Magnolias are large trees used for shade or screening in the home landscape. Their showy cream-white flowers fill the air with their fragrance in the summer. The large evergreen leaves of magnolias are glossy green with reddish-brown fuzz on the undersides. Fuzzy brown cones ripen in late fall and winter with bright red seeds. The most common magnolia tree leaf diseases are leaf spots or blights, powdery mildew and sooty mold. None of these diseases are fatal, although they cause unsightly foliage.
Fungi, the bacterium Pseudomonas syringae, or the algae cephaleurus cause leaf spots on magnolia trees. Symptoms include discolored raised spots or blotches, yellowing leaves and leaf loss. Magnolia trees are more susceptible to these diseases during periods of high humidity and high temperatures. No chemical treatment is recommended for leaf spots.
Powdery mildew also occurs on magnolia leaves during periods of high humidity. Small dusty white or gray spots grow and merge together to form a coating on affected leaves and stems. In severe cases, the leaves and stems may become distorted or stunted. Apply a fungicide for powdery mildew on magnolias according to the manufacturer's directions.
Sooty mold is a black fuzzy fungus that covers magnolia leaves. The fungus grows on honeydew, which is secreted by aphids, mealybugs, scale insects and whiteflies. Use soapy water to wash the sooty mold off affected foliage. Apply a fungicide for sooty mold on magnolias following the manufacturer's instructions to kill the fungus. Use an insecticide according to the manufacturer's directions to destroy the insects.
Magnolias are slow-growing trees, eventually reaching 80 feet in height and will be 30 to 40 feet wide. They grow in zones 7 to 10a in full sun to partial shade. They grow in any type of soil, and tolerate drought and extended periods of flooding.
Remove infected leaves, clean up debris around trees, and destroy the leaves to prevent the spread of infection. Increase the air circulation in the tree canopy with pruning.