Leaves Turning Black?

Janet McLeod Scott
Home & Garden Information Center

It can be a little disconcerting when you look at your plants expecting to see lovely green foliage and instead find that leaves have “turned black”. A charcoal-black, powdery or velvety coating on leaves and other plant parts is the result of growth of one of the commonly occurring sooty mold fungi. The good news is that despite its appearance, the fungus is not infecting plant tissue and is not causing disease. It is growing on the surface of the plant and is getting its nourishment from honeydew that is excreted by sap-sucking insects such as aphids (see below), scales, whiteflies (see below), leafhoppers, mealybugs and others. As these insects suck plant sap, they are unable to digest all of the sugar that they ingest. The excess is excreted as sugary liquid called honeydew.

Image depicts aphids, insects that commonly excrete honeydew.
R.J. Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company, www.insectimages.org

Image depicts whiteflies, insects that commonly excrete honeydew.
Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Many plants develop sooty mold problems. Any plant that is susceptible to infestation by sap-sucking insects can develop sooty mold on the honeydew that the insects excrete. Some plants that commonly exhibit sooty mold problems are crapemyrtle (Lagerstroemia indica), privet (Ligustrum spp.), holly (Ilex spp.), camellia (Camellia spp.), gardenia (Gardenia jasminoides), pines (Pinus spp.), and numerous other trees and shrubs as well as herbaceous plants and also houseplants (especially those that are taken outside for the summer).

Image depicts leaves covered in sooty mold.
Leaves with sooty mold fungus
Joseph O'Brien, USDA Forest Service, www.insectimages.org

Diagnosis of sooty mold problems is made simple by the fact that the black fungal growth can be removed by wiping with a moist paper towel, or in some cases, by peeling it off to reveal an undamaged plant surface. Since the cause of the problem is insect activity, control measures should be directed at the insects rather than the fungus. Important first steps include identifying the plant that is infested and the insect that is causing the problem. When identifying the infested plant, always look at plants growing above it in addition to the plant itself. With a sufficiently heavy infestation of sap-sucking insects, the sticky honeydew may drip from the infested plant to non-infested plants growing below (as well as onto objects such as lawn furniture, fencing, decks, etc.).

Image depicts sooty mold commonly assoicated with lobate lac scale.
Sooty mold coating on leaf
F.W. Howard, University of Florida, www.insectimages.org

While mainly a cosmetic problem, when sooty mold is severe it can reduce the vigor of plants by blocking sunlight necessary for photosynthesis (food production). On the other hand, insect feeding can seriously damage a plant. Between insect feeding and reduced photosynthesis, the plant may be more susceptible to other insect, disease, and environmental problems.

For current recommendations on how to control insects that ultimately cause sooty mold problems, contact the Home & Garden Information Center at its toll free number, 1-888-656-9988 (9 am to 1 pm, M-F). Once the insect is under control, the sooty mold will slowly wear off. You can speed up the process by hosing down the affected plant and when feasible, by wiping individual leaves and stems.

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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.