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What is Powdery Mildew and Why is it Bad for Cannabis Plants?

what is powdery mildew and why is it bad for cannabis plants

What is powdery mildew (PM) and why is PM bad for cannabis plants?

Powdery Mildew is a quite common fungal disease that affects indoor and outdoor gardeners of every plant type, including cannabis. PM can hit your plants at any stage of growth. Even seedlings and clones can get PM. In fact, clones with PM are a common occurrence because when mother plants are infected with PM, their clones are infected too. If you are bringing new clones into your grow, always screen them rigorously and do not accept any clones with PM.

Visually, you will first notice powdery mildew by the white residue spots on the leaves or branches or stems of your plant. They can be spotted as tiny slivers or large swaths. Although powdery mildew is likely to appear on the top sides of leaves, these spores can develop all over stems, flowers and the undersides of leaves as well. If you look at the white residue closely, you will notice they appear slightly raised and fuzzy.

Powdery mildew will affect both foliage and flowers and can devastate cannabis gardens quickly if not treated, but sometimes you can’t see it. Most powdery mildews have a 4-7 day post inoculation window where it remains invisible to the naked eye. Be forewarned that spores replicate and spread quickly! This is just one more reason why spending time in your garden and carefully observing your plants remains a vital factor in your success.

Some cannabis varieties are more resistant to powdery mildew than others, but all plants should be treated with PM preventives during the vegetative growth phase because getting PM in mid-to-late flower is almost an impossible situation. If you ignore PM prevention for your vegetative plants you will certainly have a more difficult time fighting PM during the flowering phase.

Part of what makes powdery mildew control so difficult with flowering plants is that fungicidal sprays have an extremely undesirable effect on the bud quality. Not only can sprays immediately burn the delicate pistils on your developing flowers, but It’s likely you will taste and smell the spray in the final product. No one will be fooled.

PM spreads through microscopic airborne spores or infected garden tools like scissors. The spores can also hitch hike on your clothing, enjoying their free ride as you work throughout your garden.

As the spores reproduce, travel and infect plants throughout your garden, these plants have an increasingly difficult time absorbing nutrients. PM leaves your plants weak with yellowing, dying leaves. Left unchecked, it will take down one or even all of your plants!

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Every caregiver or home grower at one time or another comes face to face with powdery mildew on their plants. It is easy to diagnose once you see it, but you have to remain vigilant. If you aren’t looking for it, you will miss it while it spreads and in a week it can overtake your whole garden. It only takes a few days for an infestation to dust your crop.

What happens to the plants that are infested with PM? You must throw your infected, unsafe, unhealthy cannabis in the garbage.

While it may seem extreme to throw away all your hard work, no one should consume powdery mildew-infested buds. Your respiratory system is designed to handle the inevitable spore inhalation involved in daily life, but you could consume millions of spores at once if you smoke infected plant material. Especially concerning is that many people use cannabis for healing and may already have a compromised immune system. Safe, powdery mildew-free cannabis protects your health and the health of anyone who shares your medicine.

If you can maintain a powdery mildew-free garden, it makes you a cultivation expert. You’ll need proper strategies and a good understanding of how to prevent and kill it.

The best time to begin treating your garden for powdery mildew is when you don’t have it!  Employing these prevention best practices makes it tough for the spores to take root in your garden. Cultivators rely on a multi-pronged approach to keep this pathogen at bay.  With a few adjustments to your environment and prevention treatment, your plants can remain healthy and fungus-free throughout their life cycle.

Focus on prevention with air circulation, pruning, proper humidity, foliar sprays and air purification with Airo Home Hobby Grow.

  1. Maximize Air Circulation

Use circulating fans to move the air gently around your plants. Wind prevents the spores from multiplying and landing securely on plant surfaces. Look for the leaves of your plants to be lightly dancing from the fans air circulation.

  1. Keep Plants Pruned

Look for dark, dense areas in your canopy and thin them out by pruning, so you can see light pass through them.  You might prune leaves or branches. Since powdery mildew thrives in still air and humid places, this practice is highly recommended for indoor and outdoor gardens.

  1. Regulate Humidity Targets 

Since PM thrives in a humid environment, use a dehumidifier to keep your humidity low. While plants prefer higher humidity during their vegetative growth period and flowering plants prefer a lower humidity, you still need to be aware of the powdery mildew factor. Powdery mildew takes off at a humidity of 55% and higher, so maintaining low humidity ranges (40 – 55%) puts a dent in powdery mildew’s vigor.

  1. Remove Infected Plant Parts 

Carefully cut out any infected plant parts, including leaves, flowers, stems, or entire branches  and put them immediately in a plastic bag and tie it up. Your challenge here is to avoid disturbing the spores so they don’t float up into the air and right onto the surrounding plants. Take the bag of infected material as far away from the garden as possible. You don’t want those spores following you back into your grow space!

Always clean your scissors afterwards with 70% isopropyl alcohol. If you’ve been exposed to PM, be sure to change your clothes and shower before visiting any other gardens or continuing work in your own.

  1. Use Foliar Sprays

Mix any of these solutions for a safe and effective foliar spray to drench the leaves and branches and stems before or during a PM outbreak.  Apply it once weekly or as necessary to prevent and kill powdery mildew.

HOUSEHOLD PRODUCTS TO PREVENT AND KILL PM (Available at the drugstore, grocery store, online, etc.)

  • 1:1 ratio of 3% hydrogen peroxide solution and water
  • 1:3 ratio of 70% isopropyl alcohol and water


  • Serenade (comes ready to use or as a concentrate)
  • Plant Therapy (comes ready to use or as a concentrate)
  • Trifecta (comes ready to use or as a concentrate)

In an infestation, rotate the sprays listed above every two days.  Switching up your control methods improves their efficacy. For preventative maintenance, you can spray once or twice a week depending on the environment.

  1. Air Purification

Regular spraying and pruning add time to your gardening chores and unfortunately don’t always succeed in keeping powdery mildew at bay.

Cannabis gardens aren’t the only spaces where fungal diseases can wreak havoc. Hospitals have to destroy airborne fungus, bacteria and viruses 24 hours a day to keep their patients healthy. NASA also has to ensure their space shuttle air quality remains high, which is why they developed safe, effective air sanitization technology. Airo Home Hobby Grow makes this same technology available to the home grower!

Airo Home Hobby Grow is the only PCO Air Purification system to kill powdery mildew available in the marketplace for home growers. Air Purification through PCO (Photocatalytic Oxidation) was originally created for NASA scientists and is used in indoor agriculture and medical treatment facilities.

How does it work? Harmful spores, bacteria and pathogens are eliminated when air reaches the bio-conversion reactor bed. The powdery mildew destruction process occurs on a microscopic level. Airo Home Hobby Grow literally kills powdery mildew and is an emission-free technology. It is completely safe and will not harm your plants, pets or people.

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Visit or call (844) 247-3913 to learn more or order a Airo Home Hobby Grow unit for your home, hobby, medical or personal grow.

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