I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
The black stuff you are seeing on your orange trees leaves is called sooty mold. It grows on sugary substance is secreted by certain insects such as scale, mealy bugs, whiteflies, or aphids. If you can determine which of these insects is causing it a control recommendation can be made which will then result in the elimination of the city model as well. Search the plants for small insects are strange looking material such as scale and take a sample to your County Extension office. They should be able to identify it for you free of charge.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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