Powdery mildew appears as a white or grayish-white powder that covers the top surface of the magnolia leaf, though it might also affect the stems and the flowers. Though it can be easily brushed away, it quickly becomes prevalent enough that this is not an option. As the powdery mildew infests the magnolia tree, the leaves become twisted and warped, turning yellow or brown. A severe infection of powdery mildew can cause leaf drop and prevent the buds from opening. The type of fungus that causes powdery mildew on magnolias does not affect trees other than magnolias.
Powdery mildew is typically most obvious during the late summer and the early fall. During the winter, powdery mildew takes the form of tiny black spheres which cluster in stems, in dormant buds and in the leaf litter on the ground. When the days are growing warmer but the nights are still cool, they begin to infect and spread throughout the magnolia tree.
Practice good garden sanitation to prevent powdery mildew from taking a hold in your garden. Do not allow leaf litter to sit on the ground over the winter, and do not use leaf litter as a compost or a mulch, especially if there has been powdery mildew in the year before. Give your magnolia plenty of space to spread, so that it does not easily pick up fungi from other trees. Check the spread of the mature magnolia that you are planting and give it at least that much space.
Prune out any affected shoots that show powdery mildew, taking care to dispose of them away from your property. When the infection is severe, use a chemical fungicide that is approved for your area. A fungicide will kill the powdery mildew, but you should spray the entire tree, not just the affected area. Spray every one to two weeks to keep younger magnolias fully covered and protected from the powdery mildew.