Monday, August 24, 2015

My crape myrtles are covered with black on the leaves.

Sooty mildew on Eucalyptus
Bidgee [CC BY 3.0 (]

Q. My crape myrtles are covered with black on the leaves. It won’t wash or wipe off. What is it and how can I get rid of it? Will it kill my crape myrtles? 

A. The black stuff is called sooty mold or sooty mildew which includes several species of fungi that thrive in honeydew. Honeydew is a sticky secretion from the bodies of aphids, whiteflies, scale insects and similar pests.  Honeydew reminds me of tiny droplets of corn syrup spray. Wherever it falls – on other plants or even your walkway – sooty mold will appear.

It’s futile to try removing the existing mold on leaves, but you can prevent it from appearing on new ones. Get rid of the insects. You’ll get rid of the mold. Shop your local garden center for insecticides labeled for the target insects. If mold occurs on ornamental plants, I prefer products containing systemic insecticide/fertilizer combinations. If applications are begun early in the season, you may prevent insect and sooty mold infestations all summer long.

Sooty mold will not kill your crape myrtles. However, the black film does interfere with exposure to sunlight, which is essential to photosynthesis. Repeated infestations can weaken your plants.

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