Choose another country or region to see content specific to your location.

Most Common Pests In Cannabis: Powdery Mildew

14 February 2023
Powdery mildew comes from fungus and fills your leaves with white fuzz, inhibiting photosynthesis.
14 February 2023
6 min read
 Most Common Pests In Cannabis: Powdery Mildew

Read more
  • 1. What is powdery mildew?
  • 2. What does powdery mildew look like?
  • 3. Where is powdery mildew found?
  • 4. What does powdery mildew do?
  • 5. Powdery mildew symptoms
  • 6. How to prevent it?
  • 7. How to deal with it?
  • 8. In conclusion

Powdery Mildew comes from different types of fungus, it slowly starts developing on your cannabis leaves and if left without care for too long can completely take over your plant. Thankfully, there are various steps that growers can take to prevent it from taking hold of their cannabis seeds in the first place. Above all else, high levels of humidity allow this pathogen to gain a foothold; tight crowding of plants and overwatering also play a fundamental role in the onset of this disease. If you're tired of having to deal with this, this article is just for you!

1. What is powdery mildew?

Powdery mildew (or White Powdery Mildew) is a type of fungus that coats your plant's leaves, can inhibit photosynthesis and rot buds. Even though it can seriously affect your plants, you can easily get rid of it without too much hassle with homemade or store-bought products, like Neem oil. Powdery Mildew is truly one of the worst of all the fungal diseases that can attack a cannabis plant. If left unchecked it can destroy a full weed crop in only a few days, depending on the severity of the outbreak. Powdery Mildew spores are spread through airborne contamination, which basically means that they float through the air until they come in contact with a plant and take hold from there.

The spores can also be by animal or human contact with the plant, so if you think you have an outbreak you should quarantine the affected plant or plants as soon as possible. Be sure to always clean your hands and tools before you come back into contact with healthy plants. This type of fungus can affect a wide variety of plants, including cannabis. It develops what looks like white fuzz on top of your cannabis plant leaves and can reproduce both sexually and asexually, spreading spores all throughout your growing space, spreading to other plants around super fast.

2. What does powdery mildew look like?

Powdery mildew looks like flour or fuzz, it’s a white powder that forms circular patches all over the leaves. 


Most Common Pests In Cannabis: Powdery Mildew

Powdery mildew looks like a white fuzz that slowly starts to take over your plants.

When the mildew starts to take over your plants you’ll easily see a layer of white spores on top of the leaves. They can easily be spotted because of the contrast with the bright green cannabis leaves.

3. Where is powdery mildew found?

This fungus is mostly found on top of the cannabis leaves in the early stages and can be found all over after it spreads, also affecting the buds. If you see your buds with a fine white layer of white powder, it won't take long until they start to brown and smell bad. Depending on the severity of a Powdery Mildew outbreak, it can sometimes be difficult to identify - especially in the early stages of an outbreak. 


Most Common Pests In Cannabis: White Mildew

When mildew reaches the buds they start to rot and at that point, you should discard them.

You should always keep a close eye on your plants, and keeping a grow log can be a great way of staying on top of any grow room issues. Also, have in mind that when powdery mildew reaches the buds you should discard them, even if they are not completely rotten because they can be toxic for us.

4. What does powdery mildew do?

Powdery mildew slowly covers the surface of cannabis leaves, this will ultimately inhibit photosynthesis. This will cause the leaves to yellow, turn brown, and ultimately die due to the lack of energy. A wide range of fungi causes the disease in different species of plants. In the case of cannabis, that disease can spread in several different ways, including windblown spores known as conidia, and through mycelial mats present on crop debris. Powdery mildew fungi have both an asexual cycle and a sexual cycle. The sexual cycle starts off with conidia making direct contact with the surface of a cannabis leaf. When conditions are just right, in that there’s plenty of moisture in the air, the conidia send out fine filamentous threads known as hyphae.

These strands start to occupy the surface of the leaf, resulting in a white-to-grey appearance. However, the hyphae don’t just reside on the surface. They puncture leaf cells and start to feed off of the nutrients within. Towards the end of the sexual cycle, they develop specialized reproductive structures that form and release more conidia that go on to infect other nearby plants. So, what about the asexual cycle? Well, the same reproductive structures that make conidia are also capable of creating a structure known as a cleistothecium that is able to overwinter on plant tissue—this makes powdery mildew a rather pernicious pathogen that often proves difficult to totally eradicate. When conditions are right in the spring, this ball-like mass releases ascospores that are able to infect leaf surfaces and cells by growing out into hyphal threads. 

5. Powdery mildew symptoms

In the first couple of days, you won’t be able to see anything but the spores are slowly infecting your plant. After a couple of days, you’ll start to see white spots on the leaves, these spots start to spread to the rest of the leaves through spores and they end up looking completely white and fuzzy.


Most Common Pests In Cannabis: White Mildew on Cannabis Leaf

Mildew can easily spread to other plants around through spores.

In more serious cases, the buds will also be infected and they will start to rot, having white fuzz all over and ultimately starting to turn brown and smelling bad.

6. How to prevent it?

It is really easy to get powdery mildew so to prevent it you should have a good growing environment, always checking (and adjusting if necessary) humidity levels and airflow.

Because this is a fungus, it thrives in humid places where there’s no airflow to evaporate the water.

Even though you can have a relatively high humidity level (in the seedling stage, for example), you have to provide a good airflow because the powdery mildew spores cannot settle down where the air is being moved. You should also try to avoid extreme temperature fluctuations. The optimal temperature range for cannabis growth is 20-30 °C (70-85 °F) for vegetative growth and 18-26 °C (65-80°F) for flowering. The smaller the temperature variation, the harder it will be for Powdery Mildew to take hold.

If you keep experiencing this pathogen indoors, then you should work to prevent it from day one of the growing cycle. Because the fungi that cause this disease thrive in cool and moist conditions, you should aim to elevate temperature (to a reasonable degree) and minimise excess moisture. You can keep temperatures at the desired level by using a heater. To keep moisture down, employ both a dehumidifier and fans. Use a hygrometer—a device that measures both temperature and humidity—to monitor and stay on top of both of these variables.

7. How to deal with it?

There are several ways of eliminating powdery mildew and if you spot it early, it should be very simple to do so. First of all, it is recommended you remove the infected plants from your growing room, although if it’s widely spread it’s likely all your plants already have it.

You can use Neem oil but this can be bad if used on the flowering stage, for more safe options you can spray milk or baking soda mixed with water in the following ratio:


Most Common Pests In Cannabis: Mix to eliminate powdery mildew

You should start with less harmful ways to deal with mildew before moving to other ways.

  • Mix 900ml of water with 100ml of milk
  • Mix 1 tablespoons of baking soda with 2L of water

You should spray once or twice on the affected leaves without overdoing it because it can have a negative effect on your plants. If the infestation has taken a strong grip on multiple plants, it may be best to cut your losses and scrap the entire crop. Trying to maintain a cannabis crop with a high Powdery Mildew infestation rate can put the rest of your grow op at risk. 

8. In Conclusion

Powdery mildew is a fairly simple pest to take care of and by having a fan moving the air in your growing space you can easily prevent it.

If you have powdery mildew in your growing room, make sure you try the less harmful ways of dealing with it before trying anything else.


This post was most recently updated on March 22, 2022.