We enjoy magnolia trees for their beautiful early spring flowers, but in summer they sometimes get downright ugly. Gardeners may notice a fuzzy black coating on branches or a sticky glaze that attracts ants and wasps. “That’s known as honeydew,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle.
“It’s an excretion from scale insects, and the black stuff is a fungus growing on the honeydew.” The black mold is not in itself a disease and is harmless to the plant, although it may be unattractive to the gardener. These results of magnolia scale insects are easier to spot than the insects themselves.
A magnolia scale insect (Neolecanium cornuparvum) lies flat against the bark of a tree or shrub, sucking sap from the plant and dripping honeydew onto branches below. The insect has a protective covering built up from waxy secretions that makes it impervious to insecticides. “You have to look closely to see them along a branch,” Yiesla said.
“They look like oval bumps or overlapping scales about half an inch long.” An established magnolia tree can usually tolerate a small infestation of scale insects, but a large untreated population can divert enough sap to weaken the tree and eventually cause it to decline. “If you see honeydew or sooty mold on your magnolia, look along the branches for scale insects,” Yiesla said.
If the infestation is small and is localized to a few branches, you can just prune them out. Dispose of the infested.