Hello Elaine,

It looks like your young magnolia tree has a fungal infection called powdery mildew. It is a contagious fungal disease that infects plants such as magnolia trees. It appears as a powdery or fuzzy white growth on leaves. This fungus causes the tree's foliage to discolor, wilt and fall off the branches. 

Powdery mildew can be treated but most universities recommend treatment only if it is a high value tree. Although it is unsightly it will rarely kill a plant. It is best to manage the disease as fungicides will not cure a current infection or kill the virus. 

One means of preventing further infection and reduce the spread to other plants is to make sure to remove all leaves that fall off the tree and dispose of them in the trash. You could also trim off the affected leaves. Make sure to disinfect your pruners after each use so you don't inadvertently transfer the disease to another part of the tree or other plant. 

Here are some links to powdery mildew and other diseases that can affect magnolias https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/magnolia/spotsonleaves.html.   https://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden#:~:text=Many%20different%20fungicides%20are%20effective,and%20horticultural%20oils%20are%20recommended.

I hope this helps. Please let us know if you have additional questions.