Powdery mildew is one of two pervasive pathogens that harm marijuana plants. The other is gray mold. Powdery mildew spores float around in the air, and also live on leaves, other surfaces, and in soil. If you’re unlucky, it finds your cannabis leaves and infects them. The infection looks like clumped, sticky sand poured onto your marijuana leaves.

If your cannabis grow room conditions become favorable for powdery mildew, and especially if you’re growing a marijuana strain prone to powdery mildew, it can ruin your bloom phase plants, although it doesn’t directly attack buds like gray mold does. Bloom phase is the prime time when powdery mildew comes on, although you sometimes see it on hydroponics clones or otherwise during veg phase.

Powdery mildew spores are carried by air currents and germinate on leaf surfaces. Liquid water on leaves inhibits spore germination. The fungus grows on the leaf surface but sends fine threads into the cells to obtain nutrients. It only takes a couple of days from the time a spore germinates to the time new spores form. High humidity favors spore formation while low humidity favors spore dispersal.

Powdery mildew likes the following conditions:

  • Temperatures between 70-80°F.
  • High relative humidity at night.
  • Low relative humidity during day.
  • Marijuana plants fed too much nitrogen.
  • Poor fan aeration and air movement.
  • Poor grow room air exchange.
  • Dense cannabis plants. Lack of light and air movement penetration.
  • Damaged leaves.
  • Marijuana strains susceptible to powdery mildew.

Preventing infection is possible, but totally stopping it once it’s started isn’t. Powdery mildew is said to “live inside and outside your marijuana plant” but it cannot be stopped using a systemic feed, and only partial control after infection is possible.

You prevent or control powdery mildew infection using the following methods:

  • Don’t allow marijuana grow room conditions that favor powdery mildew.
  • Microfilter your grow room air intake and seal the grow room so no powdery mildew spores get in.
  • Choose cannabis strains not prone to powdery mildew. Strains with a high percentage of the terpenoid limonene (such as Lemon Skunk) tend to be powdery mildew resistant.
  • If powdery mildew hits in grow phase, or in early bloom phase before buds have thickened considerably you can use a sulfur burner that burns pellet sulfur to coat the plants and the room and kills the spores.
  • If you choose to use a sulfur burner, pay attention to the following instructions:
    • Use a timer on the burner.
    • Turn all aeration and exhaust fans off.
    • To block onset of powdery mildew, use burner four hours each time twice a week. If your plants have powdery mildew, use the burner four hours per day as much as 3-4 days per week.
    • The burner should be hung 2-3 feet above plants. Anything near the burner is likely to be turned yellow.
    • Don’t go in the room while the burner is on and allow at least one hour after burner session is done before you enter room.
    • Don’t use oily or greasy foliar sprays such as Neem if you intend to use a sulfur burner.
    • Clean your HID grow lights bulbs or LED grow lights after each sulfur burn.
  • Spray with Rhino Skin potassium silicate foliar spray at 5.7 pH and 160 ppm. Do foliar spraying at times of low humidity; powdery mildew doesn’t like wet leaves and water on leaves combined with foliar spray ingredients as described in this article can kill the spores or at least slow down their dispersal and growth.
  • Spray ½ teaspoon Sodium Bicarbonate (baking soda) per quart of water on leaves 1-2 times per week when powdery mildew is present
  • Spray 1 teaspoon of Limonene per pint of water 1-2 times per week.
  • Spray 2 teaspoons apple cider vinegar per quart of water 1-2 times per week.
  • Spray 1 tablespoon of Hydrogen peroxide per gallon of water.
  • Use Neem in a foliar spray.
  • Use epsom salts in a foliar spray at the rate of 1/2 teaspoon of epsom salts per gallon of reverse osmosis water at 5.7 pH once every three days when powdery mildew is active. Sulfur is a plant nutrient and bud ripener, but I wouldn’t use it as a foliar spray past the third week of bloom phase because it stays on and in the buds and changes the taste and affects vaping and combusting the buds.
  • Reduce amount of nitrogen you’re feeding your marijuana plants.
  • Increase the light intensity your cannabis plants receive, particularly UV-B wavelengths.
  • Spray Physan 20 on your marijuana leaves, but no later than 2-3 weeks into bloom phase.
  • Decrease watering.
  • Decrease ambient humidity to 54% night and day. Maintain grow room temperature at 75°F day and 68°F night.
  • Sterilize fans, vents, ducts, exhaust fans using bleach or some other microbial toxin.
  • Increase fan and venting aeration and air movement.
  • Increase plant spacing.
  • Increase grow room air exchange rate and volume.
  • Monitor your leaves every day of your marijuana plants’ lives and immediately cut all infected marijuana leaves/stalks/stems and dispose far away from your marijuana grow room.
  • Disinfect all clothes, tools, and other materials that could have spores on them.
  • After you harvest the infected marijuana crop, remove all plants and totally disinfect room, ducts, fans, tanks, reservoirs using Physan 20, and after that, bleach.

Note that you only want to use one of the recommended foliar spray ingredients per week and no more than 2-3 days of foliar spraying except in worst case scenarios.

The old saying is “an ounce of prevention is worth a pound of buds.” Powdery mildew spores are in the air in most places during spring, summer and early autumn. Growing outdoors, you have no hope of stopping them from getting onto your plants.

Growing indoors, you can prevent them from ever reaching your plants by keeping your grow room sealed against outside pathogens.

Powdery mildew doesn’t destroy cannabis buds the way botrytis (gray mold) does. It mostly attacks leaves. If you battle it properly, you can get to harvest time and salvage some of your buds. Check out the YouTube videos embedded in this article, one of them shows grow guru Jorge Cervantes washing powdery mildew off of harvested buds!

If the buds have been sprayed and there’s residue on them, or if the mildew is too pervasive, don’t combust or vape those buds. Make a water-wash concentrate such as bubble hash from them.

My hope is you never see that sandy stuff on your leaves!