Powdery Mildew

Lyall McGovern

Lyall McGovern

>6 min read

With the warm weather approaching, the last thing you want to see is a soft layer of snow dusted across your crop, especially indoors.

I’ve seen this in April, I’ve seen this in July and especially in December. Though it may look beautiful from afar, this fungus can ruin your entire crop. Powdery Mildew, P.M., Podosphaera macularis; all names for the same plague that haunts many new growers and veterans alike. 

Powdery Mildew is easily identified as a stark white, fuzzy growth, on the top of your cannabis leaves and bracts (the buds). There are few other pathogens that look like it, and only a handful that can spread as quickly. What may start as small patches the size of a peppercorn on the lower leaves of the plant, can quickly spread to every square inch of a plant in full flower. Powdery Mildew is a parasitic fungus that lives off the energy your plant creates. It’s only found on the tops of the leaves which makes it easy to distinguish between other mildews like botrytis and downy mildew.

Powdery Mildew lives primarily under the cuticle of the leaf or bract of the cannabis plant, completely invisible to the eye. The cuticle is the top layer of “skin” on a plant. The body of powdery mildew, called the hyphae, spreads like roots within the tissues of the plant’s leaves and bracts. You may not see it, ever, if the conditions are not right for its reproduction… but it can still be there, waiting patiently. Though it has a complicated life cycle, the basic idea is that when conditions are right, the hyphae burst through the cuticle and produce conidia, the reproductive organ of the fungus. The conidia are the visible parts of the fungus you see on the leaves. Conidia is the plural stage of conidium – also known as spores.

There are many ways to prevent powdery mildew, but in this article, I’m going to focus on four actions you can follow that will almost certainly save your garden from this pest. In order from most crucial to – oh shit it already happened: clean genetics, environmental controls, defoliation, and pesticides. 

Clean Genetics – this is the big one. I can’t tell you how many clones I’ve seen brought into a facility that were destined for failure. The most effective way to ensure “clean genetics” is using a technique to propagate clones/cutting called tissue culture. Tissue culture really only requires one cell of a plant to recreate an entire plant. It’s basically magic. More often a small cutting of a leaf, smaller than a centimeter squared is used. Fresh clean cells from new growth, containing no pathogens, are used to grow a mother plant that is as clean as from seed. Assuming the facility and environment are clean and on point, cuttings/clones produced by a nursery using tissue culture should be completely free of any pathogens, like Powdery Mildew. No hyphae, no conidia, no conidium – no spores, no problems. If your starting material is free of any Powdery Mildew or other pathogens you’re off to a winning start.


Environment – this only really works if your genetics are clean to start off with. The life cycle of powdery mildew relies heavily on a perfect environment. Or in your case, an imperfect environment. Whether you have Powdery Mildew on your starting material or not, it is extremely important to control your environment to prevent the spread of the plague. Unfortunately, Powdery Mildew’s favorite temperatures, are some of your cannabis plants favourite temperatures. 15.5C (60F) to 22.2C (72F) are the most desirable temperatures for Powdery Mildew to reproduce. Generally speaking the fungus reproduces via the conidia in cool, moist, dark environments. On a microscopic level conidia look like a chain of sausage links. When temperatures rise and humidity lowers, the links break off individually and spread into the air as “conidium” looking for a new leaf to land on and infect. A more familiar term for these free flying conidium are spores. A few simple tips can help you keep your environment in check and slow or eliminate the spread of Powdery Mildew. 

  • DO NOT water your plants within 3 hours before the lights go out. When the lights go out the temperatures drop dramatically and the humidity spikes. Though your equipment may be able to handle this swing well, your plants will still transpire more rapidly after being watered. This transpiration causes micro climates with both high humidity and low temperatures inside the canopy. 
  • Do not allow your humidity to go above 70%, ever. Though you may get increased growth due to ideal VPD above 70% in certain environments, the risk is not always worth the rewards. I personally aim for 65% as my max humidity post-propagation phase. 
  • Avoid fast temperature swings. This isn’t always possible, but whatever you can do to slow the transition from warms days to cool nights is ideal. There are some advanced crop steering techniques that don’t allow for this, but for beginners this is key. 
  • If you cannot control your humidity, keep your temperatures on the higher side, day and night.
  • If you cannot control your temperatures, keep your humidity on the lower side, day and night. 
  • keep your air movement consistent, and equally distributed throughout the crop. We’ll do another article just about air movement soon. But for now, try and keep your air flowing at the top, center and bottom of your crop. This helps to avoid micro climates in different parts of your crop. 

Defoliation – you’re probably already doing this for light penetration. However, this is also a crucial step in preventing the spread of Powdery Mildew. Due to radiant heat only reaching the top of your crop from your lights, the bottom of your plants often have a radically different temperature and humidity than the top of your crop. Most of your sensors in the grow room will be also be located where it really matters, in the peak bud site areas. Because of this it is almost impossible to control the environment on the bottom 30% of your plants. If you’ve grown cannabis more than once, you probably also know that the product grown on the bottom 30% of the plant is useless. Get rid of it. Due to the morphology of the cannabis plant, there’s no good reason to keep anything alive on the bottom third of the plant. Removing leaves, and bud sites from the bottom of your plant leaves only the hardy thick-skinned branches and stalks. The cuticle of the plant here is more like bark, think fingernails instead of skin. The branches and stalk are dramatically less susceptible to powdery mildew. They can live happily in the danger zone without becoming infected. 

Pesticide – a dirty word, but sometimes you have to play dirty to win. I’ll start with a disclaimer here… Health Canada Approved Pesticides are all 100% safe for consumption and are often or always used on any organic food crop you buy at the grocery store. If you’re using something off label, you shouldn’t be growing in a regulated market. That being said, it is still ideal to avoid pesticides whenever possible. Avoiding pesticides reduces costs across the board, they are expensive to begin with, training staff on proper application, and if applied late in flower (don’t do this) can leave residual tastes on your beautiful final product. There are two routes to using pesticides, preventative and curative. Curative is a misnomer because you can never cure your plants of powdery mildew using pesticides. 

With preventative pesticide application, as far as I’m concerned there is only one option here. Venders may tell you a different story to sell their products, but Regalia MAXX is the only approved preventive pesticide on the market for cannabis. In short, this plant-derived foliar applied product uses 4 modes of action to prevent disease, and the most important one here is strengthening of the cell walls and cuticle. This makes it harder for the conidium (spores) to land on the leaf and infect it via the hyphae growing into the cells. Always use Regalia MAXX in accordance with the label. This product can be used even in the propagation phase. If you are unsure whether your genetics came in clean, I would always recommend the use of Regalia. The one down side of this product is that is leaves a yellow film on your tables, walls, and any equipment it comes into contact with. You will have to send someone into the room after the product has dried to clean up the area by hand. This product is highly effective for indoor, greenhouse, and outdoor crops.

In terms of curative, or a treatment for plants that already have evidence of Powdery Mildew, there are 3 pesticides I am very familiar with. Milstop and Sirocco (which are basically the same thing) and ZeroTol. Though there are many others on the market, these tend to be industry standards. Milstop and Sirocco can be sprayed or low volume fogged and are quite effective against the breeding cycle of powdery mildew. They mainly work by changing the pH of the leaf surface interrupting the breeding cycle of powdery mildew. They can only be sprayed at certain intervals and do not work long term without control over the environment. I would not ever recommend spraying either of these products past week 3 of flower, but sometimes you may have no choice – they can be sprayed up until the day before harvest technically. 

ZeroTol is something I prefer spraying over Milstop or Sirocco. ZeroTol is basically Hydrogen Peroxide at a concentration suitable for application to plants. ZeroTol will leave no residues or residuals, and can happily be sprayed up until the day before harvest. H2O2 or Hydrogen Peroxide is a powerful oxidizer that destroys the conidia of Powdery Mildew. The downside of this product is that your staff must suit up in full hazmat PPE with a face shield respirator, rubber boots, rubber gloves and a water proof bubble suit. These supplies can be expensive and extremely uncomfortable if you’re in a greenhouse environment. 

At the end of the day, there are many ways to control Powdery Mildew. But using these simple techniques I have avoided any incidence of Powdery Mildew at my facility for over two calendar years. I have managed to avoid step four – pesticides – for that entire period as well. 

If you have tried everything here, and are still struggling to keep your facility clean, please contact a professional at Mother Labs to see how clean genetics can change your entire grow.


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about the writer:

Lyall McGovern

master grower, Quality green

Lyall is a master in all things cannabis cultivation and production. His expertise covers compliance, quality assurance, scheduling, plant health, I.P.M., phenohunting, environmental controls, industry innovation and more! We are very lucky to bring Quality Green’s growing guru on board as our resident Mother Blog writer. Collaboration over competition is king!