I apologize for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
The stuff you are seeing is called sooty mold. Certain pesky insects, including whiteflies, scale, or aphids secrete a substance known as honeydew, which supports the growth of the black, sooty mold.
Control the insects with sprays of an appropriate insecticide such as insecticidal soap or lightweight, horticultural oil (probably better than soap for this). Direct sprays upward from beneath the plant making sure to wet the undersides of all leaves and branches. Repeat spraying will likely be necessary to get the pests under control.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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