Powdery mildew is usually first observed as small white circular spots on Cannabis leaves. They can be faint, but they begin to cover entire leaves if left unchecked and can begin to grow on buds as well.

Photo taken from White Powdery Mildew on Cannabis Plants – Identification & Solution! (n.d.). Retrieved February 12, 2020, from https://www.growweedeasy.com/cannabis-plant-problems/white-powdery-mildew

Photo taken from Is Powdery Mildew Systemic? | Medicinal Genomics. (n.d.). Retrieved February 12, 2020, from https://www.medicinalgenomics.com/powdery-mildew-systemic/

Conditions

For powdery mildew, the conditions that favor the host, also favor the pathogen. Dry leaves, warm temperatures, and moderate to high humidity. It can tolerate low humidity as well

Many websites out there will tell you that powdery mildew in Cannabis prefers cool temperatures and low humidity. Powdery mildew can be one of the most difficult diseases to control because it grows best in the same conditions as your Cannbis plants. However, PM can tolerate a wide range of RH levels, and simply lowering your RH levels will not eliminate PM risk, though it does help. In fact, low humidity can favor the spread of the disease, but high humidity can favor spore germination (although liquid water in contact with spores will inhibit germination).

Powdery Mildew on Cannabis: The Summary

What PM Species Affect Cannabis? What Environmental Conditions help control PM on Cannabis?

Much of the information on popular websites for Cannabis Powdery Mildew conflicts with the information in the scientific literature. I will report what has been published in scientific literature. Powdery mildew fungi often have a narrow host range. They are known as biotrophs, meaning that they can only live and reproduce on living hosts. Coincidentally, they also cannot be cultured in vitro because they require the living host to survive. There is a closer evolutionary relationship between the host and the parasitic fungus than that of necrotrophic fungi such as B cineria, the causal agent of bud rot.

Many species have historically been identified as capable of infecting Cannabis. In the early 1990s, McPartland reported at least 2 different species (Leveillula taurica and Sphaerotheca macularis) [1i, 2i]. In 2018, a Canadian publication described the causal agent of Cannabis Powdery Mildew in samples from drug Cannabis as belonging to the Golovinomyces cichoracearum species complex from looking at ITS sequences, which may include other species such as G. spadiceus or G. ambrosiae [1]. Subsequent studies show that G. spadecius is a common PM pathogen on Cannabis species [2, 3,]. Cannabis can also be infected by PM from closely related plant species such as hop PM, Podosphaera macularis [2].

Golovinomyces spadiceus grows best in warm, low humidity climates. It is commonly found on wild plant species such as wild sunflower [4] or plants within the same tribe, such as Zinnia flowers and various other plants [5, 6]. Cannabis can get a decent amount of infection on the flowers, and this can lead to unmarketability in the private and medical sectors. It is not recommended to consume bud infected with PM, though I do not believe there is any research as to the health effects of consuming Cannabis infected with PM. It can certainly destroy trichomes and affect the flavor and odor of your buds is you have a heavy infection.

When temperatures drop, the relative humidity of your grow area will go up because cooler air holds less water and condenses water easily [7]. The tomato-infecting PM species, which has also been reported as infecting Cannabis, is signifiantly reduced at low humidity levels (20-40% RH) [8], and this is a common recommendation for Cannabis growers. For G. spadiceus, the pathogen is presumed to act similarly, and keeping humidity under 50%, and preferably lower is ideal for controlling Powdery Mildew. For instance, one report of G. spadiceus in Cannabis says that G. spadiceus thrives in warm temperatures and moderate to high humidity [9].

The Biology of Powdery Mildew

Powdery mildew are ascomycete fungi in the order Erysiphales. As mentioned, they are obligate biotrophs. The life cycle of PM is shown below. This diagram is for grape powdery mildew, though the life cycle is the same for PM on most plants

Powdery Mildew of Grape | Ohioline. (n.d.). Retrieved February 15, 2020, from https://ohioline.osu.edu/factsheet/plpath-fru-37

Cleistothecia are structures produced in the late summer that are known as ‘overwintering structures’, meaning they help protect the sexual spores (ascospores) inside until they are released in the Spring. They are far more resilient than the conidia spores that are asexually reproduced in the disease’s main reproduction cycle that provides secondary infections during the Spring, Summer, and Fall. Ascospores are sexually produced spores, meaning that they undergo sexual recombination, whereas conidial spores have the same genotype as the isolates that formed them. Conidia begin to be produced quickly, usually within a week of initial infection, leading to rapid disease spread and exponential growth of the pathogen.

PM fungi, despite what some Cannabis websites claim, are not systemic. For a pathogen to be systemic, is has to be able to spread via the vasculature of plants. Of course, it can spread to distant parts of the plant through spread of spores or growth of mycelium over susceptible tissue. I would hypothesize that websites that claim that asymptomatic tissues are testing positive despite showing symptoms are simply identifying spores or initial infections that are not yet visible to the naked eye.

PM fungi only grows on the surface of plants, obtaining nutrients from living epidermal cells from specialized structures known as haustoria that penetrate the cell walls and invaginate the cell membranes. PM fungi use the haustoria to obtain nutrients from living cells, but also to modulate the host plant’s defense responses. All plants have an immune system, and fungi that feed on living hosts use proteins known as effectors in order to inhibit host defense responses.

Control Strategies

Climate

As mentioned, it is important to control temperature and humidity. It is recommended to keep humidity low to control PM (20-40%RH). If PM is not such a big problem, keeping RH so low may not be recommended. I tend to try to keep RH in my grow tents around 40% RH. In addition to humidity in your grow area, it is important to have good airflow and ventilation. Every plant makes a microclimate around leaves [10]. The trichomes, hydrophobic leaf cuticle, and leaf transpiration create a small layer of air around leaves that is relatively still and higher humidity than surrounding air. Since this is the climate that PM spores and mycelium actually grows and reproduces in, it is important to disturb this microclimate with fans that gently disturb the leaves as well as to have high levels of air exchange in your grow area.

It is difficult to use temperature as a control method, as temperatures between 50 and 90 Farenheit (10-32 Celsius) can be conducive to PM growth and spread. Unfortunately, the high and low temperatures that may inhibit PM are also very stressful to Cannabis plants. I would recommend maintaining your normal temperatures (70-85 Farenheit or so), and focusing on humidity, circulation, and fresh air exchange in terms of climate control.

Genetic Resistance

One thing to consider is that the longer time a plant takes to achieve maturity, the more time PM has to develop and spread. Choosing plants with short flowering periods such as indica-heavy hybrids or short total growth periods such as autoflowering plants may be a good choice in reducing the risk of harvesting buds infected with PM.

PM disease resistance can indeed be bred for. Unless a major resistance gene is identified, it is likely that all resistance is multigene, quantitative resistance. Unfortunately there is not public research available as to which strains have high PM resistance. Research that has been done is likely by private companies and breeders screening commercial strains for PM resistance and keeping information in-house for a commercial advantage. Unfortunately, the strain selection here must be based on knowledge spread around the growing community on forums and growing websites such as this. It is important to note that these are anecdotes and have not been verified with any experiments.

Besides indica heavy strains being a better choice due to flowering time, it also appears that many strains with Afghani heritage are more resistant to PM than most strains. In general, landrace sativa strains from equitorial regions have higher mold (bud rot and fusarium) resistance than Afghanis. However, these strains also appear to be more susceptible to PM than many Afghani-dominant strains.

Here is a list of strains that I see popping up a lot when people discuss PM-resistant strains:

Bubba Kush L.A. Confidential GDP Purple Punch Northern Lights White Widow Super Skunk White Russian Grape Ape Purple Kush

The one thing in common with all of these strains? Afghani Heritage. In fact, some of the most resistant strains also tend to have the most Afghani genetics. If you are selecting strains for PM resistance, a good rule of thumb is to maximize the Afghani landrance genetics by choose strains that are not just polyhybrids, but have had recent crosses with Afghani strains. Pairing proper selection of genetics with proper environmental controls will likely prevent you from having to deal with PM outbreaks. However, if you are still dealing with problems, you will have to move on to chemical control methods.

Cultural

A few tips can help prevent a wide range of pest issues. If you are so able, isolate your grow area and use HEPA filters in your grow room, preferrably in your air intake. This will prevent spores from accessing your plants. UV lamps can also be installed in your ducting that are effective at killing airborne spores. All tools that you use for trimming, defoliating, or any plant manipulation should be soaked in a 70% alcohol solution before every use. After each cut, it should also be sprayed with the same solution and wiped dry. If possible, keep a box of nitrile or latex gloves close by and always wear them when entering the grow area. It would benefit you to not wear your outside clothes in your grow room. Have a pair of dedicated clothes for your grow area that you wash regularly.

Finally, sterilize your grow area after each harvest. Using 10% bleach can be one good way, but I would recommend cleaning your grow room once with a bleach solution, wipe it dry, and then do a second cleaning with a ‘Quat’ soap. If you follow the cultural, environmental, and strain selection guidelines I have put forth, you will likely not need any targeted fungicides (which are not approved for use in Cannabis at least at a commercial level, due to bureaucratic and legal reasons). However, I do recommend putting together a pest control spray program for your plants as well, for use in vegetative growth and even early flowering.

Chemical

Unfortunately for commercial growers, there are very few registered fungicides that can be used on Cannabis. On the federal level, no pesticides have been approved for drug Cannabis. However, states have approved certain fungicides that the EPA has approved for hemp, which became federally legal in 2018.

If Cannabis is being sold on the legal markets and tests positive for an unapproved pesticide, it cannot be sold. Most of the fungicides on the market for Cannabis are not nearly as effective as fungicides used, for instance, in grape production for control of PM. The approved fungicides are generally untargeted, broad spectrum, and diversity of FRAC groups are not represented. Below is the list of approved fungicides in California:

• Bacillus amyloliquefaciens strain D747

• Cloves and clove oil

• Corn oil

• Cottonseed oil

• Gliocladium virens

• Neem oil

• Peppermint and peppermint oil

• Potassium bicarbonate

• Potassium silicate

• Rosemary and rosemary oil

• Sodium bicarbonate

• Reynoutria sachalinensis extract

• Trichoderma harzianum

As you can tell, there are various fungi and bacteria that can be sprayed as biocontrol agents, plant extracts and oils, and some basic salts that can be sprayed. .

Compare what is available to be used on Cannabis with what is approved to be used for PM on another smokeable crop, tobacco, and you will see how handicapped the Cannabis industry is in terms of pest control options: Mancozeb, Terramaster, Azoxystrobin, Copper based fungicides, Actigard, Agri-Mycin, Manzate, Orondis, Aliette. It is likely that similar pesticides will eventually be approved for use in Cannabis on a federal level, though not until it is federally legal and goes through rigorous studies for pesticide safety. The EPA will not approve any pesticides for Cannabis if it remains a scheduled drug.

What do I use to address a PM outbreak and help prevent PM infection?

First, I want to address one common home remedy: milk. A lot of people swear by using diluted milk as a control method. In my experience, this is the least effective method out there. If you want to use something that basifies your leaf surface, I recommend using a baking soda (sodium bicarbonate) solution.

Bicarbonates

Many homegrowers swear by using baking soda or other bicarbonates (such as potassium bicarbonate which may be more effective than Sodium Bicarbonate). I believe that when used as a preventative, it is far more effective than to stop an outbreak. In my experience, once an outbreak starts, it may behoove you to go with more aggressive measures. Bicarbonate ions work by increasing the pH of the leaf surface which inhibits fungal growth/spore germination. It is best to include a spreader/sticker to your baking soda solution such as vegetable oil and dish soap without antibiotics.

Recipe:

3 tbsp baking soda

1 tbsp vegetable oil

a few drops of dish soap without antibiotics to emulsify the mixture.

*If you can get a nonionic surfactant such as Yucca extract or CocoWet, I would recommend that over dish soap which may have some amount of phytotoxicity. I would also recommend potassium bicarbonate over baking soda*

Spray liberally on your leaves, coating the tops and bottoms. This can be used on buds through harvest as well, though I would recommend rinsing your buds with water before harvesting.

Apply once per week as a preventative.

Neem Oil Products:

I would recommend to only use these during vegetative growth, pull back on use when buds begin to visibly form. Some have reported allergies to this product, and it may pose other health risks if ingested [11, 12].

During vegetative growth, neem oil should be sprayed as a preventative every 7-14 days depending on how aggressive you are trying to be in your disease control.

Generally it is recommended to use 2 tbsp of 70% neem oil concentrate for each gallon of water. If you experience negative reactions in your plants, try diluting it further to about 1 tbsp/gal. I like to add just a couple of drops of dishwashing soap to help emulsify the oil.

Use a one-hand pressure sprayer to fully coat the tops and bottoms of all of the leaves of your plant.

Spray your plants at night, just after the sun sets so that there is plenty of time for the leaves to dry If it is not dry by the time the sum comes out, your leaves can get sunburned quite easily, make sure you have plenty of fans moving and drying your leaves

About 3 days after your neem oil application, rinse your leaves with water or a solution of citric acid pesticide such as Nuke Em by Flying Skull or other comparable products. I like to rinse just to prevent buildup of oils, but this is not necessarily required, in fact having the oil on the leaves can help deter insects. You could also just do a rinse right before your next application.

If you are experiencing an outbreak, begin using it more frequently, up to once every 5 days.

Purified azadirachtin products, while useful for insects, are not as useful for PM. The main ingredients in neem oil that are effective against PM are the triglycerides and terpenes that are not present in products like Azamax.

Stop using it when you see buds beginning to form (not right when flowering starts, but you don’t want neem residue on your buds)

Citric Acid

Some have reported that citric acid sprays are not as effective at controlling powdery mildew outbreaks as compared to other fungicides, but I believe that they can be effective when used in an IPM program with other fungicides. It is important to use what we have available since there are few targeted pesticides available for use in Cannabis.

Some studies have shown that citric acid can significantly reduce the incidence of PM [13] (though there are no studies on Cannabis specifically)

Citric acid sprays (especially solutions such as Nuke Em that also have insecticidal soaps and yeast) have the added bonus of helping to control insects including aphids, whiteflies, and arthropods such as mites.

Finally, citric acid may be able to increase yield of your plants, including dry flower weight [14, 15].

You can continue to use citric acid all through flower, and some people even spray it on their buds at harvest to help prevent postharvest bud rot without any noticeable change in flavor or bud quality.

I would not use this in an IPM program along with baking soda solutions, because both affect the pH of your leaf surface in different directions.

Plant Extract Oils

There are some approved plant extracts described in my list of approved pesticides. I have never tried these for powdery mildew, but it may be worth trying. There are some products that have premixed a variety of different oils.

For instance, Trifecta crop control has the following ingredients:

14.0%……….Thyme Oil

10.0%……….Clove Oil

9.0%…………Garlic Oil

4.0%…………Peppermint Oil

3.0%…………Corn Oil

3.0%…………Geraniol

2.0%…………Citric Acid

2.0%…………Rosemary Oil

However, this product, as well as neem oil, has the potential for causing some foliar stress symptoms until the plant becomes acclimated. It would be interesting to make a mix of Trifecta as well as Neem oil and replacing the pure neem spray with this mixture.

Again, I am not sure as to the efficacy of mixing this with neem. I have seen many forums of people claiming that this is a helpful product for controlling insects, though I have not seen much information for how it works on PM.

Reynoutria sachalinensis Extract

Reynoutria sachalinensis is a plant from which extracts are made. Extracts are classified as ‘plant activators’, meaning that the compounds stimulate the SAR (systemic acquired resistance) and ISR (induced systemic resistance) responses of plants. In short, this means that resistance genes in the plant are induced prior to any pathogen recognition, essentially making the plant more resistant to attack. I definitely recommend using this, and I believe it can be worked into a good IPM program. This, much like citric acid sprays, can be used up to the day of harvest.

B. subtilis Spray

Finally, I recommend adding a biofungicide (applying a spray with living microorganisms). The most common biofungicides utlize Bacillus subtilis bacteria or Trichoderma harzianum fungi. In general, B. subtilis is used as a foliar spray, and T. harzianum is used as a soil soak, mainly to control soil pathogens. However, there is evidence that using T. harzianum to the soil can actually increase the efficacy of B. subtilis foliar sprays [16,17], though I recommend sticking to B. subtilis sprays unless you are growing outdoors and concerned about soil pathogens. Such products include Serenade and Cease.

Keep in mind that B. subtilis in not labeled for Cannabis use in CA, if you want to use a Bacillus spray that is on-label, use B. amyloliquefaciens such as Revitalize or Triathlon.

Hydrogen Peroxide

Hydrogen Peroxide can be good for when an outbreak is actively occurring. If I could recommend one product for sanitation purposes, it would be Zerotol. Not only does it have hydrogen peroxide, but it also has peroxyacetic acid. Peroxyacetic acid forms when acetic acid (vinegar) reacts with hydrogen peroxide. Both of these are strong oxidizing agents that will kill the fungus on contact without harming your plants. As these products degrade, they will form water, carbon dioxide, and water. Peroxyacetic acid can be corrosive and dangerous in high concentrations, so be sure to dilute it according to producer recommendations. In the midst of an outbreak, this can be used every 3-5 days to help control the pathogen outbreak. This is a particularly effective tool for when an outbreak occurs in flower.

One preventative IPM program centered around PM might look like this:

Day 1: Neem Oil or Azadirachtin Spray/Plant Oil Extract Spray (or mixture)

Day 4: Citric Acid/Insecticidal Soap spray such as Nuke Em (to rinse neem oil) or sodium bicarbonate mixture outlined previously.

Day 7: Regalia Spray

Day 10: Restart Cycle.

Another rotation might look like:

Day 1: Neem Oil or Azadirachtin Spray/Plant Oil Extract Spray (or mixture)

Day 4: Citric Acid/Insecticidal Soap spray such as Nuke Em (to rinse neem oil) or sodium bicarbonate mixture outlined previously.

Day 7: Serenade Spray

Day 10: Restart Cycle.

You can also make tank mixes of Serenade and Regalia, or replace the neem oil with one or the other, especially after stopping using oil sprays in flower.

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