The Q&A Archives: Rhododendron problem

Question: I have an Anna Rose Whitney Rhododendron, 2 years old. In late winter/early spring I noticed the leaves covered with a gray to blackish waxy film. I can scrap off the waxy film with my fingernail, but it won't wash off with water. What is it & what do I do to correct? Plant does not seem to be suffering--has lots of flower buds, but the film makes the shrub look ugly. Thanks.

Answer: 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.

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.

Or, if your rhododendron is planted out in the open, it may have an infestation of aphids, mealybugs, scale or whiteflies.

Sooty mold can be washed off plants, but unless the causal insects are controlled, it will reappear. 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. Remember -- look for insects not only on the affected plants, but overstory plants as well for infestation when sooty mold appears.

At the first sign of aphids, mealybugs or whiteflies an insecticide spray may be used to control the pests.

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