Q.Our Established Crepe Myrtle Tree In Dropping Leave And Flowers And Now The Branches Are Quite Sparse. What Can We Do?
It was fertilized in the spring. It is mulched. It gets regularly watered by the sprinkler system. We do not see mold on it.

Certified GKH Gardening Expert
Unfortunately, it is hard to say without seeing more of the plant, and the issues such as insides of branches and any growth still on the tree.
Taking samples to your local garden centers or extension service can help since they can see the issue in person. This link will help you to find your nearest extension service:
https://www.gardeningknowhow.com/extension-search
I suspect, possibly, Verticillium wilt, or crape myrtle blight.
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-tree-problems.htm
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-tip-blight.htm
This collection of articles will give you more information on crepe myrtles:
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle