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. By learning more about this type of mold and what causes it, prevention is relatively simple.
By OG Team

What exactly is powdery mildew?

Powdery mildew is a fungal disease. This white mold 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 the future.

What causes powdery mildew?

The cannabis plant's metabolism must be out of balance for any kind of pathogen to attack. 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 mold and disease.

If your garden 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 garden 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.

How to beat existing powdery mildew

Powdery mildew can have devastating effects, especially if it’s not treated quickly. Fortunately, if caught early enough, a PM infection is manageable and treatable. These are step-by-step instructions for dealing with mildew once it strikes:

1. Clean with high-pH water

Clean the affected surfaces using water with a high pH level. By doing this you will render the cannabis plant leaves and surfaces uninhabitable to the fungal spores – at least in the short term.

Use paper towels or a clean sponge/cloth and cold water. Once you have wiped down your marijuana plants, ensure that you dispose of the item used to clean your plant. The fungal spores stick to any material or surface and can spread to other plants in the vicinity of your house or garden.

2. Prune if necessary

Use a good pair of scissors and cut off-dry or yellowing leaves. By doing this, you allow your plant to redirect its energy. Dispose of any cuttings and leaves in order to keep the fungal spores from spreading. Be sure to catch the infection early enough to prevent having to prune too much, which will shock your plants. Remember, you don’t want air to be trapped in humid pockets – open up, allowing for light penetration and airflow.

3. 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. This will stop any of the fungal spores from splashing upwards onto your plants while you water them.

4. Treat your plants

Start treating your plants with an antifungal agent.  Essential oils are great alternative options and are easy to use. Most fungicide treatments need to be mixed with water and applied directly to your cannabis plants.  Neem oil can be used as an alternative. Don’t get it on your cannabis buds. If treating plants indoors, you’ll want to temporarily stop your airflow system to prevent spores from spreading further.

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 in soil.

If you have ever dealt with powdery mildew before, you know how difficult it can be to treat it 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, it’s relatively easy to reduce your risk of a mildew infection. One thing that can help is monitoring the humidity levels in your garden and ensuring the space gets adequate airflow and ventilation.

The best way to get air moving in your grow space is by using fans and a ventilation system. 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!).

In order to control the humidity of your 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

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.

Powdery mildew is a common fungal pathogen, however, there are a few others to look out for. Stay tuned for our next article on the various molds and fungi to look out for. 

In the meantime, Moby Dick, Cookies Auto, and 707 Truthband are great mold-resistant strains to try. 



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