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 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. 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.
Organic Fungicides to Stop Powdery Mildew
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.
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.
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.
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.
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.
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.
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.
Furthermore, neem oil makes buds oily and smelly—another reason to avoid it altogether, especially during the cannabis plant’s flowering cycle.
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.
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.
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.
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.
Researchers speculate that the enzymes Trichoderma produces help the plant resist powdery mildew, among other infections.
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.
Methods of increasing Si in the soil or root medium include diatomaceous earth, greensand, pyrophyllite clay, and high-silica fertilizers such as Pro-TeKt.
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.
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
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.
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:Hemp Diseases and Pests: Management and Biological Control (Amazon link) medicinalgenomics.com/powdery-mildew newcropsorganics.ces.ncsu.edu/2010/07/the-cornell-formula-fungicide-an-example-of-why-you-need-to-check-out-your-internet-information-sources/ merryjane.com/health/the-curious-case-of-cannnabis-hyperemesis-syndrome link.springer.com/article/10.1023/A:1009919417481 apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-93-3-0281
Additional references are linked in-article.
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