This looks like powdery mildew. We are sending a lot of it this year. 

Once you see the mildew there isn’t much you can do to manage it other than make sure that your plants have good air circulation.  Your best control is to prune out the worst of the leaves and make sure you create room for air to circulate. Clean up all debris in the fall.

It affects many plants including annuals, perennials and vegetables. Powdery mildew is mostly cosmetic and will not kill your plants, but a heavy infestation will diminish their ability to photosynthesize. This is not a big issue for annuals as they don’t have to grow strong roots to live through the winter.

https://hgic.clemson.edu/factsheet/powdery-mildew/

https://extension.colostate.edu/topic-areas/yard-garden/powdery-mildews-2-902/