My verbena bonariensis has white leaves

[Maggiethecat]

First time growing them and notice white lower leaves, suspect this is mildew.
Do I just remove the leaves and hope for the best?

Yes I would. It’s tough as old boots

I had this last year. I removed the affected leaves and soap-sprayed the rest. It still has a few white leaves this year but is growing vigorously anyway.

Sounds like powdery mildew. Worse when the plant is dry at the roots but the weather is relatively humid. A good watering helps.