I talk about Black Sooty Mold a lot on the Gardenline. Quite often, the first advice I give is to solve the insect problem that usually causes it. But in this email tip of the week, let's work from the beginning. WHAT IS BLACK SOOTY MOLD? It's a sooty, gray-black, velvety, often crust-like coating may develop on the leaves or needles, fruits, and branches of certain plants. The coating is actually the growth of one of several species of black-colored fungi or molds. The coating can be removed easily by rubbing the leaf between the fingers, thus exposing the green leaf tissue below.
Sooty molds grow only on the plant surface and will not kill plants. In fact, sooty molds often grow on sidewalks or fences under infested trees. Sooty molds are normally considered to be a cosmetic or aesthetic problem. In extremely severe cases, it is possible for the black growth to block enough sunlight to interfere with photosynthesis. In such cases, leaves or needles, fruits and new shoots may be smaller, or less intensely colored. Respiration can be reduced through the physical closure of stomates by the molds' vegetative growth. Under drought conditions, plants affected with sooty mold will wilt more rapidly than unaffected plants. If plant vigor has been reduced, the plant may also be predisposed to further injury by other insects, diseases or environmental stresses.
Sucking insects are the primary cause of sooty mold growth. Many plant sap-sucking insects feed on leaves and stems of trees and shrubs. These sucking insects often produce excessive, watery excrement rich in sugars. This excrement is called honeydew. Excreted honeydew often falls on leaves or needles, branches, fruits or anything else immediately underneath the infested area of the plant. It is on this excretion that the sooty mold fungi grow. Sometimes plants not actually infested by insects may be affected if a tree above them is being attacked by a honeydew producing insect and the honeydew drops onto them.
The major types of honeydew producing insects in our region are: Aphids, Scale and Whiteflies.
Control sooty molds by controlling the honeydew producing insect. Chemical control is most commonly used to manage these sucking insects, although biological and cultural control strategies are available for some aphids and scales. We can do that orgnanically with Soapy Water Sprays, which also help to break up the black sooty mold. Or with chemical controls such as Permethrin, Cypermethrin and Malathion.
If you have black sooty mold on trees/shrubs, but no insects then it's coming from above. In these cases, you probably won't/can't/don't need to treat the big trees, but you do need to remove the black sooty mold. In this instance, I've always recommended Consan Triple Action 20. It breaks up the black sooty mold and can be washed off with plain water later. If you need to stay organic, plain old soapy water will break up the black sooty mold, but you will need to come back and rinse of the soapy residue with plain water so that the soap doesn't hurt the plant's respiration too.
Until next week, here's to
Great Gardening from the GardenLine, heard
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