Black Sooty Mold is a velvety gray-black crust-like coating on the leaves, needles, fruits, or branches of certain plants. The fungi can be easily removed by rubbing it with a finger.
Sooty molds grow only on the surface and will not kill plants. They also often grow on sidewalks or fences under infested trees.
Sooty molds are normally considered to be nothing more than a cosmetic or aesthetic problem, but in extremely severe cases, it is possible for the black growth to block enough sunlight to interfere with photosynthesis. In such cases, leaves, needles, fruits and new shoots may be smaller or less intensely colored. Respiration can be reduced through the closure of stomates, and under drought conditions, plants affected will wilt more rapidly than unaffected plants. If plant vigor has been reduced, the plant may also be predisposed to further injury by insects, diseases or other environmental stresses.
Insect Association
Sap-sucking insects like aphids, scale and whiteflies are the primary cause of sooty mold. As the insects feed on the leaves and stems of trees and shrubs, they often produce excessive, watery excrement rich in sugars. It is on this waste, called honeydew, that the fungi grow. Sometimes a plant not actually infested by insects may be affected if honeydew drops onto it from a tree above.
Control
Sooty molds are controlled by managing the honeydew-producing insect. Chemical controls such as permethrin, cypermethrin and malathion are the most common, although biological and cultural strategies are available for some aphids and scales. Soapy water sprays, for example, help to break up the black sooty mold.
In cases where honeydew is dropping onto plants from an infested tree above, you probably won't, can't or need to treat the big tree, but you do need to remove the black sooty mold on the smaller plants. For this, I recommended Consan Triple Action 20. It breaks up the mold and can be washed off with plain water later.
If you need to stay organic, plain old soapy water will do, but you will need to rinse off the soapy residue with plain water so that it doesn't hurt the plant's respiration.