Answer: Without being able to inspect the leaves personally, I can't really diagnose the problem. Could be a fungal or bacterial disease, or what you're seeing might actually be sooty mold which is a by-product of insect feeding. I think the best course of action is to take some of the affected leaves to your local cooperative extension office, Master Gardener Clinic or well-staffed nursery for onsite diagnosis. Helpful folks there can also suggest remedies. Best wishes with your magnolia!
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