Silver Spots on Magnolias

Scale Insects on Magnolia

• If your tree branches develop whitish-silver spots that are about 1/8 to 1/2 inch in diameter, and the leaf surfaces develop a condition call sooty mold (gray fuzzy covering), then it is likely that your magnolia tree is infected with Magnolia Scale (Neolecanium cornuparvum). This insect attaches to the plant and inserts its syringe-like mouthpart into the tree limbs to feed on the sap. As the insects mature around July in Northern temperate climates, the silvery-white wax coating wears off and they change to a pinkish-brown color. Males are smaller than females. The offspring, known as crawlers, are brown and they turn black after feeding. The insect overwinters in this stage.

Varieties Susceptible to Scale

• Species susceptible to scale include Saucer Magnolia (Magnolia soulangeana), hardy in U.S. Department of Agriculture plant hardiness zone 4; Cucumbertree Magnolia (Magnolia acuminata), hardy in USDA zone 4; Star Magnolia (Magnolia stellata), in zone 4; and Lily Magnolia (Magnolia liliiflora), hardy in USDA zones 5 to 8. Tulip Trees (Liriodendron tulipfera), hardy in USDA zones 5 to 9a, have also been reported as susceptible.

Contol of Magnolia Scale

• Application of dormant horticultural oil is an effective measure to kill overwintering nymphs. Apply the oil in the late fall or early spring. After the insects develop their waxy coating, insecticides are less effective. Insecticide applications are most effective during the period of crawler emergence late in the summer. Many insecticides are effective against crawlers, including malathion, insecticidal soaps, chloropyrifos and carbonyl.

Powdery Mildew

• If there are diffuse silver spots on your magnolia leaves, it is likely that there is an early infection of powdery mildew disease, caused by the fungi Microsphaera or Phyllactinia. These fungi grow on the top surface cells of the leaves leaving a powdery silver coating of spores and vegetative mycelia. They can also attack new, unfolding leaves in the trees, stopping growth. Powdery mildew fungi don't often kill plants, but their appearance is diminished and thus also their aesthetic value. Flower production can also be affected. The disease is worse in locations and warm climates where the leaves are wet for extended periods.

Control of Powdery Mildew

• Powdery mildew is controlled by horticultural oils applied it in the late fall or early spring. This kills spores that can survive the winter and reinfect the trees. Sulfur products are also effective, but they can't be applied when temperatures exceed 90 degrees Fahrenheit or within two weeks of horticultural oil applications. A biological fungicide made with the bacterium Bacillus subtilis helps provide protection from infection and has some ability to kill existing infections. Synthetic fungicides made with myclobutanil have both eradicant and protectant properties. Protectant fungicides like sulfur and Bacillus subtilis need to be applied early during the infection cycle, so look closely for disease presence. Afterwards, repeat applications every week to 10 days during conditions of favorable disease development.