Mildew, more properly called "Powdery mildew", is, like the name says, "powdery". This is what the onset of Powdery mildew looks like on a zinnia.

ZM

if you look to the newest growth ... it looks unaffected ....

so it makes me wonder.. what happened a few weeks back ....

location seems to add info usually.. could there have been a very light frost???

ken

Thank you for the replies!

The temperature dropped drastically but I’m pretty sure it didn’t drop below freezing. It was also raining quite a bit over the past week

i asked if there was a frost ... that is much less severe than a freeze??? ... how cold did it get???

ken

The lowest temperature it got to was 3C or 37F

In my experience, coleus shows resentment of cold temperatures at around 40F. It is a fully tropical plant, native to equatorial locations. Without a doubt, the chilly temperatures have damaged your very sensitive plants.

Thanks all!