Hi- how spaced out are your plants? Is there decent airflow? 

I'm going to resend my earlier answer with mistakes corrected, and my apology.  My cell phone likes to autocorrect to different words.  Here's what I intended to send you, and, I will incude a picture this time. 

Thank you for the picture! Unfortunately, powdery mildew spores could come from miles away, but airflow is a very common reason it would spread, which is why I asked that first. If everything infected isn't removed right away, it's likely to stick around and reinfect the plant again later. The mulch should probably be removed, as it could be infected. This plant likes dry, well-draining soil, and full sun. How often and when are you watering it? 

There are a few methods that may work, alone or in combination- cutting it down to 6 inches and removing any mildew in the fall,- this would be my first choice, along with new mulch, not too close to this plant, 

-or: planting something not prone to powdery mildew in that place next year, depending on the variety of blanket flower you could collect it's seeds, or only keeping the best portion of your plant for one bed. Fungicides or horticulture oils are options, last ditch efforts, if you were open to them, in an attempt to prevent it next year. 

Some resources:

https://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden 

https://hort.extension.wisc.edu/articles/blanket-flower-gaillardia-spp/ 

https://ipm.ucanr.edu/home-and-landscape/powdery-mildew-on-ornamentals/pest-notes/#gsc.tab=0 

You can cut them down and remove the mildewy leaves a few weeks before you move them. The goal being to keep as many of the mildew spores away from the new location and not upsetting the plant by changing everything too quickly. Happy planting!

You're welcome!