My (limited) experience thus far is that morning sun is very important to preventing powdery mildew. Avoiding overhead watering is also helpful. Hopefully someone will have a suggestion for how to get rid of powdery mildew once it shows up.
The other thing to avoid is grass thrown by lawn mowers. Grass has plenty of fungus and mold that potentially transfer readily when mowing the lawn - with wet grass being a double whammy as it sticks and subsequently provides a bit of extended moisture and shade for whatever it is transferring.
I personally have not seen any evidence that itohs are more resistant to anything that plagues peonies - but then, I only had a pair of them last year. (One of my itohs was by far the most impacted by mold or fungus - I ended up cutting it down in early September. But it was also a yearling and was almost buried in grass debris.)
EDIT: I came across this in my notes - I am not sure where I got it from or I would credit the person / website.
POWDERY MILDEW REMOVER
To 1 gallon of water add 1 tablespoon of baking soda, 3 tablespoons of canola oil, and 1/2 teaspoon of dish soap. I seem to recall that it was important to get the underside of the leaves.
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