Thanks for the question.

Barberry is often infected by a fugus that causes white spots/areas to appear on its leaves. It is called powdery mildew. It is possible that this is what produced these white areas. See:

https://extension.umn.edu/plant-diseases/powdery-mildew-trees-and-shrubs - :~:text=Powdery mildew is a common,young leaves and green shoots.

https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/barberry/leavesdiscolored.html

https://pnwhandbooks.org/plantdisease/host-disease/oregon-grape-berberis-aquifolium-powdery-mildew

However I am not fully confident in my assessment. If you really want to get to the bottom of things, you should submit leaf samples to the Plant Disease Clinic of the University of Minnesota. Before submitting anything, first contact them to be sure you are submitting things in an appropriate manner. See:

https://pdc.umn.edu/

Good luck. Thanks for using our forum.