I have never seen this or any other disease/pest on Crape Myrtles. Has weather been unusually wet or humid? Also, post a picture if possible.

I, too, had a mildew problem on my crepe myrtle, for the first time ever. I just pruned the affected branches & the mildew still took over. It actually spread to an adjacent plant that I cared more about (than the c.m.), so rather than spray & risk more plants, the crepe myrtle had to go!

I asked around, and a couple of gardening-savvy people told me to just ignore it, and it would go away. And it did. I'm sorry your mildew problem was so bad that your Myrtle had to go.