Powdery Mildew fungus on marijuana plants

In recent years, the marijuana plant is becoming increasingly sensitive to the attack of the fungus called Powdery Mildew, also known as Oidium or White Mold.

It’s a parasitic fungus of the family of the erysiphales, which attacks the aerial parts of the plants.

Although this infection is relatively easy to cure, it can bring serious consequences, because it can cause damage to crops leaving them absolutely useless. In a first phase it develops on the leafs, to extend then to the petioles, stems and finally the buds, completely destroying the resin and leaving the marijuana plants totally unusable for smoking, since the spores of the fungus are solid matters that we would absorb during its inhalation.

Bud infected with Powdery Mildew

Bud infected with Powdery Mildew

This pest affects both indoor cannabis as well as outdoor or greenhouse crops; for its propagation it needs strong humidity variations, a situation found very commonly in crops under artificial lights and in greenhouses. Outdoors the plague is more seasonal and usually appears more often in spring and autumn.

Powdery Mildew is a fungus that is easily diagnosed. It manifests as a typical white powdery that can be confused with dust on leafs and buds. In a second phase the attacked leafs become yellow and end up drying.

Powdery Mildew is an external fungus and its behaviour is very similar to that of sucking insects, since its small spores are carried by the wind and when they lay down on a leaf they put down rootlets that suck and absorb nutrients from the plant.

Powdery Mildew in indoor marijuana crops

Indoors, as we mentioned before, Powdery Mildew is an increasingly common disease and in some cases it’s necessary to take preventive measures from the beginning of the crop, but what will help us more to avoid this pest is to control the climate of our growing space. The aim is to prevent that the relative humidity of our growroom neither exceeds 65% nor goes down under 40%. To do this perhaps we will need a humidifier, to avoid that – with the lights turned on – the humidity doesn’t descend too much due to the effect of the heat from the bulbs. During the night hours of our plants, we should adjust the ventilation so that plants don’t produce condensation inside the growing room. In extreme cases we can use a de-humidifier. Another detail that can help to prevent the propagation of the fungus is to leave enough space between the marijuana plants, because shady areas are formed in the lower parts of the plants when they are too close, what favours the appearance of Powdery Mildew.

If you can’t avoid the ups and downs of your hygrometer, you can use preventive phytosanitary treatments.

Powdery Mildew in outdoor or greenhouse crops

Fungal infection of Powdery Mildew

Fungal infection of Powdery Mildew

In outdoor cannabis plants, the plague of Powdery Mildew appears with a certain seasonality, being Spring and especially Autumn the seasons with more propensity to get this disease. To  prevent it, we should keep plants free of dried leafs and avoid planting them in areas with few hours of sunshine. If we grow in plant pots it’s convenient to turn the pot to make sure that all parts of the plants have a good exposure to sunlight. We also should avoid placing the plants too close to one another. It’s important that the air can circulate between them.

In greenhouses we should try to have a proper air circulation. For this, it’s recommendable to install some ventilators inside the greenhouse to force the air movement.

In autumn it’s recommended to realize preventive biological phytosanitary treatments, specially if we live in areas whit fog and mist.

Control of the Powdery Mildew

Within the biological treatments for the control of Powdery Mildew, we can find products like Propolix, Oidioprot from Ecoprotect or Ospo. They are two products of proven efficacy, but due to the high resistance of the fungus, we should make the treatment each 10/12 days until 10-15 days before harvesting our plants.

Macro of cannabis trichomes degraded by Powdery Mildew

Macro of cannabis trichomes degraded by Powdery Mildew

There are also chemical synthesis products on the market like the Anti-Powdery Mildew from Compo, which can be used if we have exhausted the possibilities of biological origin, always taking into account that we have to respect the safety term that in this case is about 30 days.

Although in many books and gardening manuals it’s recommended the use of sulfur as a biological remedy to control powdery mildew, we do not recommend to use it with cannabis plants, given that even if we make it with great precocity, the smell of sulfur stays impregnated in the marijuana plant.

June 17, 2014 | Cannabis pests and diseases

54 comments on “Powdery Mildew fungus on marijuana plants

  1. Chris Walters

    I’m trying a vinegar based spray and I still have sporadic mildew spots! Any suggestions?

    1. Dani Alchimia

      Hello Chris,

      The first option would be raising a bit the concentration of vinegar. Another option is sublimated sulphur, or spraying your plants with natural orange juice. You can also try some of the products listed in this post, like Propolix or Oidioprot. As the last option, you can try systemic fungicides like penconazole, which are highly efficient. However, we don’t recommend using chemicals since they are dangerous and harmful for both our health and environment.
      If you have powdery mildew in your area, you will see it often. Get used to the idea of fighting against it, and treat your plants from seedlings regardless if you see powdery mildew traces on them. If you have a garden or backyard you’ll have to treat all plants too, since spores can easily enter your indoor growing space from the outside. The point is having clean plants so they can flower without the need to spray their flowers during the blooming period.

      Hope it helped,

      Best vibes!

  2. Bert Shumaker

    u have to vaccinate while in vegging

    1. Dani Alchimia

      Hi Bert,

      That’s something I always say to novice growers: it doesn’t matter if you don’t see molds or insects, always use some kind of preventive during vegging/pre-flowering and you’ll make sure to harvest your plants healthy.

      Thanks for your comment,

      Best vibes!

  3. Glen

    Excellent point Dani regarding prevention. Recently my grow became infected with powdery mildew and man is it hard to get rid of. I ended up using Green Cleaner….
    Worked get for me but sadly it was too late for a few of the ladies

    1. Dani Alchimia

      Hi Glen,

      I had issues with powdery mildew for some years, although somehow it has vanished for the last 2 years. What worked best for me was penconazole, I used it twice during the pre-flowering stage and the plants were free from powdery mildew for all the bloom poeriod. Notice it is a systemic chemical.
      Still, I’ll always recommend preventive treatments with organic fungicides like Oidioprot.

      All the best!

  4. steve

    can you smoke white powder if on your marijuana plants?

    1. Dani Alchimia

      Hi Steve,

      Fungus spores can really hurt your lungs if you smoke them. What you should do if you have plants with fungus is extracting the resin (dry sift, bho, etc.), mixing the resin with ethanol (Qwet) and then filtering with a 0.2 micron syringe filter. In this way you remove all spores and have a very nive cannabis concentrate.

      Hope it helped!

  5. Brendan

    I’m just going to say one thing… for powder mildew to exist you need both high humidity and cool night time Temps… If you eliminate one of those problem solved!

    1. Dani Alchimia

      Hi Brendan,

      Thanks for your contribution!

      All the best! 😉

  6. big snapz

    Can I smoke my buds if they have a lil p.m on them

    1. Dani Alchimia

      Hi big snapz,

      Smoking buds with fungi is never recommended. If you have a considerable amount of infected buds, I would suggest an extraction using ethanol and then filter it with a 0,2 micron syringe filter. In this way, you remove any trace of spores in your resin. We’ll soon publish an article on this subject.

      Hope it helped!

  7. dale

    I come acrossed a product called CU H2O I have powdery w really bad and was about to lose my cRop this thing works really no I use it for preventative for 6 months I have been happy

  8. dale

    I come acrossed a product called CU H2O I have powdery mildew really bad and was about to lose my crop this stuff is amazing now I use it as a preventative I have been happy so far

    1. Dani Alchimia

      Hi dale,

      Glad to hear that, as you know powdery mildew can be devastating if not treated on time. Good for you!

      All the best.

  9. jack nickoleson

    Dang I didn’t know there was a up to date forum out here I’m actually shocked. It looks like you had a little trouble getting that out there Dale lol but belive it or not I’m in central MI and I have too come across that powdery mildew evil $#%t lol took my whole crop in 2013 they really didn’t have much at the time to get rid of the issue so I had to let it go any ways just the other day my buddy asked if I knew what the white powder on the plants was and I said yup u gotta find something fast he went and grabed cuh2o the next morning we took a peek and belive it or not it was gone

    1. Dani Alchimia

      Hi jack,

      Thanks for your comment, we’ll surely take a look at that product.

      Best vibes!

  10. Terry Ayers

    i am in my 6 wk of flowerying og a 9 to 1o wk plant i got some pm how can i get rid of it

    1. Dani Alchimia

      Hi terry,

      Normally, we don’t recommend spraying the plants in bloom, but if you think that the infection will cause problems you can spray your plants with some organic fungicide (not chemical, you’re too close to harvest). I know it is a hard work, but the ideal would be cleaning the infected leaves manually with a cloth soaked in water and some hydrogen peroxide, and then spray the fungicide only on the leaves. Also, try to lower a little bit the humidity in your grow room (removing the lower parts of the plants and the largest, infected fan leaves can help).

      Best of luck!

  11. JoeR

    Mixture of milk and water sprayed on plant works wonders

    1. Dani Alchimia

      Hi JoeR,

      Thanks for the tip dude, I’ll try that for sure! 😉

      All the best!

  12. Josh vandergaurd

    Hey! Dale! I tried out cuh2o after i seen you post something in here about it. Man o man, in about a week my pm problems were gone! I searched it on amazon and it popped right up, i figured i would try it cause well ive tried damn near everything else. Its natural, i can spray it on my crop litterally up untill the day of harvest. Doesnt mess with the taste or smell of my buds either. And plus it works as a micro nutrient! So i mean not only did i remove my pm issues i gave my plants some much needed nutrients in the process! 😀

  13. Nancy Peace

    We just got a few babies and have noticed some spots of mold or mildew on the leaves. We want to stay as natural as possible and would like the vinegar solution remedy.

  14. Nancy Peace

    We just got a few babies and have noticed some white spots of mold or mildew on the leaves.We want to stay as natural as possible and would appreciate the recipe for a vinegar solution.

    1. Tim

      Hi Nancy, to deal with powdery mildew in the most natural way possible, it’s usually necessary to combine various methods. I haven’t personally used vinegar, but I’ve read about people using 2-3 tablespoons of Apple Cider Vinegar per gallon of water, but be sure to get organic, unfiltered vinegar for maximum efficacy. Other tried and tested natural options are availaable, such as products like Propolix and Oidioprot. I’ve had good results using compost tea as a foliar spray or alternating spraying a weak solution of Potassium Bicarbonate with a 1.5% solution of H2O2. Some growers fight mildew by simply washing the spores off the leaves with a strong spray of water, while others use a diluted milk solution! There are plenty of different options and opinions, and the jury is still out on many of these homemade remedies, more research is needed. Good luck!

  15. stoppowderymildew

    Great Post. Thanks for sharing this post. powdery mildew is very common in plants. Can we use UV Techonlogy for treating powdery mildew?

    1. Dani Alchimia

      Hi stoppowderymildew,

      Yes, you can use UV lamps to treat several infections caused by fungi like powdery mildew or botrytis.


  16. susan gibson

    I used water and milk at a 60/40 ratio worked GREAT!!!!

    1. Dani Alchimia

      Hi susan,

      Thanks a lot for your tip! I’ve also heard orange juice works, although I’ve never tried…


  17. susan gibson

    Good Luck!! The milk is supposed to act as an antibacterial when the sun hits it so use this method in the morning.

  18. stoppowderymildew

    Thanks for sharing this information with us. Can we use UV technology for treating the powdery mildew.

    1. Dani Alchimia

      Hi stoppowderymildew,

      From what I know, UV lights can be used against all fungi that grows from the outside in, being powdery mildew one of them.


  19. bill simmons

    I did not know about PM until today, and I finished harvesting and hanging my buds two days ago. The leaves were covered with the stuff. Must I trash it all?? Thanks…

    1. Dani Alchimia

      Hi bill,

      If the leaves were covered with pm, then it’s possible that it also infected your buds. When I’ve been in your case, I’ve thrown everything away.

      Notice there are some methods to make extracts with plants infected by pm (you can dry sieve the resin, mix it with ethanol and filter the liquid mix with a 0,2 micron filter for syringes), although I’ve never seen a microbiological analysis demonstrating that this method really works.

      Hope it helped!

  20. stoppowderymildew

    Great post. Thanks for sharing this detailed information about powdery mildew. Please share use of UV technology for treating powdery mildew.

    1. Dani Alchimia

      Hi stoppowderymildew,

      Thanks for your input mate, I’ve read a lot about UV light to kill spores but never tried…luckily I don’t see PM too often in my area!!


  21. Brian

    Thanks everyone for all the helpful knowledge.. DALE and JACK..?? Was interested too in what you guys called “cuh2o”.. Didn’t see it anywhere online anywhere so was just wondering.. and everyone else that Named off a few I see and much appreciated.. happy growing

  22. Justine

    Can I wash my buds after harvesting to get rid of pm or can I make cannabis butter with it and use it?

    1. Tim Alchimia

      Hi Justine, thanks for your question, sorry for the delay in replying. We would never recommend consuming contaminated cannabis in any form, even though cooking may kill any fungus or spores, the mycotoxins produced by the mould will remain and can cause health problems, especially in anyone with increased sensitivity or a compromised immune system.

      Despite this, every year at harvest time you’ll find growers washing their freshly-cut buds in a dilute solution of Hydrogen Peroxide solution in an attempt to clean them of powdery mildew, a technique made famous by Jorge Cervantes. We can’t really recommend this approach, as it also seems unlikely to remove the mycotoxins. If in doubt, always think of your health first, it’s just not worth taking the risk of making oneself unwell.

      All the best

  23. Jon

    Is the PM always visible? ,,,in other words if I see on the sun and sugar leaves and not on the bud …viewed with a 60x loop…I do not see it in the bud center..I If I snip the contamination away (after several rinsing with “cider”) the buds appear clean ..is that realistic thinking or only hopeful?

    1. Tim Alchimia

      Hi Jon, thanks for the question. In short, if you’re not seeing it with a 60x loop, it’s not there. I’ve had plenty of plants with PM spots on the shade and sugar leaves, but nothing on the buds. I think you’ll be fine.

      May I ask how you’re using cider to rinse the plants? Cider or cider vinegar? What dilution? Are you also spraying the buds to rinse them, or just the leaves?

      In previous seasons I’ve rinsed my outdoor plants down with a mix of Aloe Vera juice and Epsom Salts (1/2 to 1 cup aloe juice & 1 tbspn Epsom Salts per gallon) in the week previous to harvest, which seems to work really well to get rid of any contamination.

      All the best for your grow and happy harvesting!

  24. Jon

    Hi Tim…Thanks for getting back so soon…we do need quick answers and truly appreciate your help.
    we are right at harvest (I feel) for several plants PM has got the sun/sugar leaves and in some cases appear in some small locations in the bud (I dried a sample and took a look at it with 60x) ..i could only see a “wisp” of “web” mainly near the edges. Am I still OK?
    I am using “Raw Unfiltered Apple Cider Vinegar with Mother” 8 tsp to a gallon. I spray everything …..I try and “flood the bud”.

    I want to be really aggressive at this late hour…I have trimmed the sun and some sugar leaves.

    Should increase the strength of cider? or should I use your Aloe/Epsom method?

  25. Jon

    Tim …OOPS! I have an additional question…I was thinking I would cut down the buds and immediately “dip” them in “cleaner and then hang to dry..is this possible?

    1. Tim Alchimia

      Hi again Jon, no problem, we’re here to help! Firstly, thanks for the info on the ACV treatment, I’d been researching but all the recipes I’d found were more diluted, like 2-3 teaspoons per gallon, have you noticed any ill effects from using a stronger solution so far? I’m definitely curious to try this myself. I think it’s almost the only organic anti-pm treatment I haven’t tested yet!

      The Aloe/Epsom method might be a good idea to try, yes, just simply for the reason of alternating treatments. Another option is Hydrogen Peroxide, I occasionally use it myself, the dilution I’ve used was 1 cup of 3% (10 volumes) in a gallon of water. This amount can be increased for more effective disinfection if you don’t see any ill effects on the plants. This might be the best idea for a lats spray, to leave the plants really clean before hanging them to dry.

      As for dipping, it’s definitely possible, I’ve seen it done, in fact Jorge Cervantes has some videos somewhere on YouTube with instructions about this. Personally I’ve never tried it because: A. I don’t have a vessel large enough to dip my branches, and; B. I’m actually quite uncomfortable with the idea of dipping all my plants in the same, potentially pathogen-laden water. I’m much happier to thoroughly spray my plants down a day or even a few hours before harvest, to give them time to dry a little before I take them into the drying room.

      I think you’re on the right track though, between throughly washing down plants and trimming away any visible PM, I imagine you’ll get good results! All the best, good luck and happy harvesting!

  26. Jon

    Tim…so I used 10 tsp Raw apple cider vinegar to one gal water…so I trimmed like a mad man ..and sprayed like crazy 2x +over.& over..I then took in a branch I broke and quick dried the buds at 160 degrees in a vacuum…The outside was laced with PM …I broke it open the same on the inside…so did I ruin the high? the only way to know was to take a hit…I took a one small toke off off of 2 different kinds I had quick dried…surprisingly good taste (I had never dried in that method before) and definitely stony !! so I usually make “dabs” the ‘vacuum with heat” is part of my dab system…so I read online that if I filter with a 45 micron filter (I use “ever-clear” booze to extract) they say it should remove the spores?? I ordered a system for filtering and I will post back my end results. with 10 plants I have a lot to lose. but now on another note, YOU have got me spooked about smoking it and my wife thinks I was nuts to test it..but I had to know ..you know?….so whats your prognosis.doc?

    1. Tim Alchimia

      Hi Jon, damn, I’m sorry to hear that the PM had got inside the buds too. Good for you for testing it though, you’re not crazy, but all the same if it’s really badly coated then I wouldn’t smoke any more of it!

      I’m not sure about filtering out the spores etc with 45 microns, I’d definitely err on the smaller side myself and use 25 microns to be safe. Having said that, I have never tried a solvent extraction with PM infected material, so I can’t really say. It ought to work though, and the alcohol is 100% sure to kill any spores too.

      I have, however, made ice water hash with material that had some PM, and I’ve spoken with a well-known US hash-maker about the subject too, and we’re both in agreement that the water separates the mould from the resin. The mould spores float on the water where they can be scooped off with a spoon, while the trichomes fall to the bottom of the bag. This particular hash-maker has tried this multiple times and sent for lab tests run on the finished hash, where no trace of PM was found. Utmost care must be taken during drying to avoid re-appearance of mould.

      I think either method ought to work fine for you. Did you try using Hydrogen Peroxide to wash the plants down before harvest? That might be a good idea as a last step before hanging them to dry, it should kill off any spores so the problem doesn’t extend while the plants are drying.

      Good luck and keep us posted!

  27. Relampago

    Would buds lightly stricken with PM present a health problem if used for making oil/butter for edibles?

    1. Tim Alchimia

      Hi Relampago, thanks for your question. As unsightly as PM is, it doesn’t normally represent a health risk in itself, so should be fine for edibles. You can filter the infused butter or oil through a 25 micron bubble bag, or even a coffee filter if you want to be extra-safe and remove any spores.

      As far as I understand it, the health risk from Powdery Mildew is very low even when it’s on cannabis that’s being smoked, but the micotoxins produced by the fungus can certainly cause problems in people with immune system problems.

      I hope that helps clear things up, all the best.

  28. brethren

    Hi there. How do you feel about green cure or just spraying a hydrogen peroxide/ h2o mix 2weeks before harvest? Started noticing some PM on fans creeping to sugars. I have been exfoliating those particular leaves and was ready to spray or even willing to meticulously wipe down the leaves/buds with the peroxide mix. Thanks for you help and input and site!!

    1. Tim Alchimia

      Hi brethren, thanks for the comment and question. Personally I have no problem using Hydrogen Peroxide right up till the day of harvest. I’m not so sure about Greencure though, as it’s Potassium Bicarbonate which may leave some residue so soon before the end of the flowering period. I’d use it up to, let’s say a week or ten days before harvest, and then use H2O2 during the rest of the time, which as well as helping with PM, will also go some way to removing any residue from Greencure.

      All the best and happy harvesting!

  29. Wap

    Peroride to water ratio and will water milk mix work for indoor grow

    1. Tim Alchimia

      Thanks Wap, those are good tips. What ratio do you use in either case? For Hydrogen Peroxide I use 1 cup of 3% strength in a gallon of water, and up to 2 cups in severe cases. I’ve never tried milk, although I’ve heard it’s important to use full-fat and that raw, unpasteurised milk is best. All the best and happy growing!

  30. islandbob

    do you know what the mix ratio would be using certified 35 food grade hydrogen Peroxide?

    1. Tim Alchimia

      Hey Bob, thanks for the question. You can add 1 teaspoon of 35% Hydrogen Peroxide per litre of water as a start. If you see it’s not working then increase the concentration a very small amount at a time, always with great caution. I hope that helps, all the best and happy growing!

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