one of my Japanese Maples, which seems to have a pretty bad case of powdery
mildew (fine white powder on the underside of the leaves). I don't really know
what has caused it - I have 2 maples and the other is fine (it is a bit more
developed, perhaps more disease resistant).
they get good part sun and virtually no wind,

There's your answer. To prevent powdery mildew, you need a little more sun, and
more air movement.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)