magnolia tree leaf drop
The damage to the leaves looks like old damage and no control is necessary. Some reasons for decline and leaf drop include poor soils, poor drainage, soil compaction, lack of sunlight, etc. See our publication on these types of problems http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG86%20Common%20Abio...
In the shade the tree can thin out. Magnolias are surface rooted and very competitive for water and nutrients. Keep your magnolia well watered during dry periods. Mulch with an organic mulch about two inches in depth and keep away from the base of the stems. All you can do is prune dead wood and nurture or replace the tree.