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There’s nothing more troublesome than moldy plants. But, what causes powdery mildew and how do you get rid of it? This segment of our Cannabis Diseases series goes over everything you need to know about powdery mildew. 

What is powdery mildew?

Cannabis Diseases 1 1 Powdery Mildew Can Destroy Your Plants & Make You Sick
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Powdery mildew is a fungal infection. Various kinds of fungi can create powdery mildew. When conditions are just right, a white or gray mold begins to show up on your plant’s leaves. It looks like sections of your plants have been covered in tiny white snowflakes or, as the name suggests, a kind of powder.

Indoor and outdoor plants are both susceptible to powdery mildew, and it’s one of the most common diseases that growers face.

Attentive growers can catch this mildew early on by examining fan leaves. If you notice that the tops of your leaves begin to show small bumps, it’s a sign that a mildew outbreak might be just around the corner. If left untreated, plant leaves will turn yellow, wilt, and die.

Consuming plants infected by powdery mildew is a health hazard. Not only with buds taste rank and hit extremely harsh, you’re opening yourself up to infection and illness. Medical marijuana growers want to be extremely cautious about powdery mildew. If you have a compromised immune system, this stuff can really do some damage to your health.

When are your plants at risk for powdery mildew?

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Unfortunately, wind and ventilation systems can spread these mildew spores around. This is terrible news for growers. Brushing up against a plant with powdery mildew and then touching another plant can also spread the infection. This kind of mildew is so easy to pass along that prevention is the best way to combat the disease. Once it gets started, it can be incredibly difficult to stop.

Some risk factors for powdery mildew include:

  • Overwatering, getting leaves of the plant too wet
  • Improper ventilation, which creates high humidity
  • Humidity over 55%
  • Rain
  • Young plants, which aren’t yet well-established
  • Plants that are overcrowded, trapping moisture

How to treat powdery mildew

1. Remove infected plants

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If your plants are indoors or grown in pots, it’s very easy to move them. If one of your plants has been infected, separating it from your other plants is a great idea. Having a quarantine area picked out in advance in case of a mildew emergency will help speed up your reaction time in the event of a mildew attack.

Take your plant somewhere that won’t be exposed to too much wind or jostling. This can spread the spores around.  If mold has spread to more than one leaf or branch, you can prune those branches back and wait to see if others continue to mold.

2. Prune immediately

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If you notice some small bumps on the top of a leaf, or if you see any fuzz, you need to take action right away. Powdery mildew can spread like rapid fire. From one leaf to the next, from one plant to another. Cut off the leaf or branch that has been infected as soon as possible. Do it carefully so you avoid spreading the spores around.

3. Switch off the fans

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If you’re indoors, temporarily turn off your fans while you prune or remove the infected plant from the grow room. You want to avoid creating wind that will spread spores to other plants. To avoid risking your crop, you want to be as cautious as possible. A moldy crop can be quite costly.

4. DIY spray solutions

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After pruning, if you feel that your plant is still salvageable you can treat your plants with a DIY spray. Opt for two teaspoons of apple cider vinegar per one quart of water. Put it in a sterilized sprayer bottle and mist the leaves. This is a super easy way to prevent powdery mildew as well. You can use it pre-emptively if you feel like your plants might be at risk.

Baking soda and water make for another great spray. Try one teaspoon of baking soda per quart of water. Many people also find success with a 40% milk to 60% water mixture. You can also do a gentle wash like that shown in the Jorge Cervantes video above.

Powdery mildew prevention

1. Make a game plan before you grow

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People invest a lot of time and money into cannabis crops. Since so many resources are involved in cultivation, it’s devastating when something goes awry. Plan for the worst prior to starting your grow. If one patch of crop becomes infected with some kind of pathogen, do you have a few backup plants at a different location? Drafting up a game plan in the event of an emergency will help you respond to unexpected hiccups a lot faster. This can save you from a ton of stress, financial burden, and perhaps save your entire crop.

Drafting up a game plan in the event of an emergency will help you respond to unexpected hiccups a lot faster. This can save you from a ton of stress, financial burden, and perhaps save your entire crop.

2. Ventilation and moisture control

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Especially for indoor grows, ventilation is key. If your grow room is too humid and doesn’t have enough air flow, you’re practically begging for mildew. Fungi flourish in damp, humid environments.

Think of a tropical rainforest. Fungi dominate the forest floor, encouraged by the intense heat and moisture in the environment. You don’t want to re-create this habitat for your cannabis plants. Similarly, if they’re out in the rain without long stretches of hot sun to dry out the buds, fungus will take over.

If outdoors, make a game plan for rain and bad weather. Do you have a simple greenhouse shelter that will protect them from excess water? Pay attention to the weather forecast. Invest in a Farmer’s Almanac to give you a sense of what the growing season will be like.

3. Give your plants space

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A lot of growers try to cram as many plants into a space as possible. This is not good. To keep your plants healthy, disease-free, and thriving, you want to leave at least ten inches (25cm) to a foot and a half (46cm) of space between plants. The more space the better.

More space also allows your plant to grow a little larger. Some people go as little as six inches apart, but that’s pushing it a bit in terms of optimal disease control.

4. Foliar sprays

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Bacteria and fungus are not bad for plants. In fact, a truly healthy plant thrives thanks to helpful colonies of bacteria and fungus on leaves and inside the soil. When a plant develops a fungal or bacterial disease, harmful microbes outnumber the good. When a plant has access to enough healthy soil and leaf microbiology, it is better able to defend itself from pathogens like powdery mildew.

Treating the leaves of your plants with a foliar spray made from a healthy, high-quality compost or worm casting tea coats the leaves of your plants with a kind of external immune system. This spray acts as kind of a defensive wall that ensures that harmful bacteria and fungi are unable to take over your plant.

If you do use a foliar spray, there are some vital rules to remember. Always give your plants time to dry out completely, and perhaps turn up the fans after application. You don’t want humidity to rise after spraying the leaves. You want to treat them and make sure they dry off quickly. Otherwise, you risk encouraging mildew.

Also, spraying with the lights out is thought to be best. Just make sure you have plenty of air flow. Never spray on the cannabis buds themselves. Lightly coat the leaves only.

Some other things to consider

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  • Always thoroughly wash and clean any tools you use to deal with an infected plant
  • To help you better monitor your plants, you can invest in an inexpensive magnifier with a light
  • Cannabis testing from a local testing lab will also tell you whether or not your bud contains any unseen mold or mildew

Powdery mildew is a true pain for growers. Always take preventative measures throughout the growing process, even if your grow is just a small one. Planning ahead will save time, stress, and perhaps even your entire crop.

Do you have any powdery mildew tips? Share them with us on social media or in the comments below. We’d love to hear from you!

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Delilah Butterfield

Delilah Butterfield is a Pacific Northwest native with a passion for cannabis and natural health. Contact her on Twitter @delilahbfield.
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