Powdery Mildew (also known as Wet Powdery Mildew) is a plague to cannabis plants, once it is established it is almost impossible to get rid of; that is why this article will teach you how to prevent powdery mildew (PM). If you have a bad infection my recommendation is to kill all plants, sanitize with bleach/heat or ozone and start fresh. It may seem harsh but any fungicides will not be things you want to ingest, and natural remedies just don’t work.

Powdery mildew (PM) is a fungal infection that shows itself as round, talcum like white-grey spots that rub off when touched. There are two parts to a powdery mildew infection, and once the white spots begin to show it is usually too late for remediation.

Pictured above is a horrible PM infection. This grower did not defoliate, had minimal airflow, was growing in overcrowded tents. Just goes to show you to be careful what you buy on the black market. These plants should have been culled a long time ago.

Preventing PM in Indoor Grows

There are a number of ways to prevent PM in indoor grows. I do not use any fungicides in my grow and methods like spraying peroxide and burning sulphur once a fungal infection has been established only seem to draw out the painful process and damage an already weak plant that is under attack. I have seen people spray their plants down with peroxide, wipe off all the PM only to return the next day with an outbreak twice as bad.

I will state this again, once you have an infection my recommendation is to kill everything and start fresh. I like to shock-treat the room (after plants have been removed) with ozone, which has been shown to denature PM spores.

Air Circulation

One of the best things you can do to prevent PM is having strong air circulation that prevents stagnant high-humidity micro climates from developing in your canopy. Make sure your oscillating fans are moving air in all areas of the room. If you have wall mounted fans consider adding box style floor fans to circulate air beneath the canopy.

Defoliation

Defoliation goes hand in hand with air circulation. This is especially true when growing in DWC where the growth is so explosive. Defoliate frequently so that the canopy has room to breath and there are no thick shaded areas with stagnant air flow. Remove all growth underneath the canopy. Clean legs are happy legs.

VPD (Vapor Pressure Deficit)

Tracking VPD is one of the best ways to prevent PM before it can ever take hold. I use the SensorPush system with their WiFi gateway to track VPD. An important and often overlooked aspect of VPD is nighttime VPD. Nighttime VPD will always be lower due to lights being off, and the lower temperature. This is fine, but nighttime VPD should never dip below 0.6 kPa or you are asking for trouble with fungal growth.

VPD is one of those things that can prevent PM but may make it worse if an infection has already taken root. This is because the fungus needs moist climates to propegate but dry climates help spread the spores. So if you already have a bad infection correcting your VPD can make things worse by helping spread the spores everywhere.

If you are new to VPD please read my Vapor Pressure Deficit article to learn more about how you can use this metric to improve your grow. The picture above shows my grow room before and after adding a dehumidifier. Interestingly enough a week prior I was using a humidifier in the very same room, which shows how much plants can increase the humidity through transpiration.

Air Intake Filter

PM spores are always in the outside air, especially in the fall when temperatures and humidity start dropping, drying the spores out and making them airborne. This is why it is imperative to have an air intake filter to prevent these spores from getting into your grow room in the first place.

Clean Clothes in the Grow Room

Same as with the air intake filter, you need to take steps to keep the PM spores from coming into your grow room in the first place. Never let pets into your grow space, always wear clean clothes and shower if you’ve been visiting any outdoor or commercial grows or even your local grow store. Limit who you have visit the room. Ideally just show them a live view via your Wyze cam.

Preventing PM in Outdoor Grows

With outdoor grows you have next to no control over the environment. This means that you must find mildew resistant strains that perform well in your outdoor environment. Typically this also means growing strains with shorter flowering times, as wet fall days present a breeding ground for PM and mold.

Interestingly enough I have seen plants next to each other with one covered in PM while another plant had absolutely no PM on it. Just like with any plant, some varieties are more resistant to PM taking hold on its tissue. If you have a mildew resistant plant you may want to consider seeding it out to preserve those genetics.