I had tremendous success with perennial blue verbena when planted 4 years ago, but I did over water them. By mid or late summer they were infested with tiny whitish flies that swarmed only when disturbed and a white or light gray powdery material on leaves and stems. I am guessing powdery mildew and aphids but not positive.
It got worse each year and last summer they looked terrible by early July. Has my soil become infested and need to be replaced? I live in zone 7 but have had a colder than normal winter,