Answer: I'll bet it is sooty mold which is not a fungus, but looks like one. When insects such as aphids, scales and mealbugs feed on the fluids in the tissues of the leaves they excrete a sticky substance called honeydew. This material is high enough in sugar contact that it attracts fungal spores, which turns it black. It is a coating on the leaves that can be scraped off with a fingernail or washed off with water. If you control the insect pests, the sooty mold will weather away. Check the leaves and stems for presence of insect pests. Most can be hosed off with a strong stream of water. Best wishes with your citrus tree!
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