Certified GKH Gardening Expert
This is the time of year for bacterial infections! Usually, this is the result of saturated soils that remain that way for quite some time.
You can attempt to use a fungicide, but it won't completely cure the issue, usually. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Other than that, keeping all heavily infected growth cut, and keeping debris away from the area on the ground will help to kill off the bacteria.
A dry out will be helpful, if the weather cooperates.
Here are some articles that will help: https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-diseases.htm