How to Identify Powdery Mildew

Powdery Mildew is a combination of different fungi that affects a number of plants including cannabis and is one of the most prevalent problems growers face each day.  It can be devastating to a crop if gone unchecked, eventually consuming the surface area of a plant and preventing the process of photosynthesis and effectively killing your tree.

So in order to help prevent that, we’ve rounded up a few helpful tricks and tips to help dissuade any infestations from taking root. 


Powdery Mildew can lay dormant within a plant’s leaf membrane for weeks until environmental conditions reach optimum levels, the earliest stages of infestation appear as small bumps, typically starting on the underside of the fan leaf. After about 4-6 days, a thin layer of white powdery mold spreads across the top part of the fan leaf entirely and can run both up and down the stems if untreated, potentially spreading to other neighboring plants.


There is no question that prevention is the best route in maintaining a healthy environment for your cannabis to grow. As mentioned earlier, Powdery Mildew can lay dormant within a plant for extremely large periods of time – this environmental specificity can be leveraged by ensuring that optimal conditions for the fungus are never met.

If you are planting indoors this tactic is extremely effective.

  • Make sure your temperature is between 72-75 degrees F
  • Keep the humidity below 48% (spores begin to activate at around 55% humidity)
  • Have plenty of airflow and a good ventilation
  • Water your plants at the beginning of its light cycle to promote evaporation of residual moisture
  • Leave plenty of space between your plants – consider trimming specific parts of the plant to prevent contact and overshading
  • Sterilize vents, fans, and ducts routinely to prevent spore buildup/spread

Other good practices would be to check the condition of your plant’s leaves routinely to spot early signs of fungal infection. Spraying topical applications of Neem oil, Fish oil, Apple Cider Vinegar mix (2tsp of ACV + 1 quart of H2O), or a 3:2 ratio of H2O:Milk (yes milk) can help prevent infestation.

If you are planting outdoors, use the above topical applications, and make sure to think carefully about how to reduce these main conditions:

  • Increased humidity above 55%
  • Stagnant Air – No Ventilation – plant where there is a breeze
  • Leaf to Leaf Contact – space them out generously
  • Any form of dampness – consider shaking plants to throw off residual moisture

At the earliest sign of infestation, it would probably be best to remove and burn the plant to limit chances of the infection from spreading to the rest of the your crop. Unfortunately once a tree is infected it cannot be fully “cured”, only treated. Although treatment would allow for the plant’s healthy branches to grow correctly, treatment alone would still put the healthy parts of the plant at risk and any other neighboring plants at risk of infection – the risk of doing so may not be worth the pay-off.

Another tactic to consider is genetic testing. Clones and seeds can be tested for mildew with a PCR DNA test while the fungi is still invisible to the human eye. The most effective manner of utilizing this testing is to test a Mother plant before receiving an intake of clones, or (if you have the funds) to test each clone that is received.

Good practices can maintain healthy plants, so try some of these tactics in preventing inoculation and we wish you the best of luck!





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