Ask the Master Gardener: Sooty mold likely caused by insects

By: Trish Grenfell Placer County Master Gardener
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Question: The trunk, branches and leaves of my pyracanthas (firethorns) are again this year turning absolutely black. It did the same thing last year, losing many of its leaves by mid summer. I noticed that it improved after the summer heated up, so I hoped that the shrub had healed itself. Apparently that is not the case. I bet I have you stumped! Answer: If the plant looks like it is covered in soot, you likely have been hit with sooty mold. This term is used to describe several black-colored fungi that usually don’t harm the plant unless the black growth is substantially blocking the sunlight and inhibiting photosynthesis. Since it grows only on the plant’s surface, you can actually rub it off with your fingers. To dislodge the mold growth from your bushes, apply water with a strong spray. If that doesn’t work, the Ohio State University Extension advises a mild soap solution (one teaspoon per gallon) be sprayed. But before you spray with soap, do at trial run. Spray a small area and then wait a week to see if the soap solution damaged the plant. You are not home free; you don’t know what caused the mold in the first place. It is highly likely that your pyracanthas are infected with sucking insects. Sooty mold grows on honeydew secretions on plant parts and other surfaces. These secretions come from a number of insects that produce the honeydew needed by sooty molds to grow aphids, mealybugs, soft scales, whiteflies, leafhoppers and psyllids (including eucalyptus lerp psyllid). Both the immature and adult stages of these insects feed by sucking sap from plants, producing honeydew. Therefore, control of sooty mold begins with management of the sucking insect creating the honeydew. After finding and identifying that insect, refer to for eradication methods. Hint: Pyracantha has been known to harbor apple and bean aphids, as well as these soft scales: European Fruit Lecanium, Black Scale, and Kuno Scale. Have gardening questions? Call the Master Gardener hotline at (530) 889-7388.