Your Magnolia appears to have Leaf Scorch which is caused by drought and/or heat stress.  

Not enough water reaches the leaf margin to keep up with transpiration or too much salt from overfertilization has been concentrated in the leaf margins.  

Your magnolia needs a consistent water supply of 2 to 3 inches of water weekly.  To insure it is getting the proper amount monitor the rainfall as well as added water.  This can be done with a rain gauge or other container.  To maintain the soil moisture and insulate the roots from occasional scorching heat, apply mulch  around the base of the tree covering the root zone.  Be sure to keep the mulch away from the trunk.  Mulching also suppresses weed growth which will compete for the available water.

Be sure no chemicals you may use on lawn or garden for weeds come into contact with your magnolia by runoff or by wind drift.

Prune the dead and broken branches.

Monitor your magnolia for improvement.  If it continues to decline possible root problems can be explored.

Thank you for your question