I have seen the dreaded pm a couple times in my day and I've found the best preventative is a sulfur burner. I use mine every 4 weeks as a preventative treatment, for the plants and the room, and never see the stuff. Spray's work on the plants but don't fill the room therefore they don't take care of all of the spores and the pm will return...
here's my program: if there is a pm outbreak the first thing I do is a spray with some stuff called greencure. I do this when the lights are off and I wait an hour or two after they go off to let the plants cool down, then I soak them top to bottom. 3 days later I fire up my sulfur burner, again I wait a couple hours after the lights go off then fire it up for 4 hours (my area is 10x10x7), then I unplug the burner and let the room stew for a couple hours before turning on my ventilation and fans. the dark period would be like this - 2 hours with lights off and fan's/ventilation on - 4 hours with sulfur burner on and fan's/ventilation off - 2 hours everything off - 4 hours fan's/ventilation on. Use this up to the 4th week in flowering. It's very important to clean all surfaces whenever you have the chance to.
one thing I'd be careful of is using a sulfur burner within a couple weeks of spraying you plants with neem oil.
I've used this program in veg and bloom and it works. It's also og.