mamashakesit,

The old saying, "An ounce of prevention is worth a pound of cure" applies to mildew and other plant diseases in general. I like to spray my zinnias with a product called GreenCure®. It is safe to use and controls a long list of plant diseases, including Powdery Mildew and Downy Mildew. GreenCure is better than homemade formulas and can be used on many ornamentals, including Verbena. Incidentally, I am not in any way associated with GreenCure, other than as a user of the product. There are, of course, other products that you can use against mildew on Verbenas.

ZM