The smallest members of the magnolia family, star magnolia (Magnolia stellata) bushes stand only 10 feet high at maturity. In early spring, fragrant, dazzling white flowers cover the bushes' rounded forms. These magnolias can be small garden show-stoppers, unless their attractive leaves and foliage become discolored with black mold. Black mold on a magnolia bush presents two problems--the mold itself, and the magnolia scale insects attracting it.
Up to 1/2 inch across, magnolia scale insects are common throughout the Eastern United States. They overwinter and develop from larvae to adults on the magnolia's twigs. The mature insects have mouth parts tailored to sucking fluids from the shrub's leaves. Heavy scale infestations cover a magnolia's leaves and branches with the insects.
Scale and Honeydew
Scale insects feeding on magnolia bush leaves produce clear, liquid digestive waste called "honeydew." The leaves, branches and trunks of trees with severe scale infestations can become covered with honeydew. Because it's clear, however, new honeydew may go unnoticed.
The black mold that appears on magnolia bushes with scale is really a fungus. Known as sooty mold, the fungus reaches magnolias as wind-blown spores. Once there, it consumes the honeydew. It does not feed directly on the magnolia.
The effects of black sooty mold on magnolia bushes extend beyond merely cosmetic. If the mold covers the leaves as well as the branches and trunk, it will interfere with photosynthesis and deprive the plants of food. Magnolia bushes already suffering from compromised leaf function because of their scale investigation may experience branch die back, severe weakening or death. The mold often remains on the plants after elimination of the insects.
Controlling sooty mold is a matter of controlling the honeydew-producing scale. Ridding your magnolia tree of the infestation, however, may require several years, cautions the North Carolina Cooperative Extension. While several treatment options exist, some are useful only when the insects are crawling on the trees in search of leaves.
Dormant oil used at the correct temperature will treat scale from late winter into summer when used according to the manufacturer's recommendations. Horticultural oil will suffocate scale nymphs in the overwintering stage. They also loosen the sooty mold. Insects will die with direct exposure to insecticidal soap, but may damage the magnolia if applied to new leaves.
An application of the systemic insecticide imidacloprid to the soil around a magnolia several weeks in advance of the scale nymphs' migration may be effective. The imidacloprid travels from the roots to the leaves, where the insects ingest it. Watching the magnolia with a soap and water solution will remove mold remaining after the insects are destroyed.