humidity... powdery mldew can occur during cold or warm weather... still air and high humidlty are the conditions for pm outbreaks.

to treat the problem... 1 oz of bicarb soda, a couple of drops of insecticidal soap and 1 gallon of water - mix and spray plants lightly, either lift the lights or move the plants out until dry. Don't treat at lights out... the plants need to dry off quickly.

Prevention - REDUCE HUMIDITY

keep extraction fans going during lights out... if this makes it too cold, put extraction on a timer set for 15 mins on and 15 off

reorganise your watering so that you are not putting so much water out during the dark period.