What is Powdery Mildew?

Powdery Mildew is a fungus that infects many different plant varieties, including cannabis. As long as you treat it early, PM on cannabis isn't anything to panic over and you'll still bring your plant to harvest. Don't be lazy in treating it though; Powdery Mildew can spread rapidly if you don't treat it early.

What does Powdery Mildew look like?

You'll first notice PM as little white or gray splotches on your leaves. Like its name, PM is powdery or dusty. Note that sometimes you'll have little white specks of bird poop on your outdoor cannabis plant. The difference between bird poop and PM is that bird poop stains your leaves and can't be brushed off. Powdery Mildew will easily brush off but don't make a habit of touching it; you'll just be spreading more spores around. Hot, dry weather can lead to a breakout of Powdery Mildew although symptoms can begin at anytime.
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Example of Powdery Mildew. By Madan subedi01 (Own work),via Wikimedia Commons

How does Cannabis get infected with Powdery Mildew?

Powdery Mildew spores are always floating around in the air. Humid conditions with inadequate airflow create an environment for PM infection. Cannabis strains with lots of overlapping leaves should be pruned to avoid Powdery Mildew.

Strong, healthy marijuana plants will resist PM infection much like a healthy human will resist flu infection. If your cannabis is unhealthy due to pest infestation, lack of nutrition or other system stressors, PM is more likely to infect it. If you prune your cannabis with dirty garden tools, PM spores can infect it.

Your cannabis can be infected with Powdery Mildew yet never show symptoms. Some cloneries with bad hygiene practices might have PM floating around everywhere yet still sell you dirty little clones...you won't know they have Powdery Mildew until much later when your plant experiences a powdery mildew breakout (yet another reason to avoid clones and grow from seeds). Below is a guide to help you limit your likelihood of getting Powdery Mildew.

Guide to Prevent Powdery Mildew on Outdoor Cannabis

  • Regularly wash your hands and gardening tools to avoid transmitting Powdery Mildew spores.
  • Prune your Cannabis to avoid bad airflow and overlapping leaves.
  • Remove dead foliage around your grow area. Sweep regularly to get rid of old pollen and dust. Mold and other bad things grow on dead stuff.
  • Don't water at night. Watering at night or as the sun goes down increases the local humidity around your plant.
  • Avoid touching the PM. Touching the PM releases a shitload of spores into the air, which can infect neighboring leaves or plants.

How do I get rid of Powdery Mildew on Cannabis?

There are quite a few organic options to get rid of Powdery Mildew on marijuana plants, including store-bought organic sprays and stuff you can mix at home. The two approaches that have worked best for me are:
  • Spraying PM with a homemade mixture of milk and water
Mix 1 part milk with 9 parts water. Don't sweat it if you're a little off on the ratio, it doesn't have to be exact. What is important is that you spray this solution directly on the powdery mildew, and in direct sunlight. The sunlight interacts with the milk to kill off the Powdery Mildew spores. You may need to spray over a couple days to kill off all the PM. If your plant is late into flower I would avoid spraying too much since it'll leave a milk residue on your buds. Note that once the PM is gone, your leaf will be stained by the PM but is otherwise healthy.
  • Manual removal of PM-infected leaves
I personally don't like removing leaves unless I absolutely have to. If I have really stubborn PM that won't die from the milk and water solution, I'll cut off the affected leaves. If you're removing leaves, do it gently to avoid transmitting the spores to other parts of the plant. Don't compost the leaves, just throw them away.