Q.Fruitless Mulberry Tree
20 year old beautiful tree which went through male pollen phase and leaf growth with completely normal richly green leaves across the full tree. Last few days leaves have begun to fall off the tree in large quantities. I checked for all the symptoms of the usual diseases (mildew, cankers, nematodes, root rot etc.) and found on conditions which correspond to the conditions described.
I will assume that the leaves are yellowing first, then dropping off the tree.
I do think the most likely cause is lack of water.
Have you had dry weather? The Fruitless Mulberry Tree has a very large root system and needs additional water during dry periods.
You may also need to have a soil test done to determine if your tree has a nutrient deficiency.
Here are links with more information on care and disease issues.