Depending on the type of system you are running, you could try 15 minutes on, and 15 minutes off. Lowering your humidity may help get rid of the White Powder Mildew. I have had success with spraying the plants with 9 parts water and 1 part whole milk. Wet the leaves very well, but be sure to have extra ventilation to dry plants quickly. Gets rid of the WPM by changing the ph of the leaves somehow.
Good Luck!