Powdery mildew is a common fungal disease that thrives in moderate temperatures and humid conditions. In other words, it’s the stuff of nightmares for cannabis growers. Powdery mildew can take out your entire harvest if left untreated. Keep reading to learn how to:

  • identify the signs of powdery mildew contamination
  • prevent powdery mildew from contaminating your weed
  • and manage powdery mildew post-harvest 

We’ll help you catch powdery mildew on weed (before it catches you). 

What Does Powdery Mildew on Weed Look Like?

Powdery mildew looks like a coat of white or light beige dust. Powdery mildew tends to impact cannabis leaves first, but it can move to the buds and can even attack the stems. If you see discoloration on your cannabis leaves or smell sweat, ammonia, or rot, scan your cannabis plants carefully — your cannabis garden may be hiding a severe infestation of powdery mildew.  

The scope of the mildew coat increases as the disease progresses. In its early stages, the mildew will appear as just a few spots on the plant’s leaf. The spots will become larger and more prevalent as the mildew grows (until your plant’s foliage is entirely covered in the dust-like fungus). Powdery mildew may not appear very scary when it first emerges, but it’s an aggressive and extremely dangerous threat to your cannabis garden.

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3 Places to Check for Powdery Mildew on Your Cannabis Plants 

The best way to save your plants from powdery mildew is early detection. Regularly scout your weed in the following areas to prevent an infestation: 

1. Interior Leaves

The shadiest leaves (the ones closest to the plant’s main stem) are the most vulnerable to mildew. The shade increases humidity and protects immature mildew spores from disruptive air flow, allowing them to develop and spread. The interior leaves are also the most hidden from view, so you’ll need to be especially intentional about monitoring them for mildew.  

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2. Both Sides of the Leaf

Powdery mildew tends to favor one side of the leaf rather than appearing on both sides. For example, the top of the leaf may appear perfectly healthy while its underside shows signs of mildew contamination.

The backside of a green cannabis leaf Source: Shutterstock
It takes time, but checking both sides of your plant’s leaves is essential for the early detection of powdery mildew. 

3. Buds Post Harvest

Powdery mildew consumes the entire cannabis plant at its most advanced state, including the buds. By the time you see flour-like specks on your buds, you probably already know that your plant has a powdery mildew problem. All the same, look for pale discoloration on your flower to make sure you don’t accidentally ingest powdery mildew (or give your friends contaminated weed). 

Is Powdery Mildew Dangerous on Cannabis? 

Touching and smoking powdery mildew probably won’t kill you, but we highly discourage the inhalation of this common allergen. If you’re allergic to mold, beware of the following symptoms as a result of exposure to powdery mildew (a type of mold): 

  • Itching 
  • Congestion
  • Sneezing 
  • Runny nose 
  • Dry skin 
  • Sinus pain
  • Pulmonary inflammation 
  • Wheezing 
  • Nausea
  • Vomiting 

Powdery mildew is especially dangerous for immuno-compromised medical marijuana patients, such as those taking chemotherapy or people managing AIDS, diabetes, asthma, eczema, rheumatoid arthritis, and lupus.   Even if you don’t have a medical condition, buds covered in powdery mildew smell and taste “off.” If you grow your own weed (or spend money on top shelf buds), make sure they’re clear of mold and mildew so you get the experience you’re after. 

3 Reasons There’s Mildew on Your Bud 

Mildew growth is preventable if you avoid creating mold-happy conditions. If you spot powdery mildew on your weed, chances are you’ve got one of the following issues in your grow space

1. The air is too still. 

Powdery mildew is an airborne fungal pathogen, so movement, contact, and even ventilation can help it spread. However, the spores develop best in environments with little airflow. That’s because the lack of ventilation protects their spores from agitation, allowing them to grow. Continuous ventilation (with a fan, for example) can stop mildew from maturing and spreading.  

2. There’s too much humidity.

The ideal temperature for powdery mildew growth is between 75 and 85 degrees Fahrenheit, which is also the ideal temperature for many cannabis strains. The trend applies for humidity levels as well. Mildew loves humidity above 55 percent.

A hand holding a humidity sensor checking a cannabis plant Source: Shutterstock
Cannabis plants love RH levels between 60 and 80 percent. Keeping your grow space well ventilated and the RH levels between 50 and 60 percent will protect your cannabis plants from powdery mildew.  

3. Your plant’s leaves are touching.

Contact between leaves accelerates powdery mildew’s spread because it reduces airflow and makes the journey from one leaf to another much easier for the spores to make. Reduce the risk of powdery mildew by thinning out your plant’s foliage and making sure that there’s adequate space between each of your plants. 

How to Handle Mildewy Weed

The best way to handle mildewy weed is to prevent it. The following methods are easy hacks for preventing the growth of powdery mildew on weed:

  • Thin out plant foliage
  • Use a fan to improve air circulation
  • Burn sulphur in your grow room 
  • Keep your humidity as low as your cannabis plants can tolerate 

Even the most vigilant and careful cannabis growers still find themselves with infected plants, so don’t beat yourself up if you spot a case in your cannabis garden. The good news is that early detection can save your crop. Continuously scan your plants for mildew and take immediate action to stop the spread. If you see powdery mildew, prune the diseased leaves immediately. Clean your shears afterward so that you don’t inadvertently spread more spores. Reduce the temperature in your grow space to 45 to 55 degrees Fahrenheit to slow the mildew’s growth until you get the contamination under control.  You can also use the following treatments to kill existing mildew:

  • Hydrogen dioxide
  • Milk and water solution (¼ milk and ¾ water) 
  • Potassium bicarbonate
  • Baking soda solution (1 tbsp baking soda, 1 tsp castile soap, and 1 gallon water) 

If you lose your harvest to powdery mildew, take it as a hard-earned lesson for your next batch. Then find the dispensary with the highest reviews near you to get fresh buds in the meantime.