A thin black substance -- sooty mold -- on orange tree leaves is a sign of pest infestation. Pests with piercing and sucking mouthparts, such as aphids and whiteflies, excrete a sugary liquid, or honeydew, on citrus leaves, twigs and stems. Sooty mold -- a black fungus -- feeds on the honeydew. Sooty mold generally does not negatively impact orange trees or fruit production. When severe, however, sooty mold interferes with photosynthesis and reduces fruit production.
Beneficial insects, including big-eyed bugs, lacewings and minute pirate bugs, feed on whiteflies, aphids and other harmful insects. Often an increase in whitefly and other harmful pest populations indicates a recent reduction in beneficial insect populations due to insecticide use, severe rainfall or a build-up of dust on plants, such as might occur near a construction site. Plant small-flowered plants, such as daisies and cosmos, and refrain from using pesticides to increase beneficial insects around orange trees.
Reduce the prevalence of sooty mold on orange trees by repelling whiteflies and aphids with reflective mulch. Spray clear plastic mulch with silver spray paint or purchase aluminum sided mulch from a garden center. To lay mulch, remove weeds from the base of the orange tree. Lay the mulch with the reflective side up and bury the edges of the mulch with dirt. Remove the mulch when temperatures rise above 85 degrees Fahrenheit.
Control flying insects around orange trees with yellow sticky traps. If your citrus tree is already infested with whiteflies, hang several sticky traps in each tree on the level of the swarming pests, but out of direct sunlight. To make your own sticky traps, paint 1/4-inch plywood with bright yellow paint. Coat the dried paint with one part petroleum jelly and one part dish-washing liquid.
Horticultural oils control aphid and whitefly populations and loosen sooty mold from orange tree leaves. Spray horticultural oil on the tops and undersides of orange tree leaves at a dilution rate of .05 to 1 percent, or 2 1/2 tablespoons to 1 gallon of water. Do not apply horticultural oil when temperatures are warmer than 85 degrees Fahrenheit or in combination with sulfur treatments.