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Sooty Mold

Plant disease

Description​

Sooty mold is a common name for several dark-colored fungi, not a single species. They grow on sugary honeydew left by sap-feeding insects such as aphids, whiteflies, scale insects, and mealybugs. Affected leaves, stems, and sometimes fruit develop a black coating that can look like soot. The fungi grow on the surface rather than invading plant tissue, so the coating itself usually does little direct harm. Heavy buildup, however, can block light from leaves and reduce photosynthesis; badly affected plants may yellow, grow poorly, or shed leaves. Fruit may become unattractive but is often usable after thorough washing, provided it is otherwise sound. Sooty mold can also appear on plants beneath an infested tree, where honeydew drips onto their foliage. Controlling the source of honeydew is the key to lasting control: cleaning the coating alone will not prevent it from returning.

Symptoms​

  • Black sooty coating
  • Yellowing leaves
  • Stunted growth
  • Leaves dropping

How to Identify​

Look for a dark gray or black, soot-like film on leaf surfaces, stems, or fruit. It often wipes or rubs off, revealing largely intact tissue underneath. Check the undersides of leaves and along stems for aphids, whiteflies, scale insects, or mealybugs, and feel for sticky honeydew. Ants visiting the plant can be another clue, though they do not cause the mold. Unlike black leaf-spot diseases, sooty mold forms a surface coating rather than distinct spots within damaged tissue. Powdery mildew is typically white or pale gray. Dirt may also wipe off, but recurring black growth together with honeydew strongly suggests sooty mold.

Prevention​

Inspect plants regularly, especially leaf undersides and new growth, so sap-feeding insects can be addressed before honeydew accumulates. Encourage lady beetles, lacewings, and other beneficial insects by avoiding unnecessary broad-spectrum insecticides. Keep plants appropriately watered and avoid excessive nitrogen fertilizer, which can promote tender growth attractive to some pests. Prune crowded growth when suitable for the plant to make inspection and pest control easier. If ants are tending honeydew-producing insects, manage them with appropriate barriers or other targeted methods. Check branches above affected plants: honeydew may be falling from an infested tree rather than coming from the plant with the black coating. Quarantine and inspect new plants before placing them near established ones.

Treatment​

First, find and control the insects producing honeydew. A firm spray of water can dislodge many aphids and whiteflies; repeat as needed, directing water at leaf undersides. Small scale or mealybug infestations can be removed by hand or with a soft brush. Once honeydew production stops, wash sooty mold from accessible leaves and fruit with water and a soft cloth; avoid scrubbing delicate foliage. Rain and new growth will gradually clear what remains. If pests persist, use an insecticidal soap or horticultural oil labeled for the plant and pest, following all label directions. Thorough coverage is important, and repeat applications may be needed. Test a small area first if the plant’s sensitivity is uncertain, and do not spray heat- or drought-stressed plants. Fungicides are usually unnecessary because the mold grows on honeydew; spraying it without controlling the insects will not solve the problem.

Plants Affected​

Basswood, Littleleaf Linden, Southern Magnolia, Tulip Poplar, Tulip Tree, Yellow Poplar

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