Learn the most effective organic methods used to eliminate powdery mildew on cannabis, and how to stop further powdery mildew infections from occurring.
What is Powdery Mildew?
Powdery mildew (PM or WPM) is one of the most common fungal diseases that affects cannabis.
The species of powdery mildews that infect cannabis are in fact different from the species that infect other plants. There are at least two unique species that infect cannabis.
MacPartland et al. reported the species L.taurica and P.macularis (formerly known as S.macularis). These species are obligate biotrophs, which means 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.
Organic Fungicides to Stop Powdery Mildew
Overview: A lot of organic fungicide products for powdery mildew are simply potassium bicarbonate and a sticker agent, plus a flashy label on the bottle. 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 in water. Spray directly onto pm infected spots.
Cannabis Guru Ed Rosenthol prefers to mix an ounce of potassium bicarbonate in a gallon of water together along with 1.5 cups of milk.
Overview: Bacillus subtilis is a naturally occurring strain of bacteria that can be found in soils and in the gastrointestinal tract of humans. When bacillus subtilis is applied to powdery mildew, the bacteria feed on the powdery mildew as a nutrition source, until the infection is gone and then the bacteria die off.
Application: Use 30 ml (~6 tsp) of Cease per gallon of water (non-chlorinated) for foliar sprays. Alternatively, Serenade also contains Bacillus subtilis, and comes premixed into a spray bottle. Multiple applications of bacilus subtilis may be necessary as residual powdery mildew spores from the environment may settle onto plants after the first round of bacteria dies off.
Overview: Streptomyces lydicus is the bacterium species WYEC 108 from the genus Streptomycess. The naturally-occurring bacteria is used in horticulture to cure a number of diseases, including powdery mildew. Streptomyces lydicus works by establishing itself onto the plant surfaces, and attacks the powdery mildew fungus at its binding site.
Actinovate contains Streptomyces lydicus as the active ingredient, is 100% organic, and labeled as SAFE for people, pets and the environment.
Application: Mix 1-2 teaspoons of Actinovate per gallon of non-chlorinated water, plus a sticker (optional). Spray onto mildew infected areas of the plant. The dilution ratio can be increased for heavy pm infections.
Overview: One of the most popular organic powdery mildew killers, GreenCure is a potassium bicarbonate-based broad spectrum foliar fungicide that has performed as well or better than chemical fungicides in over 200 university studies. Like other potassium bicarbonate fungicides, Green Cure works by causing an immediate dehydration of the spores and destruction of the cell walls.
Application: Mix up to 2 tablespoons of GreenCure per gallon of water, spayed on plants to come directly into contact with the powdery mildew. Repeat application weekly for 3 weeks along with preventative measures.
Plant Oil Blends
Overview: Blends of organic plant oils such as garlic oil (sulfur), cottonseed oil, corn oil and others help to control powdery mildew and other pests and diseasese, especially when combined with potassium bicarbonate as FoxFarm does with their Bush Doctor Force of Nature fungicide.
Application: One quart bottle makes 25 gallons of foliar solution. Don’t spray buds the garlic will stink.
Apple Cider Vinegar
Overview: The acetic acid contained in vinegar helps to rid plants of powdery mildew infections. It should be noted that although apple cider vinegar is strong enough to stop most moderate-level PM infections, it’s not quite the best solution for eliminating a well-developed outbreak of powdery mildew.
Application: Mix 2 teaspoons of organic apple cider vinegar into a quart of water – be careful not to overdo it, high concentrations of vinegar can burn plants.
Hydrogen Peroxide (H202)
This cleaning procedure is a good alternative to bringing pm infected buds into the drying room. Yet there has been speculative criticism of using hydrogen peroxide to wash cannabis buds, due to the susceptibility of certain terpenes to oxidization, such as the terpene limonene.
Other Ways To Kill Powdery Mildew
High Alkaline PH Water
Changing the surface PH of cannabis leaves is a technique used to create an uninhabitable environment for powdery mildew. Using reverse osmosis water with a PH of 8 is perhaps the safest solution for spraying on cannabis buds during mid-to-late flowering cycle, still the effectiveness of this method varies.
One of the most popular powdery mildew solutions is to use baking soda (sodium bicarbonate) mixed with 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 shown to help remedy powdery mildew in some trials but not others. It’s speculated that the benefit is due to the probiotic lactobacillus. However, beware of spraying flowering plants with milk, as the proteins will create an suitable environment for gray mold to grow. Low-fat skim milk is preferred, as the fats present in milk can turn rancid.
Years ago we used to recommend Neem Oil for powdery mildew prevention and control during a plant’s vegetative growth cycle.
Not anymore though, due to the 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, which is another reason to avoid it altogether, especially during the cannabis plant’s flowering cycle.
Shortwave UV radiation (UVC) kills powdery mildew. There are a few UVC lights marketed to cannabis growers, however certain amounts of UVC radiation is harmful to both humans and to plants. UVC lights must kept at minimal intensity or use protective shields to prevent overexposure. The sun produces UVC radiation, but is absorbed in the ozone layer. For more information read this UVC hazards sheet.
Of the UVC methods available, manually applying UVC lights to PM infected plant surfaces creates the most dangerous liability. Furthermore, there’s a fine line between eliminating the existing powdery mildew with UVC and causing DNA damage in plants.
The more sensible UVC solution to powdery mildew is purely preventative, 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 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 aggrievances about the harsh effects sulfur has on the taste 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 info.
Powdery Mildew Prevention
Along with application of organic fungicides, here are some tips to help prevent further powdery mildew outbreaks in cannabis.
Clean and Quarantine
Important in the prevention of powdery mildew is the quarantine of infections and elimination of residual spores, which can hide on surfaces in the growroom/garden. Also suspect are any means of reintroducing the mildew in the grow room via infected clones, clothing, ect.
Indoor growers will want to wipe all surfaces, ventilation, fans ect. top-to-bottom with a bleach and water solution to kill any spores present. Ozone generators can also be used to sterilize grow rooms, although beware these are dangerous at high concentrations.
Lowering humidity increasing airflow, and clean air filtration all play a role in the prevention/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 that infects cannabis has developed resistance to low humidity levels.
Further research is needed, but one thing is certain – powdery mildew thrives in stale air, humid environments.
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 the leaves.
Researchers speculate that the enzymes Trichoderma produces helps the plant to resistant powdery mildew, among other infections.
Silica (silicon dioxide) is created when silicon comes into contact with oxygen. Relating to powdery mildew, silica helps strengthen the epidermal layer of plants, forming fortified areas that may help to protect plants from powdery mildew infections. Note that silica is naturally alkaline and will raise the PH level 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 concerning powdery mildew is whether or not it 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 this question here- medicinalgenomics.com/powdery-mildew-systemic
In any case, the term systemic may 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 clearly should not be smoked as it has links to causing Aspergillosis.
On the subject of making extraction with powdery mildew infected buds, it’s probable that ethanol-based extraction would burn off most of the spores. Still, we can’t 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, which is a growth structure that attaches the spore to the plant tissue. The fungi then pierces the plant’s dermal tissue with a sort of taproot, called a haustorium.
The haustorium is used to absorb nutrients from inside the plant cell, which causes weakening of the leaf and reduced photosynthesis ability. The haustorium sucks up plant nutrients and transferres them to the fungus, weakening the leaf 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 though long periods and overwintering.
Research has shown roughly 20-40 dpi (days post infection) from the start of a spore infection to when its 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 ≥30°C.
These are much of the same conditions plants prefer, which makes fighting off powdery mildew all the more of a challenge.
Stagnant air, low-light greenhouses and indoor grow rooms without 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 hopping from species to species.
What does Powdery Mildew do to Cannabis?
Powdery mildew hinders ability of a plant to collect nutrients though its leaves (photosynthesis). As PM spreads though the cannabis plant, the leaves unable to collect nutrients bring the plant into a slow stunted growth stasis.
PM spreads quickly though cannabis, smothering it in white mildew – many times in just a few days. Left untreated, powdery mildew will effectively ruin a bud harvest.
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 more probable 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 to do the job. Even water bottle sprayers can be used, however it’s important that the spray gets applied to both the top and undersides of infected leaves, along with stems and stalks.
While foliar treating plants, it may be helpful to cover up the base of plants to avoid fungicide drip 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 (sometimes called a spreader-sticker) helps the spray stay on the foliage 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 further increase the resistance in strains.
Visit our Mold Resistant Strains homepage for some real good strains. Some of the best producing strains from our favorite’s grown outdoors on the Big Island of Hawaii. High mold resistance.
I recommend you check out the Shaping Fire podcast ep. 28, where I learned much of the information needed to write this article.
You may enjoy further learning about powdery mildew though these linksHemp 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
Other references are linked in article
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