What causes the leaves on crepe myrtle to turn yellow and red then they fall off?
Certified GKH Gardening Expert
This is a leaf spot disease.
This nasty fungus invades the leaves and produces spores that travel up the tree.
It thrives in warm wet weather and stagnant air.
Fungicide should be used every one to two weeks.
Here is a link about Crepe Myrtle trees and their care.