Sooty mold

Sooty molds are Ascomycete fungi which grow from the sugary honeydew secreted by aphids, scales, the whitefly, and other insects which suck sap from their host plants. The name itself is descriptive, as sooty mold is a black, powdery coating on the leaves of ornamentals such as azaleas, gardenias, camellias, crepe myrtles, and laurels. Plants located under pecan or hickory trees are more susceptible to sooty mold because honeydew-secreting insects often inhabit these trees, and the honeydew can drip down to the plants below. Look for sooty mold on new growth and leaves, since these insects prefer soft tissue. The fungus itself does little harm to the plant; it merely blocks sunlight and very rarely may stunt a plant's growth and yellow its foliage. Thus, sooty mold is essentially a cosmetic problem, as it is unsightly and can take over a plant in a matter of days or weeks. Some common genera causing sooty molds are Cladosporium, Aureobasidium, Antennariella, Limacinula, Scorias, and Capnodium.

Treatment
There are several means of treating sooty mold. In essence, they all boil down to controlling the pest(s) secreting the honeydew on the plant. Without honeydew there is no sooty mold. Some options include:

Using systemic insecticides such as orthene, malathion, or diazinon. Always follow all instructions on such fungicides, and never use these on plants which produce fruit or vegetables.

Using insecticidal soap, dish soap, or detergent dissolved in water and sprayed on the plant. Allow to sit and then use a hose to wash off the sooty mold. Most recommend one tablespoon per gallon of water. (Note: this only removes the sooty mold, and some of the insects. It does not solve the long-term insect problem.)

The above treatments do not permanently stop sooty mold or the insects which secrete honeydew. It is best, if you grow plants prone to the fungus, to use these treatments preventively.