How To Control Powdery Mildew On Your Marijuana Plants


powdery mildew marijuana

Powdery mildew is caused by a variety of fungi resulting in the appearance of a white or gray powdery growth on leaves.  Those species that attack hops also attack marijuana plants.  Powdery mildew can appear on plants indoors or out. Before the powder forms, small bumps may be noted on the topside of affected leaves.

Mildew spores are carried by the wind, air ventilation systems, clothing, pets and just about any living thing that moves about.  Mildew spores will remain dormant until environmental factors offer the optimum conditions for them to come to life.  Humidity levels over 55% and warm temperatures, along with over crowding are conducive to mildew growth. Once awakened, powdery mildew spores are likely to attack young plants first.  It will then spread over the entire plant, infecting stems and buds, in addition to leaves.


Powdery mildew hinders photosynthesis, crippling the marijuana harvest.  If left untreated, black specks will appear in the ‘powder’.  Infected buds will smell damp when infected with powdery mildew.  This fungus cannot be removed from the buds.  Powdery mildew will turn the cannabis plants yellow, then brown and will eventually die.

Once again, prevention is the key to maintaining a healthy cannabis crop.  Give the plants plenty of room to breathe; planting too closely together gives rise to activating mildew spores.  Water your marijuana at a time of day where it will receive at least five hours of light, whether natural or artificial.  Installing a UVC light in the ventilation system of indoor grow rooms will help to keep errant spores from attacking your plants.

In keeping with the purpose of this section, here are some measures you can take to eliminate the threat of powdery mildew having a devastating effect on your efforts as a cannabis gardener:

  • Make a solution of 2 teaspoons organic apple cider vinegar per quart of water.  Spray on the plants to eliminate and/or prevent mildew growth.
  • Add ½ teaspoon baking soda per quart of water.  Spray.
  • Oil sprays consisting of neem oil, sesame oil or fish oil work on powdery mildew as it does on many fungal diseases.
  • Milk sprays have been very effective in eliminating powdery mildew.  Combine 40% milk with 60% water.  The milk’s protein acts with the sun to create a natural antiseptic, killing mildew as the result.  Milk sprays should be applied in bright light every ten days as a preventative measure.  This method can be employed to resist mildew on any garden plant.

In the event powdery mildew hits you unawares, remove any infected leaves with a plastic bag, tie it tightly closed and discard into a receptacle with a tight fitting lid.  Be careful not to jolt the spores into the air during this process, thus the recommendation to use a plastic bag during the removal process.  Follow with a fungicide treatment on the stems from which the infected leaves have been removed.  Any of the above mentioned preventative treatments can be applied.  In addition you may want to try any one of the following to help heal wounded areas of the marijuana plants:

  • Cinnamon oil, garlic oil, coriander oil, clove oil, jojoba oil, cottonseed oil sprays.
  • Applications containing copper, available at any garden center.
  • Hydrogen peroxide added to water (1 tablespoon per gallon) oxidizes the fungus cell walls, killing it.
  • Limonene (oils extracted from citrus rinds) mixed with water at the rate of one teaspoon per pint offers fungicidal properties and will aid in control, but not eradication.
  • Sulfur burners as a means of control.  Stay out of the area when using this treatment as it can cause respiratory inflammation in humans.  Follow directions and wipe down all walls and windows when completed to remove any residue.

We have so far, covered fungal and bacterial enemies of the cannabis plant that attack the visible parts.  However, disease can also attack the root systems, which is what we will now address.  Strap on your boots and come on down below for a deeper look into what can be affecting a failing crop. For a great anti-mold and disease spray, check out the products at this link here.

All living organisms need a strong core to maintain optimum health.  Just as humans need to ensure healthy innards for ourselves and our pets, we need to do the same for our plants, Mary Jane included.  Maladies arising from the inside (or below ground) are harder to detect because the affected areas are not readily visible.  As a result, we are not always aware of a problem until it’s too late.  Or is it?  What are the most prevalent root diseases and how can we recognize, treat and or repair?  Let’s travel together and see what we can discover.

Want to know more about growing marijuana? Get a free guide about growing marijuana at this link here. Order some high quality marijuana seeds at this link here.



About Author

Robert Bergman is a master marijuana grower. Robert Bergman is the author of 'Marijuana Plant Care' and 'The Marijuana Grow Bible'.

  • hambone

    has anyone used or have input on using Hydrogen peroxide added to water (1 tablespoon per gallon) oxidizes the fungus cell walls, killing it.

    • Elle

      I use peroxide to control pm it works great (lights off) but the second your humidty levels drop below 50% or gets higher then 75% it’s shows right back up . Also if your ph level is to high then your screwed bc the girls don’t get the right amount of nutes and they become week and become very very succeptible to diseases and can’t fight off the pm bc obviously it’s everywhere and always going to be part of the plants life no matter what . Unless u start from a seed in a new environment new hydro equipment ditch the old shit and understand that YOU have to control your humidity levels and watch your ph levels so your girls can get the right nutrients to keep them healthy to fight off pest and diseases commen sense think about u if your underfed your body doesn’t have the strength to fight off diseases .. Then u thrown in climate control if it’s to cold or to hot or humidity is to high or to low with not enough air flow not enough lights or proper lighting and your ph is wrong your screwed bottom line the in between its sucks I’ve got it and I know I watch my plants and when they are not on point that shit just comes out of know where literally just yesterday my humidity dropped to 46% all the old growth is covered in thick powder right now but the once I got it up to at least 55% it’s started to disappear without peroxide spray . But unfortunately I’ve got to get a good pm spray bc it’s gone to far and I am a beginner grower and learned by my mistakes and documented every move I make with my girls . My mistakes have taught me it’s always going to be there and it’s all about controlling the elements in there environment . Also make sure your outside plants , bushes roses whatever has it as well even if u don’t see it yet it’s there.. my advise spray the outside plants too ! Remember it travels in the air and it also gets in your clothes and u bringing it right back inside infecting your plants “again” there environment ” again “and all that cleaning and spraying just went to shit honestly waisted time lol .. I grow from seed indoor and also make my own clones and I will buy a clone and make cuts off them with no history about the moms or how they were grown and the only thing I had to control was the pm from someone else’s grow ! Shitty but hey as long as u have control of the environment your good ! My outdoor that I started indoor..( 3 from seed and 4 from clones I made from my Buddha Tahoe OG ) only 2 months left is a bitch to control the pm but (thankfully it’s only attacks the bottom original leaves ) not any new growth or the buds have it ! :) I beleive its bc there feeding schedule and ph is on point always that keeps those girls strong enough to fight tried the peroxide on them helps for a day or two with that u need to use the SM/PM spray that works excellent and is safe but I never spray the new growth or the flowers (bud) .

  • ! Have eucalyptus trees in my backyard full of brown ladybugs that are getting in my pot plants how can I get rid of them they are eating my plants up

  • Elle

    Then go buy red lady!! Seriously lady bugs are awesome let them eat the brown ones lady bugs will devour bugs in seconds I didn’t know there was brown lady bugs :) or get some diatomaceous earth food grade only and get it from a feed store or hydro shop ! DE kills anything with a exoskeleton the bugs crawl threw it and it rips thru them like a million tiny pieces of razor blades !

  • S grand

    Are they lady bugs or baby stink bugs