it will control insects like aphids, mealybugs, and scales.
Even after the insects causing the honeydew are eliminated, sooty mold may adhere to plants for months afterwards. In some instances, it may be necessary to wash off sooty mold with a strong stream of water.
Recommendations for the use of agricultural chemicals are included in this article as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this article does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical.
Jessica Strickland is an Agriculture Extension Agent, specializing in horticulture for North Carolina Cooperative Extension in Wayne County.
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