Sooty mold is a charcoal black fungus that appears as a black coating on the surface of leaves, fruits, twigs, and branches of many deciduous and evergreen shrubs and trees. This fungus is not pathogenic to plants but obtains its nourishment from insect honeydew. Honeydew is a sweet, clear, sticky substance secreted by insects such as aphids, mealybugs, scales and whiteflies. The honeydew drops from the insects to the leaves and twigs. Wind-blown sooty mold spores (seeds) that stick to the honeydew then have a suitable medium for growth. When spores germinate, they send out black fungus strands (mycelial threads) that bring about the discoloration. A heavy coat of black mold may build up on twigs over more than one growing season.
On leaves, this coat of mold screens out light and reduces the plant's capacity to produce food. On some trees no obvious damage can be noticed. Shrubs under trees that are heavily infested with honeydew-producing insects may be seriously damaged or killed because the leaf chlorophyll cannot function properly. Azalea, Rhododendron, Pieris, Cotoneaster, holly and other low-growing shrubs, growing under shady conditions are susceptible to serious damage.
To prevent sooty mold, control the insects. The insects involved are small and may be present in large numbers before the black strands of sooty mold appear. Trees and shrubs should be observed frequently during the growing season for honeydew and insects. At the first sign of aphids, mealybugs, whiteflies, or other insects, an insecticide may be used. Contact our office for correct identification of insect problems and control recommendations.
Remember -- look for insects not only on the affected plants, but on overstory plants as well for infestation when sooty mold appears.
Sooty mold can be washed off plants, but unless the causal insects are controlled, it will reappear.
Carolyn Klass, Senior Extension Associate and Warren T. Johnson, Department of Entomology, Cornell University
4/86 slightly revised
4/88 slightly revised