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The Scourge of Powdery Mildew:

How Do You Treat It Once It’s Occurred?

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Quest is the industry leader in energy-efficient dehumidification solutions in the agriculture industry. If you’ve got questions, we’re here to help!

Can you treat powdery mildew (also known as White Powdery Mold or just “WPM” to cannabis growers) successfully once it has occurred? Sometimes. If powdery mildew has just appeared, you can sometimes kill it with natural substances. The best option is to remove visibly affected plants altogether, and then to stop further infection in its tracks with a natural treatments.

natural treatment options

copper sulfate

Copper sulfate both kills and controls powdery mildew and other bacterial and fungal infections. However, you should only apply copper sulfate before the flowering stage, and dilute it for use on young plants so that it doesn’t burn them. In addition, apply only when temperatures are below 85 degrees Fahrenheit so that leaves don’t yellow and drop off. Follow label instructions for use.

barley straw rafts and pellets

Barley straw rafts and pellets can often inhibit the growth of powdery mildew and other organisms. While it won’t kill existing spores, it will prevent the growth of new.

hydrogen peroxide

That familiar brown bottle in your grocery store, 3% strength hydrogen peroxide, is a good natural treatment for powdery mildew. You can use a mix of hydrogen peroxide and water to control disease. Apply directly to the soil and mist on leaves for best control. You can also add hydrogen peroxide to your hydroponic solution.


Milk has natural “good” bacteria that can very effectively prevent powdery mildew. Use one part milk to nine parts water. It can significantly reduce current powdery mildew infections, and will prevent new ones from taking hold.

Prevention is the best “treatment”

Unfortunately, once powdery mildew has contaminated your plants, there’s always some risk that trace amounts remain in your harvest even after treatment. Therefore, the best treatment for powdery mildew is prevention. This happens most effectively through proper control of relative humidity and other environmental factors.


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