Powdery mildew is a fungal disease that can affect many varieties of plants, including cannabis. The dreaded PM is caused by various species of fungi, which are usually attracted to humid environments with very little airflow and ventilation. Treatment is challenging, but by learning more about this type of mould and what causes it, prevention is relatively simple.
By OG Team

Powdery mildew is a fungal disease. This white mould can affect many varieties of plants, but is regularly seen by cannabis growers.

This fungal disease leaves your cannabis plants covered in a fine, white powder, which tends to build up on young leaves. It will begin to cause the leaves of your plant to curl upwards and eventually cause them to die off entirely.

The powdery mildew fungus feeds off the leaves and causes them to rot, stunting the growth of the entire marijuana plant. It has the potential to destroy buds and affect the size of your yield.

In severe cases, powdery mildew could even kill your whole cannabis plant. These fungal spores are extremely resilient. They are able to survive hidden in your garden for long periods of time and this allows them to strike any new plants in future.

What Causes Powdery Mildew?

There are many cases apart from the environment that will cause your plant to lack resistance against powdery mildew, such as a nutrient imbalance.

There may be certain deficiencies within your feeding regime that could cause low levels of silicon and calcium available to the plant. These are crucial in the structural tissues of the plant and they also act as a defensive mechanism against powdery mildew. Another cause could be too much nitrogen. This causes the plant sap to be thinned down, making it more suspectable to pathogens and pests.

Powdery mildew is caused by various species of fungi. They are usually attracted to humid environments that have very little airflow and ventilation. A very humid environment is a breeding ground for all types of mould and disease.

If your grow doesn’t get adequate airflow, heat, and humidity, it will start to build up “pockets” around your plants, which provide the perfect environment for fungal spores to form and spread. If your grow is overcrowded and the leaves from your cannabis plants overlap and touch each other, it traps the humidity and stale air, thus creating an ideal environment for fungi.

Once an infected plant touches another plant, the fungal spores will be spread. However, spores spread via wind too, which is unavoidable indoors or out.

Identify: How to Spot Powdery Mildew

PM is fairly easy to identify, white, powdery patches can be seen on either or both leaves and stems of your cannabis plants. These patches are mostly found on the upper side of the leaf, however not uncommon to be on both sides. If left too long, it can completely cover your leaves.

Younger leave on your plants will most likely turn yellow and dry out. In the case of a serve infection you can expect your plants growth to be stunted and even fatal in some cases.

Treat: How to Beat Existing Powdery Mildew

Powdery mildew can have devastating effects.Timely and effective treatment is necessary to kill mold spores as well as to prevent the spread. Fortunately, if caught early enough, a PM infection is manageable and treatable.

Most fungicide treatments need to be mixed with water and applied directly to your cannabis plants. If treating plants indoors, you’ll want to temporarily stop your airflow system to prevent spores from spreading further.  

The good news? There are a few organic methods you can use to contain the infection. These are step-by-step instructions for dealing with the fungus once it strikes:

1. Prune if necessary

Start by removing infected leaves, when caught early on, this is a must. Use a good pair of scissors and cut off dry or yellowing leaves. By doing this, you allow your plant to redirect its energy. Safely dispose of any cuttings and leaves straight into a garbage bag, in order to keep the fungal spores from spreading any further.

Be sure to check your plants regularly to catch the infection early enough to prevent having to prune too much, as this will shock your plants. If badly shocked, plants grown from feminized seeds could even turn into hermaphrodites and start producing seeds. Remember, you don’t want air to be trapped in humid pockets – open up, allowing for light penetration and airflow.

2. Wipe with high-pH water or milk

In the case of a severe infection during flowering, you will need to physically wipe down all the leaves to remove the PM. This is the best option, if the infection strikes during mid-to-late flowering, which it so often does. Just be sure not to get any of the treatment mix on your buds.

Use paper towels or a clean sponge or cloth and cold water. Wetting your item of choice ensures that the spores stick to the item instead of becoming airborne. Clean the affected surfaces, one agonizing wipe at a time, using water with a high pH level – try find something around pH 8.0 or higher. By doing this you will render the cannabis plant leaves and surfaces uninhabitable to the fungal spores (at least in the short term). You could also use a 1:9 dilution of milk to water – easy does it, ideally about every 10-14 days, not more than once a week.

Once you have wiped down your marijuana plants, ensure that you dispose of the cleaning item used. The fungal spores stick to any material or surface and can spread to other plants in the vicinity of your house or garden. Bag your clothes to be washed and clean yourself too, to further limit the spread.

However, there may come a time when painstakingly wiping down leaf after leaf, surface after surface might simply not be a realistic option for you.

3. Spray your plants

You can also spray your plants with a diluted antifungal agent. During vegetation, treatment is simple: Mix up your treatment of choice in a clean sprayer bottle and spray your plants down, ensuring that you cover both top and bottom leaf surfaces. Outdoor growers will usually wait till just before sundown while indoor growers will spray down just before the lights turn off for the day – otherwise water droplets can act as magnifying glasses and burn your precious plants.

Let’s be clear: If you’re late into flowering when powdery mildew kicks in, spraying your plants down is a bad idea! It’s a recipe for bud rot. However, a bad PM infection at this stage means you’re already in “ruined harvest” territory. If you have many plants with a lot of leaves, wiping down could keep you busy until next Christmas.

So, while spraying during late flowering is quite simply bad advice, we understand it may be your only foreseeable option. Just be sure to use a sprayer that turns out a very fine mist and, remember, act fast but tread lightly or you’ll soak the buds (causing them to rot).

Often, a severe infection of many plants late in flowering is a no-win scenario, with no easy solution, but let’s be optimistic. In this case, spraying before sunrise might be a good option, to avoid further humidity and mold issues overnight.

Essential oils of tea tree, oregano and clove are great alternative options and are easy to use. Neem oil on its own is a powerful and popular choice. All are available organically. But whichever oil you choose, do your best to not get it on your cannabis buds or it’ll affect the flavour.

Below are 3 natural products that we recommend you to try as your treatment mix:

  • Neem Oil (2 tsp per litre of water)
  • Baking soda (1 Tbsp per 5 litres of water)
  • Milk (1:9 ratio of milk to water)
  • Apple cider vinegar (2 tsp per litre of water)

Other potential spraying options include copper soap (in an early enough phase to not leave a taste behind on flowers) or diluted hydrogen peroxide a.k.a. “H2O2” (some also use this as a last resort during late-flowering). AQ sf liquid (ampelomyces quisqualis) contains a parasitic fungus indigenous to South Africa that’s sold as a biological fungicide, specifically for treating powdery mildew – it’s also organic and seemingly safe (safer than PM anyway).

How often should you spray? The general guideline is to spray every 10-14 days, but it’s not an exact science. Monitor the situation and find the dilution and frequency that work best for you. Though always best to start slow and proceed with caution when working with valuable cargo.

Some folks even opt for a “bud wash” in an H2O2 dilution, but that’s a topic for another day.

4. Don’t transplant your cannabis plants

By transplanting your plants, you will more than likely end up spreading the fungal spores to the new soil and surrounding areas – meaning you risk infecting other plants. If you absolutely have to transplant near other weed plants, add fresh compost to the top layer of soil afterwards. This will stop any of the fungal spores from splashing upwards onto your plants while you water them.

5. Remedy the environment

Killing the spores is a matter of urgency, but they’ll soon be back if you don’t address the root cause. You need to ensure your plants have proper airflow and ventilation – not remedying this will not solve the problem!

Whether growing indoors or outdoors, ventilation and airflow are key components to consider when growing cannabis. For indoor grows, having just two fans will drastically reduce your chances of developing PM. Keep in mind that two fans are a minimum requirement – just don’t create a hurricane.

To solve the problem in the long-term, some indoor growers use a sulphur burner or ozone generator to kill all spores in the room (after removing all plants and living things, including yourself). One thing is for sure, indoors or out, if the dreaded PM has affected your crop once, you’ll want to prevent it from ever reoccurring.

Prevent: Set a Future Prevention Strategy

Prevention is far better than the cure, no matter if you grow indoors, outdoors, in a greenhouse, in water or soil, on Mars or The Moon.

If you have ever dealt with powdery mildew before, you know how difficult it can be treat retroactively, once the infection has taken hold. This is why you should do your best to prevent it, proactively, instead of trying (often in vain) to treat it mid-grow.

Fortunately, if you’re proactive, it’s relatively easy to reduce your risk of a mildew infection.

1. Optimize the environment

Powdery mildew is a tough disease; however, it won’t strike just because your humidity is at an incorrect level. It usually occurs when there is a combination of factors such as poor airflow, poor ventilation, and humid conditions that allow the fungal spores to form and spread.

One thing that can help is monitoring the humidity levels of your grow and ensuring the space gets adequate airflow and ventilation. The best way to get air moving in your indoor or greenhouse grow space is by using fans (minimum two) and an effective ventilation system.

For outdoor grows, ensure that your plants aren’t clustered all together and that the air is able to move around the grow space – don’t plant too close together outdoors! Also, make sure plants are positioned to catch as much of the late-summer to early-autumn sun as possible. Know your climate.

In order to control the humidity of your indoor grow space, use a hygrometer. Monitor the figures and ensure you don’t go above limits:

  • 65–80% for seedlings
  • 60–70% for vegetative plants
  • 40–55% for flowering plants
  • 35–45% 2 weeks before harvest

Additionally, you may want to investigate where these mold spores may be coming from in the first place. In other words, clean up your yard.

2. Preventative spraying

Given how common PM is, you don’t have to wait until you spot it to spray your plants (by then, it might already be too late). You can also use the spraying method mentioned earlier (under Treatment) to ensure that plant surfaces are inhospitable to the fungus growing in the first place.

This makes complete sense because, ideally, you don’t want to be spraying your plants during flowering, at least not after a couple weeks into this growth phase. Even indoors, where you have more control, or in hot, dry climates, wetting your buds is not a good idea – we’re trying to solve one mold problem, not cause another.

Organic is always best and the same solutions identified in the Treatment section above will do the job for preventing PM too. Many people swear by using copper soap spray as a preventative measure. Others use a milk dilution. Some even use smartly-sourced worm tea to cover plants in beneficial fungi and other soil microbes.

3. Buy disease-resistant cannabis seeds

Good genes aren’t just something humans have used for evolutionary survival of the fittest. Marijuana plants have also adapted to their environments over many years. Nowadays, breeders seek to input favourable traits to provide strains with unique traits for growers and connoisseurs alike.

When it comes to a relentless pathogen like powdery mildew, there are no guarantees, especially in some moist South African growing climates. But it certainly couldn’t hurt having the power of genetics on your side.

Fortunately, Overgrow has you covered. Why not try one of buying our famous cannabis seeds with mold-resistant properties?

Green Crack and Amnesia Kush are examples of feminized seeds that offer moisture-resistance, but don’t shy away from others with generous internodal spacing or short flowering times. All autoflowering seeds have speed on their side, making them less prone to disease in general.

If you’re after something highly-medicinal and non-psychoactive (low THC), why not try Amnesia CBD or Moby Dick CBD. And if you pride yourself on being an experienced greenthumb, there’s no reason why you shouldn’t try your hand at growing one of our resin-rich, exotic regular seeds. Just be sure to use the tools we’ve given you through this article.

Check out our next article for another 4 types of mold commonly found in cannabis plants.


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