- A plethora of different fungi cause sooty mold including Cladosporium, Aureobasidium and Capnodium. The dark color is the result of the dark pigments in the wall of the hyphae of the fungus. This fungus can grow on conifers, such as pine, hardwoods and other plant species. The black sooty mold can even grow on outdoor furniture or structures. The black soot is primarily a cosmetic issue, but the underlying cause of the fungal growth will guide you to the correct treatment.
- Black sooty mold grows on sugary excretions called "honeydew" from sucking insects. Insects that excrete honeydew include aphids, scales, mealybugs and whiteflies. Pine trees are often riddled with scale insects that suck out and excrete honeydew that the black sooty mold grows on. Sooty mold lives entirely on these excretions and cannot penetrate leaf or bark tissues.
- Black sooty mold can be seen as a thin, crusty mass over the surface of the leaves. It can also grow as a thick spongy mass that encases the needles and twigs on trees. Even though the mold poses no direct threat to the tree, the soot can block light from reaching the needles, preventing photosynthesis. This interference can lead to yellowing of the needles and even twig death if the infestation is thick enough to completely block sunlight. Look for scale insects along the stems of the trees; they will not move and look like raised bumps. Treating the scales should be your primary concern. The precious sap they are sucking and excreting can weaken or even stunt the growth of your pine tree.
- Scales can be tricky to treat. They need to be sprayed during their crawler stage, and this stage can be different for the many species of scales. The pine needle scale, Chionaspis pinifoliae, is crawling in mid to late May and should be treated before it creates its hard protective coating. Many products are effective on crawling scale insects including malathion, carbaryl, acephate and insecticidal soap.
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