Need help with WPM issue. 3rd week flowering

I had powdery mildew on my last grow but bleached everything and took apart my dehumidifiers and fans and even my mini-split to deep clean everything but… it is back again but not too bad, yet. My plants got huge before I realized so I’m playing catch up on thinning them out before it’s too late! daytime humidity is 50%-60% and night time is 45%-50%… needs to be a bit lower, i know but would that be the reason I’m getting the powdery mildew or would the plants being too dense be more of the reason? Which would be more effective on stopping the mildew… thinning the plants more or getting the humidity down 5-10%? phone is dead but I’ll get some pics up showing how thick the canopy is…

Uh, yes.

Never use bleach; peroxide is your friend. In fact, a mild solution (2 or 3 tablespoons in a quart of distilled water) sprayed on plants will nuke WPM. Defoliate, increase airflow and lower RH are the only things to fight it effectively.

You won’t remove all of it unless you perform a bud wash in peroxide, which I recommend anyway.


I used bleach on the room in between grows… not on the plants.
I washed the last harvest that had pm with like a 1/4 cup of h2o2 per gallon of water, completely submerged and soaked for like 5 min then dunked them in a rinse bucket… then hung them in front of a fan for a couple hours then into the dry room. still failed microbial test. So I did something wrong or that doesn’t totally get rid of it either. any trick to getting dehumidifiers to work better? any expierence with using a mini split on dehumidify mode? almost the same as a dehumidifier but not…

So then with those humidity levels, even if there was only 1 plant… it would get pm? meaning the RH is more of a factor than overlapping leaves? I know its combination but which is the leading factor?

It’s going to be a combination of factors. Like a fire triangle. But more interwoven as well. If you have RH below 45%, WPM cannot form. That doesn’t mean there aren’t moist pockets inside flower that won’t harbor mold. Which can then be treated with peroxide and water. I don’t know of anything that will remove all microbial activity without wrecking the harvest but peroxide does a fine job of eradicating WPM in my experience. Hell; I’m still smoking flower harvested in September 2018 (although kept under cold conditions) and is still green and fresh without that ‘old weed’ note and browning of the flower when it starts to degrade.

No; I mean never use bleach: it leaves a toxic residue. Again; peroxide would be fine. If you want sterile: paracetic acid and water, but you don’t want to deal with that stuff.

I would absolutely plan on a bud wash: up the strength by 50% and make sure of good agitation in solution. I also do as Cervantes instructed and wash with a shower head.

cool, yeah i actually watched Cervantes’ video on washing on my last crop but I guess a little more hands-on time will help. what about vital oxide to sanatize the entire grow? it is a form of stabilized chlorine dioxide…

I’d stay away from all chlorine as everything I’ve seen leaves a residue. Seriously: peroxide is dirt cheap ($.84 at Wally World), goes into a spray atomizer and mist stuff down: spray fan blades, intakes, all of it and walk away.

Something Professor Bugbee mentioned in passing was having some UV in the grow space: some photosynthesis but more for UV’s disinfecting qualities to help prevent WPM. Personally I’m hoping I’m in a dry enough climate to not need radical means. We’ll see.

So: defoliate as you know you have it. Take measures to lower RH: particularly at night. I ran a dehumidifier in the grow space and piped it into a bucket: it would fill in 24 hours. But getting too low can adversely affect terpene profile in the cured flower. In my experience. But it’s a tradeoff: lose crop to bud rot or lose some of your aroma?


all good points! I run dehumidifiers all day and night… empty about 17 gallons of water per day! … 12 plants… well 12 trees!

Ok so i have had the wpm issue on a zkittles twin auto. I did a bud wash prior to dry but i threw the branches into the bucket of mix water and peroxide and let it sit there for a few mins then rinsed off and let sit in a bucket of ice cold ro water to rinse well. It took a while but i believe it got rid of everything letting it set for a minute in the mix solution. Hopefully it didnt mess up the buds but they r super sticky to touch so we shall see

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