Sooty mold indicates that your citrus trees have an insect infestation. It is usually found on the bottom of a tree's leaves. The insects that cause sooty mold--mealybugs, whiteflies and aphids--are typically more active during cooler weather. Mold forms from a honeydew secretion that the insects leave after feeding on the tree. You can remove sooty mold with store-bought insecticides or homemade solutions.
Spray both sides of infected tree leaves with Big Buster insecticide. Use a second treatment 10 to 14 days after your first application if your trees have a severe infestation.
Spray your trees with liquid Gold Copper when mold develops. Apply it only to infected leaves. Stop a large outbreak by respraying infected leaves 10 to 14 days after the first treatment.
Mix 1 cup of cooking oil with 1/2 cup of water. Place a small amount of dish soap in the mixture, then spray it on the infected leaves. This homemade spray will smother the insects.