Magnolia and Black Fungus

Magnolia Scale

Magnolia scale insects attack magnolia trees and shrubs across the Eastern United States. Their favorite targets include the star magnolia shrub and saucer, lily and cucumber magnolia trees. Mature brown or orange female magnolia scales measure as much as 1/2 inch across. They have elliptical, domelike bodies. The much smaller male scales mature into tiny, winged insects before mating and dying.

Scales feed on magnolia foliage with piercing mouth parts that drain fluids from the leaves. Heavy scale infestation may kill twigs and leaves. The insects also secrete a clear, sugar-rich liquid waste called honeydew. The waste might drench a magnolia's leaves and branches. It sometimes spreads to surrounding objects and vegetation. Bees and ants feed the sweet waste. So does a black fungus commonly known as sooty mold.

The wind deposits sooty mold spores on a magnolia's honeydew-coated leaves and branches. The spores germinate in the waste, producing dark gray or black threads called mycelia. As the mycelia spread, they cover the magnolia with black powder. Sooty mold consumes the honeydew but not the plant. Heavy layers of the fungus, however, might interfere with photosynthesis. Combined with the scale-related stress, food deficiency might eventually kill the plant.

Eliminating the scale infestation automatically deprives the sooty mold fungus of nourishing honeydew. Removing branches with the largest scale populations before treating the plant increases treatment effectiveness. Ladybugs and their larvae feed on the scales during late summer. A fall or early spring application of dormant horticultural oil kills scale larvae that overwinter on the magnolias without harming the ladybugs. Applying the oil in summer or early fall kills newly hatched larvae, but it also destroys ladybugs. Spraying in early summer won't affect adult female scales because they're protected with an impenetrable wax coating until August.

Removing magnolia scale with horticultural oil simultaneously loosens a plant’s covering of sooty mold. Rain and wind eventually clean the plant of the fungus. Spraying the plants with water speeds the process.