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white powder on cannabis leaves

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!

6 Ways to Cure Powdery Mildew on Cannabis Organically

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.

What is Powdery Mildew?

Powdery mildew (PM or WPM) is 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.

The P. macularis species found growing on cannabis are nearly 98% identical to the P. macularis species that grows on hops, according to research done by Medicinal Genomics. [2]

Organic Fungicides to Stop Powdery Mildew

Potassium bicarbonate

Overview: A lot of 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 Bacilus 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 Stretomyces 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 on cannabis buds during 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 has 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 at the ozone layer. There are UVC lights for sale marketed to cannabis growers—they must kept at 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 used improperly 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, precautions need to be taken if this method is to be used. Read this safety sheet for more information.

Powdery Mildew Prevention

Along with application of 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

Presence of the fungi Trichoderma in a plant’s rhizosphere may play a crucial role in the prevention of 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 helps 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 another 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 aide 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 .

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.

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, it’s probable that ethanol-based extraction would 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.

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 pierces 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.

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 though 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.

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 much 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 in moisture.

There isn’t much evidence to suggest powdery mildews hop from species to species.

Powdery mildew hinders the ability of a plant to collect nutrients though 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 close to impossible, getting rid of it on a young plant is easy enough. Eliminating an outbreak is easier the earlier on it’s spotted.

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.

It’s important that the spray gets 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.

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 further increase their resilience.

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 though these reference links:

Additional 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.

Ultimate guide to powdery mildew cures. How to get rid of powdery mildew on cannabis fast during veg or flowering. The best organic methods