by Sirius Fourside
Have you seen white spots on your leaves? Are your leaves dusted with round patches of powder that look 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!
White Powdery Mildew is a rapidly reproducing (both sexually AND asexually) fungus that 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.
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 the rampant growth of WPM and bud mold.
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.
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.
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.
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 – Mix 1 part milk to 3 parts water and spray liberally while your grow lights are on. The type of milk (skim, 2%, whole, etc.) doesn’t matter. Wet both sides of leaves if possible.
- Potassium bicarbonate – Dissolve 1 tbsp of potassium bicarbonate in 1 gallon of water. Optional: Add 1 tbsp vegetable oil and 3 drops of liquid dish soap. Spray plants lightly but evenly
- Baking soda (not as effective as the other 2)- Mix 1 tbsp baking soda, 1 tsp liquid dish soap, and 1 gallon of water. Spray all the leaves top and bottom
Here’s 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. Spray all the leaves top and bottom, 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!
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.
Handheld Mister/Sprayer – A mister is awesome for applying treatment. Also, it’s the best way to foliar feed your plants.
Here are the most common white powdery mildew remedies:
Milk – Mix 1 part milk to 3 parts water and spray liberally while your grow lights are on. The type of milk (skim, 2%, whole, etc.) doesn’t matter. Wet both sides of leaves if possible.
Potassium bicarbonate – Dissolve 1 tbsp of potassium bicarbonate in 1 gallon of water. Optional: Add 1 tbsp vegetable oil and 3 drops of liquid dish soap. Spray plants lightly but evenly. Do this during lights off.
Baking soda (not as effective as milk or Potassium bicarbonate )- Mix 1 tbsp baking soda, 1 tsp liquid dish soap, and 1 gallon of water. Spray all the leaves top and bottom.
Now on to some bottled cures…
Garden Safe Brand Fungicide3 – Organic (OMRI listed) – Follow the instructions on the label
Lost Coast Plant Therapy (expensive) – Kills white powdery mildew as well as several other pests and it’s safe for flowers, pets, and people. Handy to have around the grow room.
GrowSafe (expensive) – 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?
Bonus: Glookies strain is resistant to mold and poor environmental conditions. This one survived high humidity to produce great yields with strong bud effects.