Powdery Mildew on Cannabis: Signs and Fixes

You have spotted circular patches of white powder on your leaves, like a dusting of flour. That is powdery mildew. Airflow and humidity control matter more than any spray, so start there.

Cannie showing early powdery mildew — Early

Cannie showing advanced powdery mildew — Advanced

“Someone dusted me with flour! More breeze and less humidity, please.”

Where it shows

Upper leaf surfaces, as distinct circular patches.

What it looks like

Circular patches of fine white powder, like scattered flour, spreading outward until the leaf is coated. Leaves yellow underneath.

Canna GLAP illustration: White powdery patches — White powdery patches

How to confirm it

Look at upper leaf surfaces.
Circular patches of fine white powder.
Patches spread outward until the leaf is coated.
Leaves yellow underneath the powder.
Rub gently. It wipes off like dust.

Why it happens

High humidity with poor airflow, and large day-night temperature swings. It does not need wet leaves, unlike most fungal problems.

High humidity with poor airflow.
Large swings between day and night temperatures.
A dense, crowded canopy.
Spores brought in on clones or clothing. Unlike most fungi, it does not need wet leaves.

How to fix it

Cannie's quick fix, in order:

Airflow first. Stagnant air is the precondition; a fan does more than any spray.
Drop humidity below 50% and reduce the temperature swing between lights-on and lights-off.
Never smoke or cure affected flower. Remove it.

Fix airflow first. A fan does more than any spray.
Drop humidity below 50 percent.
Reduce the temperature swing between lights on and lights off.
Remove and bag heavily affected leaves.
Never smoke or cure affected flower. Remove it.

Organic options

Potassium bicarbonate

Commonly OMRI listed. Spray in veg following the label. Never spray buds in flower, and wear gloves and eye protection.

Bacillus subtilis or similar biofungicides

Use in veg as a preventive, following the label. Do not spray buds in mid to late flower.

Sulfur

A classic preventive in veg only. Never combine with oils, and never apply within the label's waiting time of an oil spray, since this burns leaves.

Airflow and humidity control

The most important tool. Fans, dehumidifiers and thinning the canopy prevent spores from taking hold.

Read and follow every product label. Organic friendly does not mean certified organic; check the listing if that matters to you.

Veg or flower?

In veg, sprays plus environment changes work well. In flower, never spray buds; control humidity and airflow, and remove affected material.

Prevent it next time

Keep air moving above and below the canopy.
Hold humidity in range and avoid big night swings.
Thin dense growth for better airflow.
Quarantine new clones.

Questions growers ask

Is powdery mildew dangerous on buds?

Do not smoke or cure affected flower. Remove it and focus on preventing spread.

Can I spray for powdery mildew in flower?

Avoid spraying buds in mid to late flower due to residue and mold risk. Focus on airflow, humidity, and removing affected leaves.

What humidity prevents powdery mildew?

Keeping humidity below about 50 percent, with good airflow, helps a lot. Stable temperatures matter too.

Where these figures come from

These points match published grow books, page by page. They are grower practice, not research: none of these books cites a study, so where science and the books differ, this page follows the science.

High humidity with poor airflow. Danko (2018), e-book p. 133. Paull and Zetta (2020), e-book p. 71 to 73.
Airflow first; a fan does more than any spray. Danko (2018), e-book p. 133. Green (2001), p. 424 to 425.
Drop humidity below 50%. Danko (2018), e-book p. 133. Paull and Zetta (2020), e-book p. 71 to 73.

Books: Danko, Cannabis: A Beginner's Guide to Growing Marijuana (Hampton Roads, 2018); Green, The Cannabis Grow Bible (4th ed., 2001); Paull and Zetta, Foolproof Guide to Growing Cannabis (2020) and The Cannabis Grow Bible (2020).