Answer: Without seeing the trees, I can't positively diagnose the problem. However, there may be aphids or scale insects on your tree. When sucking insects such as these feed on the fluids within the tissues of your trees, they exude a sticky substance called honeydew. This substance has a high enough sugar content that it promotes a bacterial or fungal growth which is what turns it black. This substance is called sooty mold and it is simply on the surface of the leaves and stems. You can scrape it off with your fingernail or rub it off with your fingers. Sooty mold can be reduced by controlling the insects responsible for the honeydew. Without knowing what pests you're dealing with, I really can't make recommendations for control. I would strongly recommend you take a few samples of the affected tree parts (leaves, stems, fruit) to your local cooperative extension office for a positive diagnosis. Once you know what the problem is, you can take corrective action. Helpful Master Gardeners at the cooperative extension office will be able to suggest control strategies.
Best wishes with your citrus trees!
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