Question
What causes crape myrtle leaves to turn brown and dry up?
How would you treat this problem?
Thanks!

Answer
If it has been hot and dry where you are, and the plant has finished blooming, I would say it is lack of water.  The plant will let some of the leaves go in order to feed the flowers.  When there is a great stress such as heat and drought the plant will have leaves that will turn brown and fall off.  Try watering a little more and see if the problem stops.  Good Luck.