Q.mulberries died on the branch immature
Cold winter followed by initial dry spring. Branches covered in berries that didn’t mature but colored quickly and turned brown.

Downtoearthdigs
Answered on June 10, 2015
Weather and it's stresses to our trees and gardens is for the most part out of our control.
The leaves appear very healthy in this picture.
I'm sure your tree will make a good recovery and produce fruit next year.
https://www.gardeningknowhow.com/ornamental/trees/fruitless-mulberry/mulberry-fruit-drop.htm
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