We have about 11 crepe myrtles and they are about 3-4 years old. They start out every spring looking beautiful and when they first start blooming, the leaves start turning brown and crunchy and they do not recover until the next spring. They get interim new growth and then it also turns brown. They get plenty of water but we have clay soil that does not drain well. I cannot see any sign of pest. Please help!!
This sounds like either the tree is not getting enough water or nutrients due to the poor soil, or there is a fungus on the leaves. Treat the tree with a fungicide, like neem oil or copper sulfate, and consider amending the soil. Here is more info: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm