Question
What are some diseases that affect crepe myrtle?

Answer
Sun:
Here in the U.S., Powdery Mildew and Cercospora leaf spot are the most significant infectious diseases.  Both are caused by fungal organisms and attack the leaves primarily. Another non-disease problem is sooty mold. It causes the leaves/stems to become black.  Sooty mold is the result of an insect infestation.
Hope this helps

Steve