I think it could be mulberry leaf spot disease caused by the fungus, Phleospora Maculans. The main symptoms are an initial appearance of small dark brown, vein limited spots. As the disease progress, these spots, enlarged, often surrounded by a diffuse chlorotic halo. In severe attacks, leaves yellowed, withered, and dropped prematurely.

https://bfw.ac.at/ws/sdis.schadenstyp...