I have a crepe myrtle, in fact 2 crepe myrtles, and in trying to clear them up have pruned them to death, cut off many leaves. New growth seems to be coming; however, I am still dealing with what is wrong. I am using ivory dishsoap, as the other sprays have had the directions not to be used more than 3 times a year and this was not helping. So I talked to a professional and was told to wash the tree off. I have done so a good many times. The leaves are yellow and some are blotchy. It is also a bit sticky in the morning, but I can see nothing. However, I felt a web sort of and saw a spider, it looked like making a web.
Certified GKH Gardening Expert
Sounds like you have spider mites. I would recommend treating the tree with neem oil. It is effective against these pests and is a systemic pesticide, so you don't need to reach every nook and cranny the mites might be hiding in. The tree will absorb the neem oil and the whole tree essentially becomes lethal for a short period of time to insects that feed on trees.