black substance on leaves - Knowledgebase Question

Denver, No
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Question by PAPA8936
August 25, 2010
within the last two weeks a black powdery substance has appeared on 2 crepe myrtles and a hibiscus leaves that are planted next to each other. I can rub a leaf and the powder comes off onto my finger. Can you advise what this is and if so what can I do to eliminate it.
Sal Romolo

Answer from NGA
August 25, 2010
The problem sounds like sooty mold and is caused indirectly by insect feeding. Aphids, mealybugs and scale insects suck the plant juices and excrete a liquid called honeydew. The sugar content of this excrement is just high enough to encourage fungal growth. That's what you're seeing on the leaves of your crepe myrtles and hibiscus. There are several control options: if the plant isn't too large, you can use a gloved hand and squish the adult insects. Or you can scrub the insects off with a soft brush, then rinse with soapy water. Or, you can apply an insecticidal soap or neem oil based insecticide (such as Rose Defense) to effectively suffocate the insects. Be sure to read the label, and apply exactly according to directions to avoid damage to your plants. The sooty mold will eventually weather away.

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