Magnolia leaves turning brown and falling off- part 2
It looks like your magnolia is showing symptoms from environmental stress, probably as a result of the current drought. Our plant pathologist looked at your photos and we do not see any signs or symptoms of a disease or pest.
The best thing to do at this point would be to water the plant regularly when we do not have rain. A light layer of mulch (2") placed around the base of the plant can help to retain moisture. Do not place the mulch right up against the branches. Water deeply at least once each week, or consider setting up a trickle irrigation system.
We've had tough weather conditions lately and a lot of plants are showing stress symptoms like this.