Hello Gardeners,
By mid-to-late summer, I usually receive numerous calls from concerned homeowners regarding a variety of plants such as crape myrtles, gardenias, citrus, tomatoes, camellias, and pecans that have leaves covered with a crusty black substance. The complaint often describes leaves yellowing or dropping and the plant appears to be dying. This black crusty substance is called sooty mold and is very common, especially during the summer months.
By mid-to-late summer, I usually receive numerous calls from concerned homeowners regarding a variety of plants such as crape myrtles, gardenias, citrus, tomatoes, camellias, and pecans that have leaves covered with a crusty black substance. The complaint often describes leaves yellowing or dropping and the plant appears to be dying. This black crusty substance is called sooty mold and is very common, especially during the summer months.
Sooty mold is caused by fungi, but there is no need to apply a fungicide since insects are the root of the problem. Sap sucking insects such as aphids, white flies, mealy bugs, and soft scale secrete a sticky shiny substance called honeydew that falls on the surface of the leaves and also on anything the is located near the affected plants including other plants, vehicles, furniture etc. The insects are usually located underneath the leaves. Sooty mold grows in the honey dew and once it covers the surface of the leaves of plants it reduces the amount of light that hits the leaf surface, thereby reducing photosynthesis and the plant’s ability to make food. The result is usually low vigor, yellow leaves, defoliation, and reduction in the quantity and quality of fruits, vegetables or blooms. If sooty mold is not controlled it could eventually kill some plants. I have personally made site visits to some homeowners whose gardenias were partially defoliated. On other occasions I have seen homeowners who are frustrated with Japanese blueberry trees covered with sooty mold, along with all of the plants underneath the trees. I highly recommend that before you place certain plants in your landscape, be aware of its pest problems. Several homeowners who have large Japanese blueberry trees told me that if they had been aware of the tree’s susceptibility to sooty mold they would not have planted the trees.
The recommendation is not to try and directly control the sooty mold but to control the above mentioned insects. The key is to detect the insect infestation when it is low which will decrease the time needed to bring it under control. Systemic insecticides such a imidacloprid or acephate, or a contact insecticide such as malathion or insecticidal soaps or horticultural oils are options to control the insects. In terms of the use of horticultural oils, you must apply and make sure you get complete coverage on the plant including underneath the leaves. A caution regarding horticultural oils, please read instructions regarding daytime temperature limits to avoid injury to plants. Once again it is very important to monitor plants weekly to insure early detection, which will lead to more effective control.
Please see picture of frost-proof gardenia which I looked at recently. These plants were showing signs of sooty mold since last year but the homeowner didn’t realize the impact of not addressing the problem. See the difference in the gardenia appearance such as fewer leaves and little or no flowers which reduces the fragrance of this Louisiana Super Plant.
Happy Gardening!!!Gerald P. Roberts
Happy Gardening!!!Gerald P. Roberts
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