Magnolia tree diseases
Compare the pictures in the attached article (link below) to your tree. Do you see any bumps that are similar? If so, that confirms you have magnolia scale. These feed on the tree sap and excess sap, called honeydew, drops to the ground. The sap sticks to the branches and a sooty, black mold will grow on this sap.
There is a treatment to kill scale insects but timing the treatment is important. In spring, horticultural oils, often called summer oils, can be applied before the flower buds swell to kill the overwintering nymphs located on the stems. Do not exceed the label rate, which is usually a 2 percent concentration of oil in the spray that is applied. Fall application of insecticide or oil spray is most effective.
This Article with pictures, gives the details: https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now
If this isn’t a match for what you have, please take pictures of the whole tree and a clear closeup of the branches and attach to this question. Thank you for using our service.