What stage of growth are they at ? How bad is it ? what have you done so far ? Are you growing inside or out ? Hydro or soil ?
Do you ventilate during dark period ? ( some pics would be great ) then I can make a siggestion on product\methods etc.
GOOD LUCK WPM really sucks, but gray powdery mildew or BOTRYTIS is DEATH and if you don't get rid of the WPM you will end up with GPM at wich point its easier to just burn your house down than to make a feeble attempt at getting rid of it.IMO..
.J.L.