Answer: What you describe sounds like sooty mold which is the result of insect feeding. You'll want to closely inspect your trees for aphids, scale insects or mealybugs. When these insect pests feed they exude a sticky substance called honeydew. It is high enough in sugar content that it turns black and will coat the upper surface of the leaves and any fruit that is directly below where the insects are feeding. It is harmless to the fruit and the leaves but it looks unsightly. Once you control the insect pests the sooty mold will wash off with rainwater or water from the hose. Best wishes with your citrus trees.
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