Powdery Mildew on Cannabis: Causes, Symptoms, and Cure

Powdery mildew, sometimes called white powdery mildew, is one of the most common and difficult diseases that cannabis growers face. 

Whether you grow from feminised seeds, regular seeds or autoflower seeds, powdery mildew can completely ruin a crop if it’s not dealt with quickly. This fast-spreading fungal infection can devastate crops, reduce potency, and compromise bud quality. 

If powdery mildew is left unchecked it can turn a thriving grow into a worthless nightmare. In this Dutch Passion guide, you’ll learn the causes of powdery mildew, how to spot symptoms early and proven cures/prevention methods to ensure your cannabis seeds grow into healthy and potent plants.

What is Powdery Mildew?

Powdery mildew is a fungal infection primarily caused by the Golovinomyces species (and related) fungi. Like many fungus types, powdery mildew thrives in warm, humid environments and spreads via airborne spores. These spores can settle on nearby cannabis plant foliage, tools, clothing etc. Powdery mildew fungal spores can survive in dirty, uncleaned grow rooms from one grow to the next. Cannabis is particularly susceptible due to the dense foliage; thick blooms and the humid conditions often found in grow spaces.

Why Powdery Mildew Is Dangerous

• It reduces photosynthesis: Powdery mildew coats leaves, blocking sunlight and weakening plants - this all leads to lower yields.
  
  
• Compromises bud quality: Stress from infection reduces cannabinoid and terpene production, resulting in less potent buds.
  
  
• Health risks: Infected buds can contain fungal spores, posing a risk of lung irritation if smoked. This is why many growers simply dispose of plants known to be infected with powdery mildew.
  
  
• Persistent spores: Fungal spores linger in grow spaces and old grow containers, surfaces etc. Without a thorough clean down there is a risk of re-infection during future grow cycles.

Causes of Powdery Mildew on Cannabis

Several environmental factors contribute to powdery mildew outbreaks:

• High humidity: Relative humidity (RH) above 55–60% creates a fertile breeding ground for fungal spores.
  
  
• Poor airflow: Stagnant air in grow tents or rooms allows spores to settle and multiply. Use of circulation fans above and below the canopy is a good way to prevent build-up of stagnant air
  
  
• Overcrowded canopy: Dense foliage with poor light penetration traps moisture and promotes fungal growth.
  
  
• Temperature swings: Large differences between day and night temperatures can encourage spore development.
  
  
• Contamination: Infected clones, un-sanitised tools or nearby plants can introduce spores to your grow. Use of 3% Hydrogen Peroxide solution is perfect for wiping down surfaces, floors, tools & containers in your grow room. This will sterilise without leaving a residue.

Expert Tip: Even indoor grows with HEPA filtration (recommended BTW) can develop powdery mildew if airflow and humidity are not carefully balanced.

Symptoms: How to Identify Powdery Mildew Early

Catching powdery mildew early is critical for effective treatment. Look for these signs:

Stage 1: Small white or grey spots on leaves, resembling flour or dust.

Stage 2: These patches then spread across fan leaves and sugar leaves, forming a powdery coating.

Stage 3: Infected leaves turn yellow, wilt, and die as the fungus spreads.

Stage 4 (Advanced): Buds become infected, rendering them unsafe for consumption.

Visual Cues

• Looks like dust marks or chalky fingerprints on the leaf surfaces.
  
  
• Early spots wipe off easily but return quickly if untreated.

How to Cure Powdery Mildew on Cannabis - Immediate Response (If Caught Early)

1. Remove Infected Leaves: Act swiftly - carefully cut and dispose of affected leaves in a sealed bag to prevent spore spread. Remove this from the grow room.
   
   
2. Increase Airflow: Add oscillating fans and prune dense areas to improve circulation.
   
   
3. Lower Humidity: Adjust to 45–50% RH using a dehumidifier if necessary.

Natural/Organic Treatments

Neem Oil Spray: Effective as a preventive but avoid using it in late flower to protect bud flavour.

Potassium Bicarbonate Spray: Kills spores on contact; mix 1 tsp per litre of water and apply as a foliar spray.

Milk Spray: A 1:9 milk-to-water spray can alter leaf pH to combat spores. Apply weekly as a preventive.

Biological ’friendly’ Fungicides: Products like Bacillus subtilis are safe and effective for organic grows. 

Expert Tip: Avoid spraying anything onto your buds in the late cannabis flowering stage—focus on prevention and canopy management to avoid contaminating your buds.

Preventing Powdery Mildew in Future Grows

Prevention is the best defence against powdery mildew. Follow these steps:

Control humidity: Maintain RH at 45–55% during the flowering stage.

Improve grow-room airflow: Use exhaust fans and oscillating fans to keep air moving. Consider investing in a higher capacity extraction fan to really improve conditions.

Prune regularly: Defoliate dense areas to improve light penetration and reduce moisture buildup. Remember to remove the old foliage promptly.

Sanitise grow space & step-up cleaning intensity: Clean tools, tents, and clothing between grow cycles to eliminate lingering spores. Liberal but careful use of dilute (3%) Hydrogen Peroxide can disinfect grow tent surfaces, plant containers etc.

Quarantine New Plants: Isolate clones or seeds for 1–2 weeks to ensure they’re free of infections. Even a trusted clone supplier can occasionally get unlucky with diseases such as powdery mildew.

Never leave old foliage in your grow room. Get rid of it quickly. If you regularly defoliate you need to remove leaves from the grow room. Allowing old leaves to collect in a bag or on the grow room floor is poor growroom hygiene that increases your risks of disease/pests.

Consider UVA & UVB supplemental lights. As well as helping to juice your buds up with terpenes and cannabinoids, UVA and in particular UVB can help reduce viability of airborne fungal spores. UVA & UVB are also useful due to their harmful effects on pests such as fungus gnats and similar.

HEPA filters are able to remove airborne particles down to the size of individual fungal spores. Many serious growers insist on HEPA filtration in their grow room to ensure the air is as pure as possible. Free from dirt, dust, spores etc.

The most likely areas for powdery mildew include:

Lower and inner fan leaves: These leaves, especially those closer to the base of the plant or tucked inside a dense canopy, are prone to powdery mildew because they receive less light and are surrounded by still air, creating a moist microclimate ideal for fungal spores. This is especially the case in grow rooms with poor extraction and without oscillating fans.

Dense canopy areas: Regions where leaves or branches are tightly packed, particularly in the middle of the plant, trap humidity and restrict airflow. This can make them prime areas for early infection. Many growers use oscillating fans above and below the grow canopy. Some direct fans vertically upwards, through the canopy.

Young, new growth: Tender, new leaves and shoots at the top of the plant are susceptible because the softer tissues can be easier for fungal spores to colonise.

Undersides of leaves: The underside of leaves often collects moisture and can be less exposed to air circulation. This makes it a common starting zone for the tell-tale white, powdery spots.

Sugar Leaves Near Buds: In the flowering stage, sugar leaves (small leaves within buds) can be affected early if humidity is high and airflow is poor around dense colas.

These parts of the plant are most vulnerable due to the higher humidity levels locally, reduced light exposure and limited air movement. All this creates ideal conditions for the fungal spores to germinate and spread.

Pro Tip: Regularly inspect lower and inner leaves during early vegetative and flowering stages. These can be among the first to display the characteristic white or gray-ish powdery spots. Pruning dense canopy areas and maintaining good airflow can help reduce the risk. Don’t be afraid to defoliate heavily, well-bred genetics can take it. Check out the following article to see how several rounds of extreme defoliation were used to cultivate some Dutch Passion autoflower seed varieties. 

Common Mistakes to Avoid

Once you have experienced the misery of a powdery mildew infection you will be especially motivated to avoid a repetition. Here are the most common mistakes you should avoid.

• Ignoring early signs: A few spots can explode into a full-blown infection within days. Hanging out with your plants is an important way to ensure all is well. Look out for any early signs. Check regularly.
  
  
• Harvesting infected buds: Smoking powdery mildew-infected buds is unsafe and can cause health issues. Bin them.
  
  
• Overusing/use of chemical fungicides: These can harm plants and leave potentially harmful residues, especially in late flower. For many growers these are simply never considered.
  
  
• Neglecting Cleanup: Failing to sanitise grow spaces after an infection risks reinfection. Good grow room hygiene is a sign of an accomplished grower.

Final Thoughts and Bonus Tips

Powdery mildew can be a worrying problem, but with vigilance and proper grow room management you can protect your cannabis crop. Regular & thorough inspection of your plants will prevent any infection getting too established. 

Maintain plant grow room conditions to the best standards you can. If you do see a dusty, powdery area on the leaves act swiftly. 
Some professional growers safeguard their plants against powdery mildew and other diseases by using UV-C lights. These are too dangerous to use near plants or people, so instead they are usually used within the air intake ducting to kill all spores, pests or other airborne nasties before they can even enter the grow room. 

It’s an expensive but highly effective option used by many commercial growers. By combining prevention, early detection & safe treatments you can keep your cannabis crop healthy and powdery mildew at bay.

Bonus Tip: Invest in a hygrometer to monitor humidity and a HEPA filter for your grow space to trap spores before they settle. Hepa filters are especially popular with professional growers, they remove almost all small particles around the size of mildew spores. 

Enjoy your grow and remember powdery mildew can be beaten and completely avoided in future.