Gardening glossary
Sooty mold
Sooty mold is the visible black or dark grey coating that develops on leaves, stems, fruit, and any surface beneath a honeydew-producing pest infestation. It is not a single organism but a community of saprophytic fungi — commonly species of Cladosporium, Aureobasidium, Alternaria, Antennariella, and Capnodium — that share an ability to grow on sugary surface deposits.
Crucially, sooty mold does not penetrate plant tissue. It anchors only to the honeydew film on the leaf surface, which means the underlying leaf is undamaged by the fungus itself. The problem is purely physical: a thick black layer reduces the light reaching the chloroplasts, lowering photosynthesis. Severely affected leaves may yellow, drop early, and reduce yields on fruiting plants. On ornamentals, the cosmetic damage to glossy foliage is reason enough to act.
Because sooty mold lives on honeydew, every case has an insect culprit somewhere above the affected surface. The most common producers indoors and out are aphids, soft scale insects, mealybugs, whiteflies, and certain psyllids and leafhoppers. Outdoor citrus, magnolia, gardenia, holly, and crepe myrtle are frequent hosts. Indoors, ficus, hoya, and orchids are common.
Treatment is two-staged. First, identify and control the insect producing the honeydew using approaches appropriate to the pest — insecticidal soap or horticultural oil for soft-bodied pests, targeted treatment during crawler emergence for scale, systemic insecticides for severe ornamental cases. Second, once the honeydew source is stopped, gently wipe established mold from leaves with a damp soft cloth and mild soap solution, or rinse outdoor plants with a strong water spray. New growth will emerge clean. Mold that remains will dry, crack, and slough away naturally within a few weeks.