I have a few suggestions:
1. Mildew is sometimes a problem on some varieties of a flowering tree in the Deep South called crape myrtle. If it's a serious problem, some people spray the leaves with Lysol.
2. If you're having an unusually cool/cloudy/rainy summer it might be a problem for this year only.
3. Some plants don't like wet leaves and preferred to be watered under their leaves. The wet leaves can mildew.
4. I don't know much about roses; you might try posting your question on a rose forum.
5. Take some of the leaves and examine them with a magnifying glass. You might find it's not mildew at all.

NE,

"3. Some plants don't like wet leaves and preferred to be watered under their leaves. The wet leaves can mildew."

A common misconception about Powdery Mildew is that PM is promoted by wet leaves. That misconception is so widespread that it is now almost an urban myth. Many supposedly authoritative books say that wet foliage promotes Powdery Mildew. Wet foliage does promote many foliage diseases, but not Powdery Mildew. Apparently most garden writers refer to the works of other garden writers and this has become a glaring example of the hazards of "Group Think".

The truth is that Powdery Mildew spores cannot germinate in water and that water prevents it. Powdery Mildew grows on dry leaves. Refer to the National Gardening Association's Q&A on Powdery Mildew on Zinnias in which they say,

"Powdery mildew is unique among common plant diseases in that it doesn't require a wet leaf surface to spread. It can thus thrive during hot, dry weather, which is why you see it appearing in August. The general advice to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces. In the case of powdery mildew, you can actually inhibit infection with frequent sprays of water."

Considering all the books and other publications that are wrong on this subject, and how widespread this urban myth actually is, its going to take a lot of effort to correct this deeply rooted misconception about Powdery Mildew. I am giving it a shot here, but it's a shot in the dark.

ZM

I agree with zenman. I have even read (and naturally I can't find them now) that wet leaves helps to inhibit powdery mildew. Never had it on hostas, though. The strain of mildew that affects zinnias and phlox can be minimized with a milk/water solution (I have had success with it, anyway) but once mildew sets in, its tough to get rid of.

tj

Funginex
Captan
Daconil
Maneb

Pick one

A safer choice is GreenCure®. It was developed at Cornell University as a superior alternative to homemade sodium bicarbonate formulas and it is effective against several plant diseases, including Powdery Mildew. I give my zinnias a frequent bath in GreenCure.

ZM

P.S. I am in no way associated with the GreenCure product or any of its vendors. I just use the stuff because it is safe.