What Is the Black Growth on My Plants?

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During these early spring months, many people are concerned about a black film coating the leaves of their trees and shrubs. This black film is referred to as 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.

The sooty mold on tree and shrub leaves lessens the aesthetic value of the plant and lowers the plant vigor by blocking sunlight, which the plant needs in order to produce food and grow. Most plants will tolerate a small insect population and light amounts of sooty mold but the combination of feeding by a large number of 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 can 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. If horticultural oils are applied to tender, new growth, damage to the plant may occur. Remember to read and follow label directions. When applying, make sure to get good coverage on the plant because these insects are often on the underside of leaves.

There are several other insecticides available for home gardeners that will help control aphids, scales, or other insects. Some examples include insecticidal soap and imidacloprid. 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 the sooty mold with a strong stream of water or soap and water. If you use soap and water to wash off the sooty mold, be careful because some plants are sensitive to soap and make sure to rinse the plant well afterwards to prevent leaving any soap residue.

Sooty mold is an example of why one would need to have an understanding of how the disease is caused. Knowing that insects cause the black film on leaves of trees and shrubs allows the gardener or homeowner to know that the insects must be controlled in order to eliminate sooty mold. With some effort in controlling the sap-sucking insects, sooty mold can be managed.

Reminder:  If you would like to learn more about Horticultural related topics, then join the “Sampson County Friends of Horticulture”. This program offers monthly “How To” Horticultural Seminars. Please call (910) 592-7161 for more information. Please call the Sampson County Cooperative Extension Center at (910) 592-7161 with your horticultural questions and to register for any upcoming events. Be sure to check out the Ask An Expert Widget at sampson.ces.ncsu.edu for any questions you may have.