Good Morning Dean, 

Thank you for contacting the U of M Extension Service. 

You might inspect your magnolia branches and trunk to see if it has scale insects. Here is a link from the U of M Extension Yard and Garden website: https://extension.umn.edu/yard-and-garden-insects/scale-insects

Many magnolia owners have had scale insect problems for the past few years. Many magnolias have been lost. 

If you do a further inspection, and the tree problem is not scale insects, it could be a leaf spot disease. Here is another link: https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs 

This information states: 

• Fungicides are not necessary unless a tree has lost all of its leaves several years in a row.
• Fungicides are protective and need to be applied before symptoms appear on the leaves.
  • Proper timing of fungicide applications can vary depending on the biology of the disease.
  • High-pressure spraying equipment is needed in order to get complete coverage of the canopy of large trees.
  • Hire a professional arborist to treat leaf spot diseases in large trees.

Good Luck!