Is this powdery mildew?

[CosyCoffee]

I've recently noticed these powdery spots on my (much beloved) hosta. Is it powdery mildew or, as I'm hoping, some limescale or such from the tapwater?

Thanks!

I don't know but, though I get powdery mildew on various plants (dahlias, geraniums), I have never seen it on hostas. What happens if you wash the leaves?

I'm going to try washing the leaves with rainwater tomorrow and see how it dries. Fingers crossed!

Looks more like limescale from hard water to me.