Powdery Mildew onset with an RH of 20%, indoor, how'd I do that?

Good evening, opened up my grow box and spotted some PM spots on larger fan leaves, I am growing indoor in a custom grow box and because it is currently winter, the ambient air rh is about 20%, Ive seen it drop as low as 15%.

I am confused as to how PM would even start in these low rh conditions. My only hunch is that my fabric pots are always wet, maybe its growing/starting in the pot and has moved to the leaves, I have an automated dripper setup doing multi feeds a day, keeping the pots moist, but the RH in the box is still measured at 20%,

Grow box is a 3x3 - 7 feet high with a 600hps dimmed down, temps 85 lights on 70 lights off, has an oscillating fan and an extraction fan.

Something I just thought as I wrote that, this also hit me. I currently have the exhaust fan set to turn off when the lights are off ( 12 / 12 ), but the oscillating fan still runs, maybe lack of exchanging air during lights off could also be my problem, darn

Greater than 10 degrees swing between lights on and off will do it

really? Im curious ive never heard of that either but I certainly have a big temp drop

The temp swing causes condensation. Also your humidity spikes at lights off. That's how you get the PM.

Your exhaust fan is off at night and your humidity is only 20%?

my exhaust runs on a humidity switch at night

I've heard this temperature swing can cause WPM. Then I have also heard that people intentionally have massive temp swings to bring out purple in certain strains. Is there a technique where temperature swings can be tolerated by the plant? Thanks and sorry to piggyback on the thread.

TurboTokes said:

Good evening, opened up my grow box and spotted some PM spots on larger fan leaves, I am growing indoor in a custom grow box and because it is currently winter, the ambient air rh is about 20%, Ive seen it drop as low as 15%.

I am confused as to how PM would even start in these low rh conditions. My only hunch is that my fabric pots are always wet, maybe its growing/starting in the pot and has moved to the leaves, I have an automated dripper setup doing multi feeds a day, keeping the pots moist, but the RH in the box is still measured at 20%,

Grow box is a 3x3 - 7 feet high with a 600hps dimmed down, temps 85 lights on 70 lights off, has an oscillating fan and an extraction fan.

Something I just thought as I wrote that, this also hit me. I currently have the exhaust fan set to turn off when the lights are off ( 12 / 12 ), but the oscillating fan still runs, maybe lack of exchanging air during lights off could also be my problem, darn

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You run the extraction fan 24/7 ?

Smokesteve said:

I've heard this temperature swing can cause WPM. Then I have also heard that people intentionally have massive temp swings to bring out purple in certain strains. Is there a technique where temperature swings can be tolerated by the plant? Thanks and sorry to piggyback on the thread.

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You can still have lower night time temperatures without having too much temp swing. For example if your lights on temp is 76*F then 68*F is cold enough without having a ton of fluctuations.

Pm is not related to humidity like we think. It plays a part but it’s a lot more factors than just keepin it dry.

Corso312 said:

You run the extraction fan 24/7 ?

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No. He's been shutting it off at lights off...

natureboygrower said:

No. He's been shutting it off at lights off...

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That's his PM cause right there.

In the winter, I run two (2) inline exhaust fans on my 4x4 flower tent. I have one (1) six inch fan with carbon filter that runs 24/7 exhausting into the room the tent is in, drawing passive air into the tent from the room. I have a dehumidifier in this room to keep the humidity down, currently set at 50%. However, I also run a second four inch inline AC Infinity fan with temp and humidity controls set to 78 degrees F and 50% humidity. This fan vents directly out the window and only kicks on when the humidity or temp gets too high in the tent.

I do this specifically to keep the six inch carbon filter fan running constantly without sending all the hot air out the window. The six inch fan keeps the air circulating in the tent, while the four inch fan exhausts excess heat and humidity as needed.

I also found I need a third inline four inch fan on one intake port drawing heated air from in front of a propane direct vent wall heater to keep the tent warm. Without the four inch fan boost, I had trouble keeping the tent over 68 degrees F with the lights off, with the inline fan running I keep the tent at 72-74 degrees F with lights off.

It was almost ten below zero here last night, but never below 72 degrees in the room.

Corso312 said:

That's his PM cause right there.

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I think so to, I will make the change tonight and have it run 24/7

Also if you have gotten PM before clean extensively everything. Especially change your filter out. The filter will collect most of the spores. Even after your grow is done PM spores are still all around the area.

TurboTokes said:

I think so to, I will make the change tonight and have it run 24/7

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I grow indoor and outdoors.. Temp swings mean dick, if there is air exchange. My indoor in basement gets 69 lights on 57 lights out under led in winter.. 81 and 70 in summer... upstairs i get 73 ish lights on..60 lights out under led in winter .. 86- 73 in summer..... Outdoor is nuts, 30 degrees to 100 degrees in a cycle..never have PM.

Temp swings can make the fan leaves- sugar leaves or bud turn blue or purple but not PM..if their is air moving

Do you have enough air movement in there? You may have areas of higher RH% if there isn't good airflow through the plants.

PM likes times of low rh% and times of high rh%. Those swings make it happy. I never saw PM back in Missouri because the rh% was never low, had to fight to keep it down around 50%.

The only things I have found that are truly effective at eliminating PM are an AirROS machine or a sulfur vaporizer (not in flower).

I have beaten this nasty mildew, it was indeed the extraction fans turning off at lights out. H202 rinsing and pulling the most affected leaves cured it near instantly and I did subsequent washed 3 days apart for a bit and then monitored with no signs or resurgence. I will h202 dunk this crop at harvest though just for good measure.

Silly mistake