How to treat (NOT prevent) Powdery Mildew on a flowering plant (week 4)?

I do not share the opinion of the other commentators. Powdery Mildew is a fungus and spreads through spores. The spores will be everywhere in your grow area already. The best thing, in the first place, is to prevent this to happen in the future by giving your ladies a good air flow and also clean/filtered air. You should probably change your carbon filter or get a stronger filter. Powdery Mildew is nothing you should smoke and if you get this infection in flower, you should most likely not finish your lady. I know it sounds hard but health goes first! In the end you have to decide on your own, I'm just telling you that you won't be able to get rid off all the spores by using organic matter. This would require chemicals and everyone knows you should never add or spray chemicals on your lady at all- and especially not into flower.

Mold grows on wet surfaces. You want to kill it, by spraying the leaves. Every time you try and 1% mold is not killed, it will use the extra water to grow back. It's better to try to lower the humidity to the minimum, and adding lots of ventilation. And keeping the leaves the dryiest the better. It's like nute burn or other leaves damage, you won't get healthy leaves back, cut the losses, but stop adding water because you will contaminate everything! Happy smokes!

I use 1/4 cup of 3% hydrogen peroxide 1 tablespoon of baking soda to a liter of water shake well and spray on leaves top and bottom. Do it every other day for a week. If I ever get mildew I always do a bud wash when I harvest.