Answer: If the black substance can be wiped off the leaves with a soapy cloth, the problem is called sooty mold. The substance is appearing because your tree has a population of insect pests. Aphids, scale insects and mealybugs feed on the fluids of the leaves and stems and excrete a sticky substance called honeydew. This substance coats the tops of the leaves directly below where they are feeding. As the honeydew ages, it turns black. Carefully inspect your tree for insect pests. Aphids can be controlled with insecticidal soap, scales and mealybugs can be dabbed with a Q-Tip dipped in rubbing alcohol. Once you control the insect pests the sooty mold will not reappear. Best wishes with your lime tree.
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