Looks like a fungal disease. Magnolias can get mildews and canker. I live in South Florida and was concerned about how mine would fair, especially during monsoon season and they're not supposed to do well down here. I added elemental sulfur to the site before planting to drop the pH (we have alkaline soil due to calcium carbonate) and planted it in full sun. It bloomed the first year, and no problems with it since. If yours are having the same problem in the same spot you might consider another location or solarizing the soil to kill any pathogens. Good luck!