I have a 5-year old Crepe Myrtle tree growing in my rock garden. Suddenly 2 days ago the leaves have been turning yellow and dropping - this began on the bottom limbs and is proceeding up the limbs towards the top. I live in Acworth, GA. I have a new 1-year old tree and 2 bushes in different areas of my yard, and they appear to be doing fine. Please advise.