Sooty mold

Sooty molds are Ascomycete fungi which grow from the sugary honeydew secreted by plants [opuntia cactus] and insects 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 formulations of neem oil, which is an organic broad spectrum pesticide, insecticide, fungicide and miticide. It is used to control insects and mites like whitefly, aphid, scale, and mealy bugs, and the fungi they cause such as sooty mold, and fungus diseases including black spot, rust, mildew, and scab. Neem oil can be used on house plants, flowers, vegetables, trees, shrubs and fruit indoors and outdoors. Neem oil is biodegradable and has not been shown to be toxic to mammals, birds, bees, earthworms, or beneficial insects.

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.