Mildew on wild bergamot?

A day after our huge rainfall of 56 mm, I had to rearrange the soil in the garden so that the lowest spot was in the middle instead of on the sides where water was accumulating the most. I was relieved to see that there was no water left 24 hours after it stopped raining.

I was surprised to see white spots on the wild bergamot (swamp milkweed). Is it mildew? Isn't it supposed to be a plant that tolerates having wet feet? I'll have to read up on that to see if there is anything I can do. 

Mildew (white spots) on the swamp milkweed?