Answer: The black coating on the leaves of your nandina is called sooty mold. It is the result of insect feeding - usually aphid feeding. As these pests feed on the fluids in the stems and leaves of your nandina they exude a sticky substance called honeydew. This substance is high enough in sugar to develop a bacterial growth which turns it black. It doesn't hurt the foliage, other than to block photosynthesis on that part of the leaf, but it does look unsightly. If you get rid of the insects (which you did), the sooty mold will rinse or wash right off the leaves. If it reappears, the insects are back. Insecticidal soap does a good job in controlling aphids. Good luck with your nandina!
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