cannabis white spots

White Powdery Mildew

by Sirius Fourside

Have you seen white spots on your leaves? Are your leaves dusted with round patches of powder that looks like flour?

If so, you’re most likely dealing with White Powdery Mildew, also known as White Powdery Mold or just “WPM” to cannabis growers.

White Powdery Mildew is usually a minor annoyance that can be easily fixed, but if you don’t catch it early, WPM can turn into a catastrophe that ruins an entire marijuana harvest!

For those who haven’t experienced WPM, imagine circular patches of a living, breathing, fuzzy, flour-looking substance showing up on your plant’s leaves without any warning. From there, the mildew can easily spread to other leaves and buds, rendering the buds unusable.

You’ll see “powder” on your leaves…

White Powdery Mildew has such an easy time spreading that even careful growers who take proper precautions can still experience it.

Luckily, the issue in the picture above was easily resolved because it was caught early and because White Powdery Mildew is completely reversible up to a point.

This article will arm you with the information to stop WPM’s proliferation before it even has a chance to take hold!

What IS White Powdery Mildew?

White Powdery Mildew is a rapidly reproducing (both sexually AND asexually) fungus who only knows how to do two things:

Eat your plants

Make more White Powdery Mildew

Fortunately, White Powdery Mildew is easy to spot since it creates white patches of fungal growth that stand out against the green leaves of a cannabis plant.

It can be removed from plants with proper treatment if spotted early on, but any buds with WPM should be discarded as they most likely contain many more spores than your eyes can see.

What causes White Powdery Mildew?

High Humidity

WPM needs moisture to thrive, but that doesn’t necessarily mean it needs water. Having a grow area with high humidity is all WPM needs to get started. This seems to be a bit problematic since young cannabis plants grow best in relatively humid environments (40% -60% RH). Luckily, high humidity usually only becomes an issue when it’s combined with the next cause (low/no airflow).

People who live in environments with extremely high humidity (such as the southern US or anywhere in the UK) can purchase a dehumidifier to control humidity in the grow area. This is especially important during the flowering phase when humidity needs to be much lower (45% rh) to prevent rampant growth of WPM and bud mold.

Low/No Airflow

White Powdery Mildew has a hard time settling in a grow room where the air is being moved. High humidity will give WPM the conditions it needs to survive, but poor airflow is what gives it the ability to settle down in the first place. In fact, a small (preferably oscillating) fan moving air in a grow area will prevent the vast majority of White Powdery Mildew woes.

Poor Ventilation

If you have WPM spores in your grow area and the air in grow area is never exchanged for fresh air, the spores get multiple chances to land on your plants and reproduce. This happens most often in conditions where cannabis is being grown in a closed, unventilated space – such as a closet – and precautions aren’t taken to exchange old stale air for new fresh air.

Leaf-on-Leaf Contact

Leaves that are touching each other will form moisture between them, and thus they become more likely to contract WPM. Untrained bushy/leafy plants with lots of new vegetative growth are especially prone since they will often have their leaves mashed up against each other as they try to reach toward the light.

Advanced growers can defoliate some of the fan leaves that are completely shaded from the grow light to make fewer choice landing spots for White Powdery Mildew. Also, defoliation frees up energy for the plant to use when done correctly and increases yields! See our article on defoliation for more info.

How to Eliminate White Powdery Mildew

As I mentioned earlier, I recently had a battle with White Powdery Mildew. Rather, it might have been a battle if I noticed it later or waited to fix the problem. That’s the one good thing about WPM: in most cases when WPM is caught early, you can remove all traces of the mildew without harming your plants.

There are quite a few products and homemade concoctions people use to treat WPM. Among the effective treatments are:

Milk (1:9 ratio of milk to water)

Baking soda (2 tablespoons per gallon of water)

Neem Oil (4 teaspoons per gallon of water)

Hydrogen Peroxide (1 teaspoon of 35% H202 per gallon of water)

SM-90 (1:5 ratio of SM-90 to water)

Rather than go into these methods, I’m going to give you the simple strategy I use that gets rid of White Powdery Mildew on the first try, every time! Here’s my trusted 3-Step White Powdery Mold cure:

Remove White Powdery Mildew from leaves – Get some water (tap water works fine) and some paper towels. Wet the paper towels and use them to gently wipe the mildew off the affected leaves whilst being careful not to jostle any leaves with spores on them. Using a wet cloth will ensure that more spores stick to the cloth instead of becoming airborne. Note: While it isn’t necessary to use paper towels, their disposability helps to curb the spread of spores from one leaf to another.

Ensure plants have proper airflow and ventilation – Even if you have absolutely no airflow or ventilation in your grow room, having just two fans will drastically reduce your chances of encountering WPM while also benefitting your plant’s overall health. One fan should be oscillating if possible and should gently blow air over your plants. All the plants need is enough air to gently rustle their leaves which will make it hard for WPM to settle down. The second fan should be in your grow room pointing outward to exchange old air with fresh air. Having a fan pointing out of your grow room will force old air out of the room, and in turn, pull new air into the room. At this point, you’ll have new air coming in, being used and circulated, then kicked out. Keep in mind that two fans is a minimum .

Treat the infected plant with one of the options below to kill spores prevent future growth – Mix up your treatment of choice in a clean sprayer/mister. We recommend Lost Coast Plant Therapy (1oz/2btsp per gallon of water) or GrowSafe (2oz/4tbsp per gallon of water) as a safe second option . Make sure to consult the instructions on your treatment of choice to find the recommended dosage. Wait until just before your lights for off for the day and mist your (newly cleaned) plants. Get all the leaves even if you don’t see WPM on them!

There you have it! If you end up running into White Powdery Mildew, give this advice a shot and you won’t have to deal with it past that first day. If you do end up using these steps, feel free to let us know if it helped you or not, or how you did it differently. When growers know just a little bit about this plant disease, it doesn’t have a chance!

White Powdery Mildew Defense

What’s the easiest way to fight against White Powdery Mildew?

Have it completely outgunned!

Get the right stuff to let White Powdery Mildew know that your grow room is off limits!

Lost Coast Plant Therapy – Kills WPM as well as a bunch of other pests and it’s safe for flowers, pets and people!

GrowSafe – Another safe-for-buds pesticide that kills WPM and other pests. OMRI listed as organic!

Note: SM-90 is no longer available! Find out more here: What happened to SM-90?

Handheld Mister/Sprayer
– A mister is awesome for applying treatment. Also, it’s the best way to foliar feed your plants!

Bonus! Papaya cannabis strain
– The strain Papaya is potent, flowers early, and – most importantly – is disease resistant!

White Powdery Mildew (aka White Powdery Mold) can be the cause of white spots on your leaves that looks like patches of flour. Learn how to get rid of it!

White Spots on Marijuana Leaves

White spots on marijuana leaves are a common issue facing cannabis growers. Two of the most common reasons for this are either white powdery mildew or spider mites. (I will only briefly address spider mites at the end of this article) This article will only deal with these white spots on leaves caused by white powdery mildew.

Powdery Mildew On Pot Leaves

A white powdery mildew infection is caused by mold spores spreading plant-by-plant. Your plants can be affected by a number of reasons, some of which are:

  • Wind
  • Air Ventilation
  • Pets
  • Any Even Humans

How Can You Recognize A Powdery Mildew Infection on Leaves?

In the early stages, WPM and a spider mite infestation can look familiar. As a result, it can be tough to find out precisely what problem you are having. It should go without saying that you need to know what an issue is before you can even think about solving the issue.

White powdery mildew (“WPM”) is a fungus that causes the white spots to appear on cannabis leaves in its early stages. Initially, the lower plant’s leaves will be affected the most. As it gets worse, the spots will enlarge, and they will eventually turn into large circles, and the mold will ultimately spread to other areas of the plants. So, I will go over the primary signs that you should look for to determine whether you have a mite or a WPM problem.

● No Bugs Around- If, after careful inspection, you don’t see any bugs when your plants are covered with white dots, you likely are having a WPM problem.

● Spot Appearances-WPM dots tend to look more like splotches or fuzzy circle patches on plant leaves. The pattern tends to be spotted, and sometimes wilted. Please note, that you are looking for white spots. So, yellow spots or brown spots on the leaves means you may have other problems than just mildew. (Yellow-spotted affected leaves are also called leaf septoria.)

● Leaf Appearance-The white spots are easy to spot as they stand out on the green marijuana plant leaves.

What Causes WPM And How To Best Prevent It

The WPM dots (not caused by spider mites) most likely happen because of improper, indoor growing conditions. Some of the more common conditions which can cause the WPM dots to happen are the following:

  • Humidity-Humidity levels are around 70% is optimum for seedlings. However, humidity at around 70% is fertile grounds for WPM for plants and flowering. (That’s why you should never grow your seedlings together with your marijuana plants) So, you need to try to get the humidity levels down to at least 40% for flowering. So, regularly check your grown room humidity with your hygrometer.
  • Mold Resistant Strain-Consider is purchasing a cannabis strain that is resistant to mold. Click Here For a useful resource for mold-resistant cannabis strains.
  • Air Circulation– When you overcrowd your plants, you will be more likely to get WPM dots. So, you need to make sure that you evenly space out your cannabis plants and buy the correct size or grow tent for the number of plants you want to grow. Also, be sure to buy a quality circulating fan for your grow area and use it properly.
  • Not Enough Light-Your plants are susceptible to WPM when they do not get enough light. So, get a quality timer, if you don’t already have one. Also, you need to make sure that your timers are working and schedule them according to the particular strain you are growing.
  • Too Much Fertilizer WPM tends to like new plants, and over-fertilizing causes plants to grow too quickly. And your cannabis plants will be more susceptible to WPM when they are growing abnormally fast. Remember, too much fertilizer can be a bad thing. So, be sure to follow the recommended amount of fertilizer for the number of plants that you have.
  • Poor Air Quality-WPM fungus spores can spread more quickly if you have low air quality in your grow area. So, you would be wise to purchase an air purification system for your grow tent or grow area.
  • Water- A dry grow area along with high humidity can cause conditions that attract mildew. So, make sure your plants are properly watered.

Why Should You Worry About White Dots and Splotches?

WPM starts out with the white dots on the top of your cannabis plants. If ignored, it will turn a white or greyish, mold, or fungi like substance on your plants. Eventually, if left unchecked, the WPM will cover a large proportion of your cannabis fan leaves. The mildew covering makes it hard for your plants to get the amount of light needed for growth as it inhibits photosynthesis. The mold will eventually turn your plants yellow, then brown and your plants will untimely will die if you do nothing about the problem.

How Can You Fix WPM

There are several methods, both organic, DIY, and commercial, methods to fix WPM. I will go over each method. Above all, to lessen the damage, you mustn’t procrastinate. To reduce the damage, you need to fix the problem before your plants get any worse and that the WPM does not spread. Many of the solutions below can also be used to prevent WPM.

DIY Methods For Fixing WPM

  • Baking Soda-Baking soda combined with liquid soap can serve as both a preventative and potentially help you fix a WPM problem. The recipe is simple as all you need is to add 1 Tbl spoon to 1/2 tsp to liquid soap (Make sure it is non-detergent dish soap) to 1 gallon of water.
  • Mouthwash– Mouthwash is designed to kill germs that cause cavities. The same properties in the mouthwash that kill germs also can work to help eliminate WPM. The formula is simply 3 parts of water to 1 part mouthwash. (Caution-be careful doing this when you apply this to younger plants as this is quite potent.)
  • Milk– Milk has surprisingly been found to help fix WPM. It is most commonly used for squash and cucumbers. However, you may want to experiment on this with cannabis. The recipes simply add 1 part of milk to 3 parts water.
  • Fungicide-Multiple types of fungicides are specifically designed to help you fix and prevent WPM. The below fungicide, from Trifecta, is a top-seller on Amazon, and it is specifically claimed to help both prevent and fix WPM.


The key to not having a problem with white dots on your plants, in the first place, is prevention. This means that you need to follow the best practices when it comes to indoor growing. First of all, especially if you are a newbie grower, I would consider starting out with cannabis strains bred for mold resistance. Then make sure you follow the best marijuana growing practices. I.E., Make sure humidity is at the proper levels, don’t over-fertilize, etc. Also, as an added bonus, it will likely not be as severe of an issue if you do use the best practices for marijuana cultivation if a WPM happens.


How To Deal With Spider Mites

Spider mites infestation is one of the most common pest infestations for cannabis. They can be recognized by their red/brown color, and oval shape. They are very small as they aren’t even 1/4 inches.

They feed on the under part of the plant leaves by ingesting a plant’s fluids by piercing the leaf. Just like WPM, a spider mite damage starts out with tiny white dots and if not treated the leaves will turn yellow and eventually die. They tend to thrive in a dry, hot environment. A neem oil mixture is one of the most common ways to treat this pest infestation. In another article, I go more in-depth with white spots on cannabis leaves caused by spider mites.


Leaf Septoria-Is a condition caused by a fungus that initially causes yellow dots on your cannabis plants and as it worsens the spots will turn brown.

Neem Oil-Neem oil comes from the neem tree and works for both preventing and curing pest infestation. It is the preferred organic, pest control solution, for marijuana growers.

White stripes on marijuana leaves are a common problems and are caused by either white powdery mold or spider mites. Find out how to . . .