I know this is not answering your question, but I've found the best solution is to select the best variety, ie a powdery mildew resistant variety. After having lots of problems in humid Sydney last year with peas, zucchini and cucumber, this year after selecting better varieties, results have been brilliant!

g'day mabb,

the milk recipe is on our site on the remedies page, give it a try lots do and swear by it.

for me i find if plants themselves are as healthy as they can be that is good drainage heaps of sun when they need it all those factors then the plant can resist disease attack.

in the 'burbs zucchinis & pumpkins and things gave me heaps of problems and sufferd a short production life because i wasn't able to get all the factors right (lots of variables). but here where they get full on sun and very good drainage i only get p/m toward the end of the production season on zuces and that is when the plant is at low ebb. and never get it on pumpkins or anything else.

len

Here is a link that might be useful: lens garden page

The milk spray, in my experience, slows it down but doesn't get rid of it. Dilute the milk (full cream) 1:10 in water. Add a tablespoon of good plain yoghurt to boost it. If using town water, fill a bucket and let it stand overnight before using it. What you're after are the bugs in the milk and the chlorine in town water kills most of them. Leaving the water exposed overnight will allow the chlorine to evaporate.
Another spray that is supposedly useful is garlic spray. Do a web search for garlic spray and antifungal to find exact quantities. Yet another that might work is weak chamomile tea. In any event, the powdery mildew shouldn't stop you from getting a crop, unless it's a really bad infestation.
And both Len and bikingchris offer good advice - make sure your soil is as healthy as it can be, plant with some spacing for aeration, and maybe try sourcing varieties that cope better with PM.
Hope this helps.

Thanks everyone, We'll be looking at more resistant varieties for next year. We have drip irrigation, and raised beds, and the beds are all on the north side, so I think that's all ok. BUT, I think maybe they're crowded... that could be a factor.

We've had humid weather in Melbourne, and a fair amount of overnight rain so even though we don't water overhead, the sky does :-)

We have some beat-a-bug (garlic, Chilli, Pyrethrum) I might give that a whirl as well as the milk spray.

Does anyone know how often you can use the milk spray? Should we do it every day until it halts its march up the plants?

I used milk spray every week or so and after rain inbetween.