Diseased Star Magnolia
What you most likely have is an insect problem, not disease. Most likely magnolia scale and you can read more here: https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now
Black sooty mold can then develop on the insects excrement. Read more here:
It can get quite ugly and the scale should be addressed to avoid the same thing
happening next year. Correct timing of control is critical and the recommendation is to involve a Certified Arborist, one who specializes in
everything woody. You can find one or more Certified Arborists in your zip code at www.treesaregood.com Click on the Find an Arborist tab.