I don't have any experience with physan 20, but I've used several other methods;
1. Serenade- this is a live bacterial culture spray, designed to eat the PM. It's best if you spray it before the PM starts taking over. It's stinky...
2. Potassium bicarbonate- this acts just like baking soda to raise the pH of the leaves you're spraying it on, to make them inhospitable for the PM to grow. Again, it works best if you get on it early. Bonus; this adds potassium to your plants, which is a nutrient.
3. Eagle 20- this is not organic, but it's the most effective. Mix according to instructions (a little goes a long way!), and spray on the plants thoroughly drenching them, like you do with the above options. This stuff nails the PM and it stays gone. The MSDS says it can be used on food crops up to a couple days before harvest, but I would try not to use it past the first week or so of bloom, and preferably you want to have it eradicated long before then!