Thanks for writing. I'm really sorry, but without being able to see your magolia in person, I can't say what the problem is. This free service really isn't suited to diagnosing plant problems like this because there are a lot of clues I miss from not getting to see the plant.
I suggest you contact the folks at your county extension office at http://web.extension.uiuc.edu/ajmpu/ --- they should be able to help better if you can bring in a sample of the affected leaves and answer any questions they have about the plant and where it's growing.
---Justin, Senior Garden Editor, BHG.com "