Q:  Why are several perennials in my garden bed turning black? Can they all be affected by the same disease?

A:  Most plant diseases are not that wide-ranging, but in this case, the black residue is not an infection. It is a fungal species, but this fungus does not cause plant disease, as it only grows on the surface. Named for its black color, sooty mold is a group of fungi that feeds on the sugars in honeydew. Honeydew is a waste secretion produced by sap-sucking insects, such as aphids, leafhoppers, soft scales, and planthoppers (which includes Spotted Lanternfly).

The numerous questions that UMD Extension receives each year about sooty mold often result from the presence of Crapemyrtle Bark Scale and Spotted Lanternfly. At this time of year, a heavy buildup of sooty mold from high populations of high-bark scale or lanternflies can be a prominent eyesore. Reduced rainfall that isn’t diluting or rinsing away the honeydew contributes to the problem, but it’s also due to increased insect feeding. (The juvenile lanternflies present earlier in the year don’t generate as much honeydew.)

Sooty mold wears off on its own over time. If the insects generating the honeydew are suppressed (which is not always necessary from a plant health standpoint), it deprives the sooty mold of its food source. If you want to try removing the residue, look for products with a horticultural oil active ingredient, which include specific directions for sooty mold treatment on the label. The spray may make the mold easier to dislodge several days later using a strong spray of plain water from a garden hose.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.

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