SOOTY MOLD   F A C T S H E E T

Introduction

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.

Symptoms

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 (Fig. 1) and twigs (Fig. 2) 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.

Figure 1: Heavy building of Sooty Mold on needles. Figure 2: Sooty Mold on a branch.

Management Strategies

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.

At the first sign of aphids, mealybugs or whiteflies an appropriately registered insecticide may be used. If scale insects are present however, proper identification of the scale pest may be required, and you may need to consult your county agricultural agent or professional arborist for the proper time to spray that pest.

Many fungicides may be registered to treat different pests on different plant(s) or groups of plants and in different sites. Always make sure the pest and the plant(s) to be treated are listed on the label of the container. The label also contains information on how to apply the pesticide as well as any precautions. Follow the label instructions for all pesticides used.

For commercial applications, please refer to the appropriate commercial pest management guidelines, or contact your local Cooperative Extension Office for more information on currently registered products.

Updated, SLJ, 3/09