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powdery mildew cannabis flowering

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!

HELP!! Powdery mildew at 42 days flower.

Surfr
Guest

I am at 42 days flower and I found some PM on all of my plants.. I cut off everything I found that had PM but I know there is stuff i cannot see. It wasn’t a crazy amount, but enough to get me worried.. I am stressing very badly here.. I don’t know what to do.. Is there anything I can spray this late in flower?? Is it too late to use a sulfur burner?? Any help is much appreciated!

bannana man
Guest
bannana man
Guest
Surfr
Guest

Yah I think it’s too late (assuming you’re running 8-10 week strains) to use sulfur or any oils and not taste it in the finished product. I would use a potassium bicarbonate product such as green cure, or potassium bicarbonate powder. Mix it with some dutch masters penetrator. Keep it on hand and spray once today, once again in 3 days and again 3 days later. So 6 days from now you would have sprayed the 3rd time and it should take care of your PM problem until you crop.

You should also address the source of the PM, high humidity, sitting water, PM on vegetation around your grow room, etc.

Surfr
Guest

Yah I think it’s too late (assuming you’re running 8-10 week strains) to use sulfur or any oils and not taste it in the finished product. I would use a potassium bicarbonate product such as green cure, or potassium bicarbonate powder. Mix it with some dutch masters penetrator. Keep it on hand and spray once today, once again in 3 days and again 3 days later. So 6 days from now you would have sprayed the 3rd time and it should take care of your PM problem until you crop.

You should also address the source of the PM, high humidity, sitting water, PM on vegetation around your grow room, etc.

Thanks ENT. I am going to check the shops around town to see if they have any greencure.. It’s crazy that I am even having this PM.. I have 3 big ass fans in a 4×8 tent so the air movement is good. Humidity is between 30%-45% NEVER above that.. I have proper intake and exhaust out of room. I am simply stumped.. I guess I am going to need to do a whole room strip down and cleaning.. Fun shit.

tcurtiss

SM90 also removes PM, make sure you have good air flow

That is what caused PM for me, I turned down the exhaust and forget to turn it back up, after a day all plants had PM

Gave the plants some SM90 and the exhaust is on full speed now

tcurtiss

How many CFM’s are you pulling out of your room?

I am using a 165 cfm for a 12 cubic foot cabinet with carbon filter attached

Maybe you need to pull more air from the tent?

Surfr
Guest

How many CFM’s are you pulling out of your room?

I am using a 165 cfm for a 12 cubic foot cabinet with carbon filter attached

Maybe you need to pull more air from the tent?

Rushoe
Kennie_B
Guest
Mr.Sputnik
Surfr
Guest
Jalisco Kid
Guest
mrdizzle

I discoverd my girls had some bad PM around week 6, my retarded ass has never had it before so I wasnt preped. I sprayed with serenade, bi-carb, 35% perxoide with a ryobi paint sprayer, and I honestly wish I would have just burnt sulfur,depending on how bad it is dude you might want to just burn sulfur. It may take some cure to get the sulfur smell gone ( I know good friends who claim they burn real late without problems) but if its gets out of control you will be worse off with powder donut buds . I have 24cans of duster infront of me and I have just spent 8hrs cleaning a 1/2lbs watching judge judy. not to mention the shit buttfucks your yeilds.

Good luck bro I would trade PM for all the mites in the world

Surfr
Guest

I discoverd my girls had some bad PM around week 6, my retarded ass has never had it before so I wasnt preped. I sprayed with serenade, bi-carb, 35% perxoide with a ryobi paint sprayer, and I honestly wish I would have just burnt sulfur,depending on how bad it is dude you might want to just burn sulfur. It may take some cure to get the sulfur smell gone ( I know good friends who claim they burn real late without problems) but if its gets out of control you will be worse off with powder donut buds . I have 24cans of duster infront of me and I have just spent 8hrs cleaning a 1/2lbs watching judge judy. not to mention the shit buttfucks your yeilds.

Good luck bro I would trade PM for all the mites in the world

I am tempted to just burn some sulfur. The shop near me rents sulfur burners for a decent price..

What do you guys think about chopping the ladies at 55 days before I leave for vacation?

Mr.Sputnik

There’s a reason why it’s comming back so fast (low airflow somewhere, high RH) so fix that problem first and foremost. Potassium bicarbonate should knock it back for longer than 3 days if you have enough airflow. Most grow stores don’t carry potassium bicarbonate and I don’t know why, it’s very cheap and very available. serenade and sulfur make your buds taste like ass.

Chopping early sucks. An h202 mix will literally eat the mildew off of your buds if they are powdery when you get home. Spray PB, then hope PM doesn’t come back real hard. If it does, spray some h202 to get rid of it and then harvest.

Surfr
Guest

The local shop just starting carrying Greencure so I picked some up today. I am going to give the ladies a very thorough spraying before lights out. Do you guys think I should spray a few days before chop to help fight off anything that might and try to attack while drying??

Thanks for the advice! Excuse my ignorance, but H2O2 is just Hydrogen peroxide, correct? What are mixture ratios for that? It does not harm the trichomes or buds at all?? Thanks!

MooseFarts

I know I’m resurrecting an old thread but I really didn’t feel like starting a new one for a similar problem. My plant started flowering a bit early and due to the weather in Ontario (cycle of hot dry days followed by hot and high humidity and then downpours rain) I’ve been getting WPM on this plant. The plant is outdoors planted in the ground so moving it and controlling humidity isn’t an option.

I have neem oil which I’ve been using about every 7-10 days as a preventative but stopped when I started seeing white hairs. My question is, given by the picture am I still early on enough to spray with neem oil or will that still have an effect on taste? I don’t want to ruin the bud with neem but I also don’t want to ruin the bud with WPM. The picture is from two days ago

Also as a side question, Is it normal to already see white hair tips turning brown?

** as an edit: seen that the picture quality dropped when posting the pic and you can’t actually see the browning of the hairs. It’s literally just the tips of the hairs going brown **

I am at 42 days flower and I found some PM on all of my plants.. I cut off everything I found that had PM but I know there is stuff i cannot see… It…