Answer: Are there black spots on the leaves or is it a black coating? Black spots can indicate a fungal disease but a black coating is probably sooty mold which is actually the result of insect feeding. Aphids, scales and mealybugs feed on the fluids in the tissues and then exude a sticky substance called honeydew which turns black. If this is the case, you'll want to control the insects with insecticidal soap sprays. Once the insect are gone the sooty mold will weather away.
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