Awhile back, I mentioned that I was experimenting with treating the
powdery mildew on the roses w/ milk, on the recommendation from
someone at Natural Gardener.

I just thought I'd post that the milk not only seems to help reduce
(but not entirely eliminate) the powdery mildew, but also the
occasional blackspot as well. We just use the 2 percent milk already
in the fridge and soak the leaves using the Ortho sprayer on its
highest volume setting (8 oz/gallon?). Of course, it does have to be
reapplied after a rain.

Apparently, the milk changes the Ph of the leaf surface and makes it
less attractive to the powdery mildew.

Anyway, hope this helps someone else!