Dry rootball + foliar feed = powdery mildew for me. Any time I spray during flower, I let it dry then spray with plain water and shake off excess. I would not recommend foliar feeding once the buds start to harden as the potential for mold goes up significantly (unless you are having herm issues, then reverse would be preferable - or horrible bug problems).I would do most of my feeding in veg and first two weeks of flower, occasionally I would give them a spray later in flower. The days I would do it in flowering were the days I didn't water, something like twice a day every week or so. one right before lights come on and one right before they go off, what ever felt right. Not only did this help with nutes, it help me control temps and humidity a little too. The main reason you want steer away from feeding in flowering is you really don't want that stuff getting your buds all wet, you could get mold, lose potency, etc. Also if the light is really intense where you spray you could get burns like MrPiston had stated.