I grow outdoors in Hawaii and have never really had a problem with PM. I have (3) 5-6' GPD currently in flower with only 2-3 weeks left before harvest. A few days ago it rained super hard for 2 days straight. On the 3rd day I noticed i had PM starting on all of the big fan leaves. I did some research and figured out what my plan of action would be. I made a mixture with 3% hydrogen peroxide from walmart. I did 3 TBS to 1 gallon of water in a Sprayer. I soaked everything including the buds with the mixture. After I finished spraying the last plant I washed the sprayer out and filled it back up with clean water. I then proceeded to wash everything down with clean water. Today is the 4th day and it seems like the plants didn't skip a beat. Everything looks super good with no PM. I got the idea from Jorge Cervantes video.
I have used the method in this video on a very large scale to much success... Since you are very close to harvest anyways, I think you have a good solution to keep the PM in check until the plants are ready. But, if I were you I would do a complete submerging when you harvest like he does in the video, as just spraying most likely is not getting ALL of it. Other than that, keep an eye on them daily to see if it gets any worse/comes back. Would love to see some pics if you can, good luck bud!
