Answer: Magnolias suffer few pests and diseases but it sounds as though your tree has developed lots of problems. Pesticides will kill insects but won't help with fungal or bacterial diseases. If you want to try to save the tree, prune away anything with spots, anything off-color, and anything that is dried out and obviously dead. If the roots are healthy, new branches and leaves will grow.
It might be better to simply dig up and dispose of the tree. It may be sickly its entire life. Replace it with a healthy specimen this fall and you'll have flowers next spring.
Best wishes with your tree.
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