A black mold that appears on the leaves and fruits of citrus trees is also known as sooty mold. It does not harm the tree, and you can wash it off the fruit. It is actually a fungus that grows on the excretory secretions of insects, so if you can get rid of the insects, you can solve your mold problem. Sometimes the mold is accompanied by an ant problem as well because ants are attracted by the mold. The trick here is to get rid of the underlying problem, which is the scales or aphids; then the black mold and ants, if present, should disappear as well.
Apply horticultural oils or hydrogen peroxide to the tree. Aphids, scales, and whiteflies are commonly the culprit when black mold appears on citrus trees. They feed on leaves of the tree, and then they secrete a sap like excretory material onto the tree. The black mold grows on this. Horticultural oils or a hydrogen peroxide spray both have the ability to get rid of the insects. They should be used first, as these may be less toxic to you and to the environment.
Reapply horticultural oil about once a month, in the late afternoon, after the hottest part of the day.
Add a copper fungicide to the try in order to control the fungus problem about every three months.
Wear clothing that does not leave any skin exposed.
Spray on a chemical insecticide. Try to cover as much of the tree as possible.
Create a hydrogen peroxide spray. Some have reported that a solution of hydrogen peroxide and water which is sprayed on the trees has helped. This should be about 1 part peroxide and 4 parts water. This can be sprayed on the trees with a garden sprayer, paying particular attention to the parts with the most mold.