Once you see PM on your plant, your plant has PM in its system and youll have to be vigilent in minimizing it during flowering.
REMEMBER TO CLEAN YOUR GROW ROOM REALLY REALLY WELL AFTER HARVEST TO PREVENT FURTHER PM ISSUES
First things first, your room is probably too humid, so you need to increase ventiliation if you can.
Then you need to spray some sort of anti-powdery mildew spray on the leaves, poor people use baking soda in water and/or milk, other options are Serenade, Neem oil, or sulfer.
If you have a lot of plants, consider purchasing or building a sulfur burner and buying a dehumidifier.
On the other hand, if you just have a tent or closet full of plants, the best answer is dunking or spraying your girls down with Dutch Master Zone and Penetrator.
In my experiance, use Dutch Master Zone and Penetrator and you wont have to worry about PM again.