Question - What causes a sticky goo on crepe myrtles?
What makes the crepe myrtles develope a sticky goo (sap?) on the whole tree?
Crepe myrtles often suffer from powdery mildew. An excellent factsheet explaining the problems that can be encountered with crepe myrtles can be found at http://hgic.clemson.edu/factsheets/HGIC2002.htm.