Magnolias are garden and landscape trees that put out bright, showy flowers each spring. The trees generally are very hardy and resistant to many types of pests and diseases. However, white spots that appear on the magnolia indicate a health problem that requires the gardener's immediate attention.
The primary cause of white spots on a magnolia tree is the fungal disease called powdery mildew. This disease is spread by spores and causes a grey or white film to cover the leaves and sometimes branches of the magnolia tree. If the disease affects the tree over the winter, flowers will become spore sites for the fungus; the blossoms will turn brown or black as they are destroyed by the powdery mildew's spores.
Causes and Treatment
Powdery mildew is spread through the air and begins growing during times of high humidity and temperatures. The spores are airborne and grow both in the soil and on the tree itself. Remove affected leaves from the tree and the ground around the tree and dispose of them to stop the fungus from spreading. Spray the magnolia tree with a fungicide to control any remaining spores and stop the fungal disease from spreading.
False Oleander Scale
A second cause of white spots on magnolia trees is the insect pest called false oleander scale, which feeds on a variety of trees. This pest is found throughout much of the southern portion of the United States. As the insect pest feeds on the magnolia tree, it will cause white spots to appear on the leaves, which weakens the tree over time. Apply a pesticide to the magnolia tree to control false oleander scale bugs.
Most diseases can be prevented through regular care of the magnolia tree. Keep the area around the tree free of standing water; make sure the soil drains to prevent fungi from growing around the roots of the tree. Clean up fallen leaves and branches to stop rotting material from attracting insect pests and housing fungi. Prune the tree annually to get rid of weak branches of the magnolia tree where diseases can take root.