Sooty molds are Ascomycete fungi which grow on plant exudates and the sugary honeydew

Honeydew (secretion)

Honeydew is a sugar-rich sticky liquid, secreted by aphids and some scale insects as they feed on plant sap. When their mouthpart penetrates the phloem, the sugary, high-pressure liquid is forced out of the gut's terminal opening. Honeydew is particularly common as a secretion in the Hemipteran...

 secreted by insects such as aphids, scales

Scale insect

The scale insects are small insects of the order Hemiptera, generally classified as the superfamily Coccoidea. There are about 8,000 species of scale insects.-Ecology:...

, the whitefly

Whitefly

The whiteflies, comprising only the family Aleyrodidae, are small hemipterans. More than 1550 species have been described. Whiteflies typically feed on the underside of plant leaves.-Agricultural threat:...

, and other insects which suck sap from their host plants. The name itself is descriptive, as sooty mold is a black, powdery coating adhering to the leaves of ornamental plants such as azaleas, gardenia

Gardenia

Gardenia is a genus of 142 species of flowering plants in the coffee family, Rubiaceae, native to the tropical and subtropical regions of Africa, southern Asia, Australasia and Oceania....

s, camellias, crepe myrtles, and laurel

Bay Laurel

The bay laurel , also known as sweet bay, bay tree, true laurel, Grecian laurel, laurel tree, or simply laurel, is an aromatic evergreen tree or large shrub with green, glossy leaves, native to the Mediterranean region. It is the source of the bay leaf used in cooking...

s. Plants located under pecan

Pecan

The pecan , Carya illinoinensis, is a species of hickory, native to south-central North America, in Mexico from Coahuila south to Jalisco and Veracruz, in the United States from southern Iowa, Illinois, Missouri, and Indiana east to western Kentucky, southwestern Ohio, North Carolina, South...

 or hickory trees are particularly susceptible to sooty mold, because honeydew-secreting insects often inhabit these trees. The honeydew can rain down on neighboring and understory plants. 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 in the garden, as it is unsightly and can coat most of a plant in a matter of days or weeks. Some common genera causing sooty molds are Cladosporium

Cladosporium

Cladosporium is a genus of fungi including some of the most common indoor and outdoor molds. Species produce olive-green to brown or black colonies, and have dark-pigmented conidia that are formed in simple or branching chains....

, Aureobasidium, Antennariella, Limacinula

Limacinula

Limacinula is a genus of fungi within the Coccodiniaceae family. The genus was first named by Franz Xaver Rudolf von Höhnel in 1907.-Species:*Limacinula anomala*Limacinula butleri*Limacinula caucasica*Limacinula citricola...

, Scorias

Scorias

Scorias is a genus of fungi within the Capnodiaceae family. The genus was first described by Elias Magnus Fries in 1832. The fungus is known as sooty mould and is found growing on honeydew on leaves of many varieties of trees and plants.-Characteristics:...

, and Capnodium

Capnodium

Capnodium is a genus of fungi within the Capnodiaceae family.-Species list: -External links:* at Index Fungorum...

.

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

Neem oil

Neem oil is a vegetable oil pressed from the fruits and seeds of the neem , an evergreen tree which is endemic to the Indian subcontinent and has been introduced to many other areas in the tropics...

, which is an organic broad spectrum pesticide, insecticide, fungicide and miticide. It is used to control mites and insects such as 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

Diazinon

Diazinon , a colorless to dark brown liquid, is a thiophosphoric acid ester developed in 1952 by Ciba-Geigy, a Swiss chemical company...

. 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.