Sooty mold

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, scale, and whiteflies. The honeydew drops from the insects to the leaves and twigs. Wind-blown sooty mold spores that stick to the honeydew then have a suitable medium for growth. When spores germinate, they send out black fungal strands (mycelial threads) that cover the plant tissue and cause the discoloration. A heavy coat of black mold may build up on needles and 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 under the thick layer of sooty mold that develops. Azalea, Rhododendron, Pieris, Cotoneaster, holly and other low-growing shrubs, growing under shady conditions are susceptible to serious damage.

Sooty mold may be washed off plants, but unless the causal insects are controlled, it may reappear. To prevent sooty mold, you need to manage 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. Remember -- look for insects not only on the affected plants--but on overstory plants as well for evidence of an infestation when sooty mold appears.