I have a mature crepe yrtle that has less than half of of its normal leaf count. The underside of its leaf has many small, dark, mostly round spots. I have another crepe about 100 feet away that is much healthier with a full leaf count. The underside of its leaf has a few dark spots. What do you think is the problem and what is the solution? How aggressive can I be with treatment of the sick crepe? Both crepes are beautiful and important landscape features in our yard. I noticed the problem today as I was doing gardening chores. I live in Sneads Ferry, NC. Thanks for your help.
Most Crepe Myrtle problems are caused by insects.
Treat with Neem Oil, this works as both an insecticide and fungicide.
Here are a few links with more information.