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Powdery Mildew, AKA The White Scourge: What Is It, How To Identify It And How To Treat It

Last updated: 26 September 2023

Powdery mildew what is it

Seeing white spots on pot leaves breaking out in your garden? You could be up against one of cannabis’ most dangerous foes. Powdery mildew is a serious threat to your plants, and you’ll need all the quickness and guile you can muster to beat it.

What Is Powdery Mildew?

Powdery mildew is a fungal plant infection and the scourge of cannabis gardens everywhere. Powdery mildew infections start as a small dot of puffy white flakes that can quickly snowball into a dying garden. Even worse, once this fungus establishes itself in your grow op, it’s extremely difficult to eradicate.

Is Powdery Mildew Dangerous?

Powdery mildew is exceptionally dangerous and can rapidly kill an entire cannabis garden unless swift action is taken. Moreover, powdery mildew infections can lie dormant in a plant for weeks, infecting the grow op without showing any symptoms. If you’re seeing white spots on marijuana in flower, it’s possible that the outbreak may trace back as far as your vegetating plants. Even a small outbreak can spread throughout a large grow room and kill all of your plants in less than a week. To make matters worse, as soon as a bud develops powdery mildew, it’s completely unsmokeable. As you can see, growers should act fast when it comes to combatting powdery mildew. Continue reading and we’ll tell you how to do so in a minute – but first, what actually causes powdery mildew?

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Powdery Mildew – Causes

There are four main culprits that contribute to powder mold growth:

  • high humidity,
  • poor airflow,
  • lack of ventilation,
  • leaf-to-leaf contact.

First, make sure to keep the ambient humidity in your garden below 60% in veg and 45% during flower. Low airflow is another contributing factor to powdery mildew that a simple oscillating fan can address. You also need to bring fresh air into your garden frequently – if you’re growing indoors, make sure to open windows or set up a ventilation system. Finally, leaf-to-leaf contact can help powdery mildew spread, so defoliation can also help.

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Powdery Mildew – Signs and Symptoms

The main symptom of a powdery mildew infection is white spots on marijuana leaves. These spots quickly spread to encompass an entire leaf before making their way to flowers and stems. If left unchecked, powdery mildew will spread across an entire plant, dry it out, and inevitably kill it. Powdery mildew spreads during nighttime hours, when there’s no light and ambient humidity tends to spike. As a result, it tends to grow rampantly during the flower phase, when there are 6 more hours of darkness than in veg. This is when the symptoms will appear more frequently and spread faster, so keep a constant eye on the state of your cannabis foliage during this period.

How To Treat Powdery Mildew

There are two schools of thought when it comes to powdery mildew. Some growers believe that treating an infection can cure a plant. However, another camp believes that powdery mildew is a “systemic infection” and can never be truly cured, only managed. Regardless of which theory is correct, the first step toward treating a powdery mildew outbreak is to remove the infected leaves. To do this, wet a paper towel and cover the infected portion of the leaf. Then, remove it from the plant entirely.

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Home-Made Solutions

DIY solutions are safe and simple ways to stop those white spots cropping up on your cannabis leaves. Sprays like milk or oil can be easy yet effective anti-powdery mildew weapons.

DIY Powdery Mildew Spray

For minor outbreaks, you can make your own anti-powdery mildew spray. Mix 1 tablespoon (14.5 milliliters) of liquid soap, 1 tablespoon (14.5 milliliters) of baking soda, and 2 tablespoons (29 milliliters) of vegetable oil with a gallon (3.5 liters) of water. Apply it liberally to the areas surrounding the infected leaves you removed.

Oil Spray

Many growers utilize oil-based sprays to combat powdery mildew outbreaks. Mixes made from neem oil, fish oil or sesame oil are all effective antifungal agents that can help turn the tide against those white spots forming on your weed leaves.

Milk Sprays

Spraying your plants with a mixture of 40% milk and 60% water is not only an effective treatment for powdery mildew, but it’s also a powerful preventative. There are two reasons for this. First, fungus likes acidic environments, and milk is a mild base. Second, milk contains compounds that can act as an antiseptic when exposed to powerful lights. As a result, you need to spray during light hours to see the results you want.

Store-Bought Solutions

Sometimes, DIY solutions just won’t cut it. If you need to bring out the big guns, there are several over-the-counter solutions that can help fight white powdery mildew on your cannabis plants.

Bonide 811 Copper 4E Fungicide

Bonide has been a household name in gardening since the 1920s. Their organic, copper-based fungicide is a lifesaver in the battle against powdery mildew growing on cannabis. It’s specially designed to be used on fruit plants up until the day of harvest. It’s also safe to be used around people and pets. At $16 per bottle, this is a relatively inexpensive solution.

Spectracide Immunox Fungicide

This multipurpose fungicide is effective in treating a broad range of mold and mildew infections. Clocking in at $21, it’s also fairly affordable. It’s effective at combating infections for up to 2 weeks. After that, your garden may require additional applications. It’s rainproof and designed for use with fruit and vegetable gardens as well – that’s important, because you’re likely going to be smoking the fruits of your labor someday.

Prevention Is Paramount

Make no mistake – if you’re seeing white powder on cannabis leaves in your garden, the situation is dire. However, it’s not hopeless. While you can recover from powdery mildew on your cannabis plants, the best way to deal with this infection is by preventing it altogether. Assess the airflow, light and humidity in your grow, and you may be able to beat powdery mildew before it rears its ugly head. Good luck!

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Teresa Mello
Thank you for the tip about powdery mold milk and neem oil
Hello Teresa! Thank you for the comment - it's nice to hear the article is useful
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