I've Grown Cannabis Indoor for 15+ Years and Only Had One Occasion of Powdery Mildew
Throughout the different places and states I've grown, my basic principles have stayed the same. These basics are what I believe keeps my flowering rooms virtually PM free, and I know they can help you do the same.
Clean Plants, Strong Genetics, Healthy Plants, Clean Habits and HEPA Filtered Air
You want plants which are clean of molds to begin with, since pm and other molds are easily shared with clones. Cannabis genetics which are naturally resistant to PM and other molds are highly preferred.
Powdery mildew and Botrytis most often come in with clones. Other molds come from the environment. Though not generally the same kind of pm, it may be possible for other plants to have pm which attacks cannabis. There is still little to no documentation of this happening yet. (thank you for spotting this, Lamanoverde)
I've watched PM and Botrytis show up in other grower's gardens for years, and it usually comes from clones. Since PM is only visible to the eye when it sporulates it's ridiculously easy to pick up PM infected clones which look perfectly healthy. Later, when the plant gets a little stressed, the PM triggers a reproduction cycle and "BAM," your plants are covered in white powdery blotches. Botrytis is possibly even worse, silently lurking and creating a network until the last few weeks before harest.
Did you know there's a company working on a product which will allow testing for PM before you can see it? I was unaware of this until about a week ago. I listed to Shango Los covering this on his "Shaping Fire" podcast. He interviews Kevin McKernan in Episode 28 - "Botrytis and Powdery Mildew in Scientific Detail"
Fascinating information, including how the PM on cannabis seems unique to cannabis, a combination of the other two types of pm on this planet. Makes me wonder if the other two types of pm are actually mutations of the cannabis pm. Did we get cannabis clones and pm shared from another planet? lol
Some strains of cannabis have a natural resistance to molds. Even different seed plants of the same strain can have different abilities to fight off mold. The more wide leaved and dense the flowers are, the more potential there is for mold germinating. Various mold spores like to germinate in warm, wet areas with little to no air movement.
The denser the flowers, the easier it is for dew to collect inside and stay shaded. This creates warm and damp areas inside the flower where the airflow does not penetrate easily. Cannabis does very well when all parts of the plant are in constant motion. Keeping high levels of air flow goes a long way toward reducing the number of places spores can germinate.
The healthier the plant is, the stronger it is against incoming infections. A healthy clone will resist molds much better than unhealthy clones of the same plant. Calcium and Silicon are both recommended for boosting plant health and resistance. Having access to the proper trace minerals is also important to full plant health.
Calcium helps the plant transport many elements. Many nutrient profiles seem to be low on enough calcium for cannabis. Calcium can be quickly and safely be boosted in the soil using calcium sulfate dihydrate. It can also be used as a foliar spray, when mixed with a bit of fulvic acid. Personally, I use about 1ml/gallon of Floralicious+.
Silicon is an element cannabis uses to strengthen cell walls. When the cell walls are stronger, spores have a more difficult time piercing or infecting them. Not enough silicon leaves your plants with weakened cell walls, while too much causes them to be brittle. It's a fine line with silicon and cannabis and I recommend approaching it carefully. I currently use Pro-TeKt at 2ml/gallon in veg. This can also be foliar sprayed with some fulvic acid as well.
Again, go easy on the silicon. A little goes a long way toward great health.
Azomite is an amazing source of trace minerals, though it does take time to break down in the soil. Mixing up soil for spring? Amending outdoor plots or raised beds? Anywhere biological activity will be working for a while, the breakdown of Azomite makes a big difference.
What you do around your plants is probably one of the most important areas for controlling mold. I strongly recommend showering, and wearing freshly laundered clothing before tending your gardens. Allowing people, equipment and animals in your rooms uncontrolled is a large source of possible infections.
Know growers who have pm or other mold issues? Keep your awareness of these sources of spores and avoid them. Keep visitors in a 'safe' area, regardless of the desire to give 'tours' of your facility. Random people can be massive sources of all kinds of possible infections. Who knows where your dog or cat goes when it's outside, right?
Did the used/refurbished equipment you just bought come from an infected grow? Does the dispensary you get rolling papers from, have PM infected plants for sale near the counter? Wash down equipment, pasteurize media, filter water.
HEPA Filtered Air
I use up to 3 full exchanges of air per minute, so the airflow is pretty strong in my flower rooms. At this rate of airflow, even a small source of mold spores can heavily dust your plants. We're talking 2 months or more of your plants filtering spores from the air. Even when nothing shows during flower, as soon as you chop and hang to dry those spores will want to germinate.
An added bonus to using HEPA filtered air is how clean the cannabis is. It's amazing how our air can contain large amounts of dust, hair, pollen (hemp and more), fabric and natural fibers and more. None of these things have amazingly awesome flavors and aromas when burned with our cannabis. The definitely do not contribute to a nice clean ash.
I've personally used HEPA class furnace filters for over 15 years while growing. My first grow box used HEPA class furnace filter, and so have all my grows since then. It's rather easy to DIY them yourself with a carboard or thin wood box, a HEPA class (MERV 12 or higher) furnace filter, and a ducting take-off collar.
The ducting collar is attached through one side of the box, while the furnace filter is taped over a hole cut in another side of the box. Tape everything up light and air-tight and you're ready to go.
Pre-built filters are available, for those who want to simply install and go. GrowBright makes a decent HEPA intake filter for several sizes of ducting. They're already light proof and install quickly.
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I've spent several years sharing cultivation information with other growers. As always, this information gets better when you contribute your own experiences. Have questions? Please feel free to post them below and I'll be sure to take a look. I'll share what I know or point you in the right direction for what you need.