Ultimate guide to powdery mildew cures. How to get rid of powdery mildew on cannabis fast during veg or flowering. The best organic methods to control powdery mildew on your marijuana plants.
Table of Contents:
What is Powdery Mildew?
Powdery mildew (PM or WPM) is a fungal disease that affects a wide range of plants. Infections appear as white powdery spots on leaves and stems.
The species of powdery mildew that affect cannabis are different from the species that affect other plants. At least two unique species are known to affect cannabis.
MacPartland et al.[1] reported the species L. taurica and P. macularis (formerly known as S. macularis). These pathogenic fungi are obligate biotrophs, meaning they need a host to grow.
According to research done by Medicinal Genomics, the P. macularis species found growing on cannabis are nearly 98% identical to the P. macularis species that grows on hops.[2]
Organic Fungicides to Stop Powdery Mildew
Potassium bicarbonate
Overview: Many organic fungicide products for powdery mildew are simply potassium bicarbonate and a wetting agent in water. Save money on overpriced liquid formulations by mixing it yourself using potassium bicarbonate powder and an organic wetting agent.
Application: Make a 0.5–2.0% solution (5–20 grams per liter) of potassium bicarbonate to water. Spray directly onto mildew-infected plant tissue. Repeat weekly.
Cannabis Guru Ed Rosenthal’s formula uses an ounce of potassium bicarbonate mixed into a gallon of water and 1½ cups of milk.
Bacillus subtilis
Overview: Bacillus subtilis is a naturally occurring, non-pathogenic, and non-toxicogenic bacterium found in soils and the gastrointestinal tract of humans. The organism feeds on powdery mildew as a nutrition source. It can be applied directly to plants and will die off after eliminating the infection.
Application: Dilute 30 ml (~6 tsp) of Cease per gallon of water (non-chlorinated) for foliar sprays. Alternatively, Serenade may be used, which also contains the bacterium. Multiple applications of Bacillus subtilis may be necessary—residual powdery mildew spores in the grow area could reinfect plants.
Bacillus amyloliquefaciens
Overview: Bacillus amyloliquefaciens is another naturally occurring, non-pathogenic, and non-toxicogenic species of bacterium in the genus Bacillus. It is used as a biocontrol agent for a range of fungal and bacterial diseases, including leaf septoria, powdery mildew, and downy mildew.
Application: Dilute 1 teaspoon of Garden Friendly Fungicide per gallon of water. Add a wetting agent (optional). Spray the solution directly onto affected plant areas. Multiple applications may be necessary.
Streptomyces lydicus
Overview: Streptomyces lydicus is a naturally occurring bacterium commonly found in soil. The Streptomyces lydicus strain WYEC 108 is used as an agent against fungal pathogens. The organism works by establishing itself on plant material, then attacking powdery mildew at its binding site.
Actinovate is an organic biological fungicide that contains Streptomyces lydicus as the active ingredient, labeled SAFE for people, pets, and the environment.
Application: Dilute 1–2 teaspoons of Actinovate per gallon of non-chlorinated water. Add a wetting agent (optional). Spray onto mildew-infected areas of the plant. Repeat as necessary. The dilution ratio can be increased for heavy PM infections.
Trifecta Crop Control
Overview: Trifecta Crop Control is a concentrated blend designed specifically for medical and legal recreational cannabis cultivators. Using nano-sized essential oils and soap, Crop Control treats and prevents molds, mildews, pests, and parasitic species. The active ingredients are thyme oil, clove oil, garlic oil, peppermint oil, corn oil, rosemary oil, geraniol, and citric acid. Inert ingredients include filtered water, soap, isopropyl alcohol, and vinegar.
Application: For preventative maintenance, dilute ½–1 oz. of Crop Control SC per gallon of water. Apply 1–2 times per week using an atomizer, misting system, or spray bottle. For infestations, use 2–4 oz. and apply every other day.
Apple Cider Vinegar
Overview: Apple cider vinegar, the same kind used in the kitchen and for home remedies, is a simple solution for controlling powdery mildew on plants. The acetic acid contained in vinegar works to control powdery mildew. Moderate strength; it won’t rid a heavy infection.
Application: Dilute 2 teaspoons of organic apple cider vinegar to a quart of water. Multiple applications may be needed. Be careful not to overdo it—high concentrations of vinegar will burn foliage.
Hydrogen Peroxide Bath (H2O2)
H2O2 is hydrogen and oxygen, nearly the same as water but with an extra oxygen atom. World-renowned cannabis expert Jorge Cervantes uses a 3% wash of hydrogen peroxide to clean off powdery mildew on freshly harvested cannabis buds. The buds are submerged in the H2O2 bath for a brief period, then washed off with pure water and hung to dry with fans.
The bud washing procedure for powdery mildew-infected plants can be a crop saver in dire circumstances. However, there has been speculation on the aftereffects of using hydrogen peroxide to wash cannabis buds due to the susceptibility of certain terpenes to oxidization, such as limonene.
Other Ways To Kill Powdery Mildew
High pH Alkaline Water
Changing the surface pH of cannabis leaves is a technique used to create an uninhabitable environment for powdery mildew. Using alkaline reverse osmosis water is perhaps the safest solution for spraying cannabis buds during the mid/late flowering cycle. However, the effectiveness of this method varies.
Baking Soda
One of the most popular powdery mildew solutions is to use baking soda (sodium bicarbonate) diluted in water as a foliar spray on plants. Baking soda works to change the surface pH of the leaf to inhibit powdery mildew growth. Although this may have limited benefit, studies have shown sodium bicarbonate to be less effective than potassium bicarbonate.[3]
Milk
Milk has been shown to help remedy powdery mildew in some trials but not others. The benefit likely comes from the probiotic lactobacillus. Just be careful of spraying flowering plants with milk—the proteins create a suitable environment for gray mold to grow. Low-fat, skim milk is preferred, as the fats present in whole milk will turn rancid.
Neem Oil
Years ago, we would recommend neem oil for powdery mildew prevention and control.
However, we cannot recommend it anymore due to various discussions linking the chemical Azadirachtin, an active ingredient found in neem oil, to a condition known as cannabis hyperemesis syndrome.[4]
Furthermore, neem oil makes buds oily and smelly—another reason to avoid it altogether, especially during the cannabis plant’s flowering cycle.
UVC Light
Shortwave UV radiation (UVC) kills powdery mildew, but it is also harmful to humans and plants at certain levels. While the sun produces UVC radiation naturally, it is absorbed by the ozone layer. There are UVC lights for sale marketed to cannabis growers—they must be kept at a minimal intensity and used with protective shields to prevent overexposure. For more information, read this UVC hazards sheet.
Of the UVC methods available, manually applying UVC light to mildew-infected plants presents the most danger. There’s a fine line between eliminating the powdery mildew with UVC and causing DNA damage in plants.
The safest UVC solution to powdery mildew is purely preventative—installed in protected germicidal air filtration units. UVC air filtration can be installed in heating, air conditioning, and ventilation systems to disinfect the air while also avoiding direct exposure to plants and humans.
Sulfur Burners
Elemental sulfur can be vaporized in sulfur burners to prevent and control powdery mildew. When misused or at too high a concentration, sulfur causes leaf damage to plants.
In best practice, avoid sulfur vaporization during the plant’s flowering cycle. There are many grievances about the harsh effects sulfur has on the smell and flavor of buds.
Sulfur burners are hazardous to humans, and precautions need to be taken if this method is to be used. Read this safety sheet for more information.
Powdery Mildew Prevention
Along with applying organic fungicides, here are some more tips to help prevent further powdery mildew outbreaks in cannabis.
Clean and Quarantine
Cleaning the grow site and quarantining plants. The goal is to quarantine plants until powdery mildew is completely eliminated, including dormant spores that may be hiding nearby. Also suspect are any means of reintroducing the mildew in the grow room via infected clones, clothing, etc.
Indoor growers will want to wipe all surfaces and grow equipment top-to-bottom with a bleach-water solution to kill any residual spores. Ozone generators can also be used to sterilize grow rooms, although beware—these are dangerous at high concentrations.
Climate Control
Lowering humidity, increasing airflow/ventilation, and sterilized air filtration, if possible, all help in the prevention and control of powdery mildew.
While it’s often recommended for indoor growers to keep their grow rooms at a relative humidity of 45% or lower, there is anecdotal evidence among growers that the species of powdery mildew which infects cannabis has developed resistance to low humidity levels.
Further research is needed, but one thing is certain—powdery mildew thrives in stale, humid environments.
Trichoderma
The presence of the fungi Trichoderma in a plant’s rhizosphere may play a crucial role in preventing powdery mildew. While there have been no studies directly related to the powdery mildew species that grow on cannabis, peer-reviewed studies of powdery mildew on other plants have shown that application of the species T. harzianum in soil resulted in a 75–90% reduction of powdery mildew infection on leaves.[5]
Researchers speculate that the enzymes Trichoderma produces help the plant resist powdery mildew, among other infections.
Trichoderma can be found in products such as Soil Blast on Amazon. However, over-application of Trichoderma may create problems with other beneficial fungi for plants—mycorrhizal fungi.
Silicon (Si)
Silica (silicon dioxide) is created when silicon comes into contact with oxygen. Relating to powdery mildew, silica aids in strengthening the epidermal layer of plants, forming fortified areas that may protect them from powdery mildew infections. Note: silica is naturally alkaline and will raise the pH of a nutrient solution.
Popular liquid formulations of silica include Armor Si by General Hydroponics and BulletProof by Cutting Edge.
Methods of increasing Si in the soil or root medium include diatomaceous earth, greensand, pyrophyllite clay, and high-silica fertilizers such as Pro-TeKt.
FAQ
Is Powdery Mildew Systemic?
One of the most commonly debated issues among cannabis growers is whether or not powdery mildew is systemic to the plant.
The leading researchers of powdery mildew in cannabis, Medicinal Genomics, aren’t quite sure of the answer yet. Read their response to the question here: medicinalgenomics.com/powdery-mildew-systemic
In any case, the term systemic might not be the right word to use.
Is Powdery Mildew Dangerous to Smoke?
There isn’t enough evidence to define any dangers of smoking powdery mildew. It isn’t considered a human pathogen, and at worst, may cause allergenic symptoms. The main gripe about powdery mildew on buds is the nasty appearance it creates.
This is not the case for another mold on cannabis, Botrytis cinerea, which should never be smoked.
As for making extraction/concentrate products with powdery mildew-infected buds, ethanol-based extraction would probably burn off most of the spores. Still, it’s hard to endorse smoking any derivatives of powdery mildew-infected buds until further research is done.
How does Powdery Mildew Infection Start?
The infection starts when a spore (conidium) lands on a leaf. Once the spore germinates, it quickly develops an appressorium—a growth structure that attaches the spore to plant tissue. The fungi then pierce the dermal tissue with its taproot, called a haustorium.
The haustorium is used to absorb nutrients from inside the plant cell, causing weakening of the leaf and reduced photosynthesis ability. The haustorium sucks up plant nutrients and transfers them to the fungi, stunting the plant and slowing growth.
How does Powdery Mildew Spread?
Once the powdery mildew infection has started, a mycelium network is quickly developed throughout the plant.
Spores of both species are spread airborne. The P. macularis spores also migrate with moving water, such as drops of water falling from leaf to leaf or blown by the wind to other plants.
Hardy spores and spore structures, called cleistothecium, can survive through long periods and overwintering.
Research has shown it takes roughly 20–40 dpi (days post-infection) from the start of a spore infection to when it produces spores of its own.
Where does Powdery Mildew Grow?
Powdery mildew spores germinate in slightly acidic conditions with temperatures ranging from 18–24°C. Once germinated, the powdery mildew fungi can withstand a wider range of climates. Infectivity of P. macularis conidia is greatly reduced at temperatures over 30°C.[5]
These are many of the same conditions plants prefer, which makes fighting off powdery mildew all the more difficult.
Stagnant air, low-light greenhouses, and indoor grow rooms lacking proper ventilation are easy places for powdery mildew to reproduce. Plants that have been overcrowded grown Sea of Green or SCROG style may be problematic, as they trap moisture.
There isn’t much evidence to suggest powdery mildews hop from species to species.
What does Powdery Mildew do to Cannabis?
Powdery mildew hinders the ability of a plant to collect nutrients through its leaves (photosynthesis). As the mildew spreads throughout the cannabis plant, the leaves become unable to collect nutrients, bringing the plant into a slow, stunted growth stasis.
Powdery mildew spreads quickly in cannabis, many times establishing itself in just a few days. Left untreated, powdery mildew will effectively ruin the crop.
While bringing back a matured plant covered in PM can be nearly impossible, getting rid of it on a young plant is easy enough. Eliminating an outbreak is easier the earlier on it’s spotted.
How to Apply Foliar Spray Fungicide
A simple garden pump sprayer like this on Amazon is all you need for most applications. Even a punctured water bottle works in a pinch. Atomizers and misters make a finer spread but can clog.
The spray must get applied to both the top and undersides of infected leaves, along with stems, branches, and stalks.
You may consider laying down a cover under plants to avoid fungicide dripping into the root zone. Although these powdery mildew remedies are organic, they still can be harmful to beneficial microorganisms in the soil.
Most remedies require more than one application to really be effective. Moisture may wash the foliar spray off; even morning dew on plants can remove foliar applications from leaves. Perhaps the best time to spray outdoors is before noon on a dry, sunny day. Indoor growers usually spray before lights out.
Using an organic wetting agent (also called a spreader-sticker) helps the spray stay on longer.
Breeding For PM Resistance
Certain strains of cannabis have natural resistances to powdery mildew infection. What cannabis breeders can do is work with these genetic lines by selective breeding to increase their resilience further.
Visit our Mold Resistant Strains homepage for a range of quality genetics. We’ve got everything from popular feminized, regular and autoflower seeds all the way to niche landraces, jungle sativas, and mountain indicas.
References
Listen to the Shaping Fire podcast, ep. 28, for an informative conversation on powdery mildew. Featuring special guest Kevin McKernan of Medicinal Genomics.
You may enjoy further learning about powdery mildew through these reference links:
[1]Hemp Diseases and Pests: Management and Biological Control (Amazon link) [2]medicinalgenomics.com/powdery-mildew [3]newcropsorganics.ces.ncsu.edu/2010/07/the-cornell-formula-fungicide-an-example-of-why-you-need-to-check-out-your-internet-information-sources/ [4]merryjane.com/health/the-curious-case-of-cannnabis-hyperemesis-syndrome [5]link.springer.com/article/10.1023/A:1009919417481 [6]apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-93-3-0281Additional references are linked in-article.
If you have any questions, experience, or comments about powdery mildew on cannabis, please comment below. Disclaimer: We do not promote or undertake in illegal activities.
I use Need tincture this year mixed with soap and all gone so quickly. I am an herbalist.
I made a spray with baking soda and dawn is this suitable?
Hi, thank you for your advice! I am having problems with my plants it first was the white dust mold and ever since I used the baking soda remade now their is white spots on the plant. Could you help to tell me what I can do to fix this? I just switched to H2o2 to spray them it’s only been a day using it but I do not want this to go any further so, any advice would be greatly appreciated thank you!
Same problem. Started with H2o2/ water/dawn. Worked ok but came back. Now using Potassium Carbonate/water/dawn. 2tbsp per gallon w/1/2 tsp dawn. Also defoliate large leaves,etc
Potassium bicarbonate is much more effective, if spraying to treat pm.3tsp potassium bicarb,3 tsp dish liquid,and a few drops of cooking oil per gallon.With some plants this may be a tad hot, but is a good treatment for pm.In any case if I get hit with pm,I usually do a h202 bath at harvest,between the two. It will save your cantaloupe….
What are you measurements of your mixture?
Neem tincture.
I have an old secret recipe,
5 gal bucket of water,
2 cups earthworm casts,
1 cup Grandmas UNsulphered molasses,
Mix(i aireate with aquarium pump),
For 24 hours, put in backpack sprayer,
Soak down leaves , a cloudy day works best
And thats it!!! 2-3 days not a spot of mold.
Now thats organic!!! Its worked on the foggy Oregon Coast for decades where many a crop gets ruined, it qorks on 3 footers. And my 20′ “trees”…
Your welcome, No charge
Thanks for your input anonymous 😉
Hey, I doubt you’ll respond… been a while lol! Does this have a negative effect of flowering plants since you’re using worm castings?
Sweet. Thanks.
Question: Why the earthworm castings? What role do they play?
plays a role in having beneficial microorganisms and fungi that will systemically save the plant from molds and pests.
Howzit! My plant is 6 weeks into flower. I’ve been noticing stem rot where a branch comes out of the main stalk. It’s happening in more than one spot.The branch would turn yellow and die off. It’s only on one plant. Have any experience with this? Genetic or some kind of bug? Mahalo for any manao.
Hey brah,
Sounds like you got stem borer problems. Common in Hawaii. Best bet is to inject Dipel [BT] (Bacillus thuringiensis) into the wound as soon as the bruise is spotted. Keep a close eye on the main stalk and spray immature plants with BT as a preventative method.
Thanks Bra! I think you are right! I see a little spot on or near where the rot is. Someone else from Hawaii on another site said it was a fungus problem. I’m growing the Kryptonite from Amsterdam seed co. Nice dense nugs. It’s suppose to be high in Thc and Cbd. 2-3 more weeks to go. I have 2 phenos of this strain and one is getting the stem borer. Thanks for your manao
Hi Nugs808. Yes that stem borer is a big problem in tropical cannabis growing. So is powdery mildew. I have seen more and more powdery mildew around the Islands decimate people’s crops, either by completely smothering the buds in white powder mold, or by the grower using harsh treatment trying to cure the mildew. How’s that Kryponite strain from Amsterdam seed co.?
no where have I seen anything about how long WPM spores are viable ?? a week, a month, a year… forever ?!?!
WPM spores can live on plant debris for a few years and still be infectious until the plant material is sterilized or completely breaks down. The exact time for how long powdery mildew spores can survive without a host is unclear and likely varies with environmental factors. Hope this helps you get an idea, good luck! 🙂
I wish it was a better answer but thanks…. I am just curious, if there is no plant material, all surfaces and equip have been cleaned, the room has been left dormant for 18 months… it is dark, cool and low humidty…
I just hate to go thru the energy, resources and time to start again only to run into this wpm problem again…
Can’t win if you don’t play, homie!
Get amongst it!
We use greencure and onyx to help our herb garden with powder mold, but only in the early stages before buds come.
Agreed for sure Dino. Curing powdery mildew on cannabis should not be done while the buds are well developed. Best way to deal with powdery mildew is to take precautions from the very beginning. Spraying regularly with organic fungicide will help the wicked powdery mildew controlled and at bay. If things start developing, step it up. Potassium bicarbonate / Green Cure, Bacillus subtilis and Actinovate (Streptomyces lydicus) all work well. Make sure to do this before the flowers have developed and with a little bit of luck you can be blessed with a clean, mold and mildew free harvest of the dank.
Check out Mites & Mildew, not sure effectiveness against mites but 3-4 treatments about same amount of days apart will rid moderate outbreak of pm. Saved some very prized mothers and new cuts. Check it out. Otherwise look at oil fungicides is my opinion or combo both. I always continue process twice as long just to be sure. Cookie strains seemed to be most affected. Cured.
Great comment Jake, thanks 🙂
Does anyone know if Actinovate can actually help PREVENT powdery mildew? My plants are just geting started and I would love to spray or soak them with something long before buds form.
I’ve been trying to kill powdery mildew from my garden for a long time.. baking soda and safers just stops the pm mold for a few days… actinovate does the trick to kill powdery mildew, but the best way to do it is to trash all the plants that are already infected with powder mold… yeah it sucks, but hey, better than smoking mold yeah?
use milk! use milk! use mikl! not low-fat, but normal
but mix 1part milk with 10 parts water!
Hi do you know strain who have resistance for powder mildew oïdium?my favorite is sour diesel but not resistant to oïdium…thank you!
Hi, I started my “Rainbow Sherbert” marijuana plant from seed. It is now in it’s 4th week of flower and was growing real nice buds. A week ago, I noticed powdery mold and used a mixture of water, dish soap, canola oil and baken soda. I sprayed the entire plant even the buds (which I saw them do on Youtube. I am now worried because some sites say you can spray the buds when the mildew is present, others say never spay buds. I had started the plant outside and it was doing great. I decided to bring her indoors because I live in the Northeast and I thought it was too cold in the evenings. I have her on a 12/12 schedule. Did I ruin my plant by getting the mixture on the buds?. Sincerely, Claudia
Can you spray Apple Cider on a plant when it starts to Flower ?
I read the warnings, but what if anything is recommended for treating plants already flowering?
Same question as Mojofoto. My buds are looking good, mildew has formed. I have a sodium bicarbonate & water solution. Can I spray on leaves and buds or should I just spray on leaves?
You will be fine using the sodium/potassium bicarbonate, add some dawn blue dish soap, just a couple drops in a litre spray bottle, it won’t harm a thing, even some peroxide and water with a couple drops of dish soap is fine. if it gets worse, there are stronger solutions.
Hi, I’m finishing about a pound indoors and have been using 5 tablespoons of hydrogen peroxide in 16 ounces of water to control small amounts of powdery mildew. I spray the worst plants completely, saturating the leaves and buds and leave for about five to ten minutes. Then I spray with regular water until the hydrogen peroxide solution is completely washed away. This kills the PM and washes most of it away. I think it’s probably a safe efficient way to clean the plants. They may lose some flavor buy are still dank and it doesn’t wash away any of the thc at least. I even happy with the results and just repeat about every other week or as necessary.
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You’re an imbecile Beau Hawkes.
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