Thank you for your question. There are several issues that magnolias have, and I'm not sure when you say "sooty," you mean some type of fungal growth. The most common of these among Southern magnolias is anthracnose, described in this Extension article. I would suggest that you take some of the leaves (in varying stages of decline) to your county Extension office at 1951 Woodlea Rd.,Tavares  (phone<personal data hidden>) to have a staff person examine them and help you with a solution, be there any.  I trust they will be helpful.  Good luck!