There are two diseases that affect magnolia trees. I am providing a link to the description, with pictures, of both.
Magnolia leaf spot is described here https://pnwhandbooks.org/plantdisease/host-disease/magnolia-leaf-spots and magnolia bacterial blight https://pnwhandbooks.org/plantdisease/host-disease/magnolia-bacterial-blight 
Note that under cultural controls, both diseases can be avoided by good air circulation and keeping the leaves dry.
Both are treated with the same chemical control and the only one that can be used by the home gardener, signified by an H surrounded by a box: Monterey Liqui-Cop at 3 Tbsp/gal water. H
Follow all label directions on the chemical control including when and how to apply.

Thank you so much for your fast response Sharon!  -Ellen 

If you had concerns about not enough water, I would expect to see wilted leaves, which I don't. The following publication has some good information on care and watering .https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1863&context=extension_histall