The black sooty mold is probably the result of sucking insect damage such as aphids, mealybugs or scale. For sucking insects spray with a suffocant such as a horticultural oil but spray in the early evening to avoid burning the foliage during the heat of the day. There are also chemical systemics specifically formulated for citrus plants as another option, but you would need to get rid of the current problem first and the systemic would be for future control. Also look for ants as they farm the sucking insects for their honeydew, excreta from the insects that leads to the black sooty mold. Use a bait that ants take back to their nests to kill the entire colony and the queen. You can also apply tanglefoot around the trunk of the tree to keep ants out of the tree.
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