Anyone who has been gardening in Texas for more than a few years knows about black sooty mold. And they probably also know it’s the sign of a bad insect infestation. Although the term “mold” is in there, it’s actually not a fungal disease.
Black sooty mold is a velvety, gray-black, crust-like coating on the leaves, needles, fruits, or branches of certain plants. It grows only on the surface and will not kill plants, and it can easily be removed by rubbing it away with a finger. It also often appears on sidewalks and fences and under infested trees.
Sooty molds are normally considered nothing more than cosmetic or aesthetic issues, but in extremely severe cases, it is possible for the growth to block enough sunlight that it interferes with photosynthesis. In those 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. If plant vigor has been reduced, the plant may also be predisposed to further injury by insects, diseases or other environmental stresses.
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. That’s a form of scale, as seen in the picture, courtesy of Southern Living Plants.
In cases where honeydew is dropping onto plants from an infested tree above, you probably won’t need to treat the tree, but you should remove the black sooty mold on the smaller plants. For this, I recommended Consan 20. It breaks up the mold and can be later washed off with plain water.
To stay organic, plain old soapy water will do. But you will need to later rinse off the soapy residue with plain water so it doesn’t hinder the plant’s respiration.