I have white mildew on a Japanese magnolia. How do I get rid of it?

Every area is different, even in the same neighborhod. You need to go to your local REAL garden center/nursery (not box store ie Home Depot, Lowes) to find out what grows best in your area & how to doctor it. You need to bring a little light pail of dirt to be analyzed, so you can put in the correct product to amend your soil to a healthy soil that the plant needs, or that you will be planting. The best thing to do is contact your State's Cooperative Extension Service. Every state has a State's Cooperative Extension Service, it is a part of your State University System. Just Google & include your state's name to get the email address &/or tel number for your state. They will let you know what to plant & how to take care of it & usually have soil testing for a small fee or free you can also buy soil testing kits in gardening stores( yup H D & Lowes) or on line. Actually the soil should be tested every Spring, & in several areas around your yard. That is how diverse soil can get. Your State's Cooperative Extension Service, is specifically there to handle all of your state's agriculture needs, be it animal, forestry, gardening, even an African Violet on a windowsill. If they are far away they can recommend a Master Gardener in your area. Feel free to contact them, that is what they are there for.

Also Pinterest.com has great suggestions.

Spray with copper sulfate.