Powdery mildew on Marijuana?

A question from a fellow grower:

I have a couple of WW Auto that are about 10 wks and several weeks into flowering. About a week ago I noticed some dusty white spots on some of the bigger leaves. Primarily on one plant. There is also a white hazy appearance to some of the small leaves of the flowers and buds and a few smaller white spots on some leaves. I’m concerned about the possibility of mold and if that is the case, want to address it as soon as possible.




powdery mildew marijuana

Powdery mildew is caused by a variety of fungi resulting in the appearance of a white or gray powdery growth on leaves. Those species that attack hops also attack marijuana plants. Powdery mildew can appear on plants indoors or out. Before the powder forms, small bumps may be noted on the topside of affected leaves.

Mildew spores are carried by the wind, air ventilation systems, clothing, pets and just about any living thing that moves about. Mildew spores will remain dormant until environmental factors offer the optimum conditions for them to come to life. Humidity levels over 55% and warm temperatures, along with over crowding are conducive to mildew growth. Once awakened, powdery mildew spores are likely to attack young plants first. It will then spread over the entire plant, infecting stems and buds, in addition to leaves.

Powdery mildew hinders photosynthesis, crippling the marijuana harvest. If left untreated, black specks will appear in the ‘powder’. Infected buds will smell damp when infected with powdery mildew. This fungus cannot be removed from the buds. Powdery mildew will turn the cannabis plants yellow, then brown and will eventually die.

Once again, prevention is the key to maintaining a healthy cannabis crop. Give the plants plenty of room to breathe; planting too closely together gives rise to activating mildew spores. Water your marijuana at a time of day where it will receive at least five hours of light, whether natural or artificial. Installing a UVC light in the ventilation system of indoor grow rooms will help to keep errant spores from attacking your plants.

In keeping with the purpose of this section, here are some measures you can take to eliminate the threat of powdery mildew having a devastating effect on your efforts as a cannabis gardener:
•Make a solution of 2 teaspoons organic apple cider vinegar per quart of water. Spray on the plants to eliminate and/or prevent mildew growth.
•Add ½ teaspoon baking soda per quart of water. Spray.
•Oil sprays consisting of neem oil, sesame oil or fish oil work on powdery mildew as it does on many fungal diseases.
•Milk sprays have been very effective in eliminating powdery mildew. Combine 40% milk with 60% water. The milk’s protein acts with the sun to create a natural antiseptic, killing mildew as the result. Milk sprays should be applied in bright light every ten days as a preventative measure. This method can be employed to resist mildew on any garden plant.

In the event powdery mildew hits you unawares, remove any infected leaves with a plastic bag, tie it tightly closed and discard into a receptacle with a tight fitting lid. Be careful not to jolt the spores into the air during this process, thus the recommendation to use a plastic bag during the removal process. Follow with a fungicide treatment on the stems from which the infected leaves have been removed. Any of the above mentioned preventative treatments can be applied. In addition you may want to try any one of the following to help heal wounded areas of the marijuana plants:
•Cinnamon oil, garlic oil, coriander oil, clove oil, jojoba oil, cottonseed oil sprays.
•Applications containing copper, available at any garden center.
•Hydrogen peroxide added to water (1 tablespoon per gallon) oxidizes the fungus cell walls, killing it.
•Limonene (oils extracted from citrus rinds) mixed with water at the rate of one teaspoon per pint offers fungicidal properties and will aid in control, but not eradication.
•Sulfur burners as a means of control. Stay out of the area when using this treatment as it can cause respiratory inflammation in humans. Follow directions and wipe down all walls and windows when completed to remove any residue.

We have so far, covered fungal and bacterial enemies of the cannabis plant that attack the visible parts. However, disease can also attack the root systems, which is what we will now address. Strap on your boots and come on down below for a deeper look into what can be affecting a failing crop. For a great anti-mold and disease spray, check out the products at this link here.

All living organisms need a strong core to maintain optimum health. Just as humans need to ensure healthy innards for ourselves and our pets, we need to do the same for our plants, Mary Jane included. Maladies arising from the inside (or below ground) are harder to detect because the affected areas are not readily visible. As a result, we are not always aware of a problem until it’s too late. Or is it? What are the most prevalent root diseases and how can we recognize, treat and or repair? Let’s travel together and see what we can discover.

I hope this helps. I lost my first two grows to this mildew



I hereby annoint you Sir Fungacide. You will be who I ask should I need to eradicate this evil menace.

I hope you never experience this mildew NEVER But I will help you and anyone else who ever dose…



I know this is an old thread BUT I have a question regarding Powdery mildew.
I recently got a clone from a friend and I ended up with powdery mildew.
I have been growing for 10 Years NEVER had an issue. But Now I do.
So now I need to try and eradicate it.
Right now I now am using a Milk spray which works well and the plants seem to like it.
But I would like to try and eliminate it totally.
I am planning on trying Marijuana Plant Protector to achieve this.
BUT How to I clean my closet??
I know this needs to be done because everything I have read says powdery mildew is hard to eliminate. BUT I would like to I clean my closet walls and floors.
Any Ideas on what to use ??? Any help will be appreciated.

I hearing about a milk and a special recipe to get rid of it. So as i know you can use any bonide products for this fight vs mildew. Well, i try to find recipe with milk and write it here. Good Luck.

Thanks for great read. I actually had powdery mildew in my garden but it did not affect my girls.

I had to move my small (ish) grow room to a new location a couple of months ago and it’s been fraught with issues.
Unfortunately I’ve been unable to get the humidity below 65 - 70% although the temp. is a near perfect 28 Celsius with a 5 deg drop at night.
I am guilty of a little over-crowding, and have four different strains on the go, only one of which is showing signs of Powdery Mildew - Purple Shaman grown from seed.
Is this mildew more likely to affect some strains more than others?
The bulk of this crop (my second in 4 months) is white widow, all of which are extremely healthy, budding well, with no signs of this disease.
I also noticed the one tall Purple Shaman has a much higher watering requirement than the other plants, several of which are the same size and bushier.
Any ideas out there about what’s going on?
I’ll be off to buy some apple cider vinegar once the supermarket opens, and I’ve recently applied a second dose of neem oil spray to all the plants, none of which seems to help the PS.
I’m really looking forward to seeing PS in the flesh, so any assistance would be bloody fantastic.

Hi I know this is an old post but i think my plants have white powdery mildew and have been uding the peroxide and water muxture 3 teaspoons per gallon. It does not look like it us working perhaps even getting worse. It has only been 2 days since i started using the mixture. Now one of the plants are turning brown around the tips. Also turnef the heat up to 90. I have a fan circulating air through the tent 24/7 and lights.

Any pointers.


White Powdery Mildew has no conchance and will attack your other plants.
Here read this…

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 grow. 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 Florida
and 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-Leaf Contact

Leaf to leaf contact = moisture = White Powdery Mildew!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 plants will often have
leaves mashed up
against each other as they try to reach 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.

White powdery mold on the stem of an outdoor plant - White powdery mold can
grow nearly anywhere on the
plant that’s exposed to air.

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
hing 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 per gallon of 35% H202)

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:

1.White Powdery Mold: Ruiner of beautiful plants.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 g
ently 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.

2.Ensure plants have proper airflow and ventilation - Even if you have absolutely no
airflow or ventilation
in your grow room, having even two fans will drastically reduce your chances of
encountering WPM while also
benefitting your plants 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.
The second fan should be
in your grow room pointing outward, pulling heat away from your plants (only needed
if you have no ventilation).
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.

3.Treat plant with SM90 to kill spores prevent future growth - Mix 1 part SM90 to 5
parts water(I’ve found 7 parts
water to be equally effective) in a clean sprayer/mister. Wait until just before your
lights for off for the day
and mist your (newly cleaned) plants. Get all the leaves! This diluted SM90 mixed will
kill any spores it touches,
and anywhere it lands becomes uninhabitable for future spores. Plus, it’s safe to use -
even during flowering - and
it smells awesome.

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
disease, it doesn’t have a chance!


Thank you!

1 Like

your very welcome ant time