Powdery mildew (PM) is a surface fungus that appears as white, powdery spots on cannabis leaves. Unlike bud rot (Botrytis), which attacks from inside the densest buds, PM grows on leaf surfaces and is visible early. The catch: PM spreads extremely fast, produces microscopic spores that travel by air, and is systemic — once a plant is infected, the fungus is inside the plant tissue even where you can't see it.
Identification
What PM looks like
- White, powdery patches on the top surface of fan leaves — like someone dusted the leaf with flour or baking powder
- Starts as small circular spots (2–5mm) and expands outward
- Usually appears on lower and interior leaves first (less airflow)
- Does NOT wipe off easily — unlike dust, PM adheres to the leaf surface. If you wipe it and it smears, it's PM. If it brushes off clean, it's dust or perlite particles
- Advanced PM yellows the leaf underneath and causes leaf curl
PM vs. trichomes
New growers sometimes confuse PM with trichomes on sugar leaves. The difference:
- Trichomes are on sugar leaves around buds, are stalked (mushroom-shaped under magnification), and are sticky to touch
- PM is on fan leaves (not usually sugar leaves initially), has no visible structure under a loupe, and feels dry/powdery
- Trichomes glitter under light. PM looks dull and matte
PM vs. other white marks
| Mark | Texture | Location | Wiping test |
|---|---|---|---|
| Powdery mildew | Powdery, matte | Leaf surface, starts lower | Smears, doesn't come off clean |
| Perlite dust | Gritty, loose | Random spots from watering | Brushes off completely |
| Calcium/mineral deposits | Hard, crusty | Near where water droplets dried | Doesn't wipe easily but isn't powdery |
| Spider mite webbing | Silky, strand-like | Underside of leaves | Pulls away in threads |
What causes PM
The conditions
PM thrives in specific conditions:
- Humidity 50–65% — PM actually prefers moderate humidity, not extreme humidity (unlike Botrytis, which needs high humidity). This is counterintuitive — you can get PM even with "normal" humidity
- Stagnant air — still air around leaf surfaces lets spores settle and germinate
- Moderate temperatures — 68–77°F is the PM sweet spot. Temperatures above 85°F inhibit PM growth (one reason outdoor summer crops often avoid it)
- Crowded canopy — dense foliage with leaf-on-leaf contact creates the still, shaded microclimate PM loves
- Weak or stressed plants — healthy plants have some natural PM resistance. Stressed plants (underwatered, underfed, root-bound) are more susceptible
How it spreads
PM spores are microscopic and airborne. They enter through:
- Open tent/room ventilation — outdoor air carries PM spores
- Contaminated clones — the #1 introduction vector. PM is systemic; a clean-looking clone from an infected mother carries PM inside its tissue
- Contaminated tools, hands, or clothing — touching an infected plant and then a clean one transfers spores
- Pets — cats and dogs can carry PM spores on their fur
Prevention
Environmental controls
- Airflow is #1 — oscillating fans moving air across the canopy surface prevent spores from settling and germinating. Stagnant air is PM's best friend
- Temperature above 80°F inhibits PM (but creates other problems). Don't raise temps just for PM — focus on airflow
- UV light exposure — some growers run a UVC light during the dark period (when the tent is empty). UV destroys PM spores on surfaces. Don't expose plants to UVC — it damages plant tissue
- Defoliate to open the canopy and improve air circulation between leaves. Remove leaves that touch each other
Preventive sprays
If you're in a PM-prone environment (past infections, humid climate, growing from clones):
- Potassium bicarbonate (MilStop, GreenCure): 1 tbsp per gallon of water, sprayed weekly. Raises the leaf surface pH, making it inhospitable for PM. Safe throughout veg and into early flower
- Neem oil: preventive spray every 7–10 days during veg. Not for use in flower (residue on buds)
- Sulfur burner: vaporized sulfur creates a thin antifungal layer on leaf surfaces. Effective but requires proper setup. Not for use within 2 weeks of harvest
Clone quarantine
- Quarantine new clones for 10–14 days in a separate space
- Inspect daily for PM spots
- Treat with a preventive potassium bicarbonate spray upon arrival
- Only introduce to the main grow after the quarantine period passes clean
Treatment
Veg treatment (aggressive, effective)
If you find PM during veg, treatment is straightforward:
- Remove affected leaves — cut off any leaf with visible PM. Bag the leaves and remove from the grow space. Don't shake them
- Spray the entire plant with potassium bicarbonate solution (1 tbsp per gallon + a few drops of castile soap as a surfactant)
- Spray every 3 days for 2 weeks — PM spores survive on surfaces and reinfect. Multiple treatments break the cycle
- Increase airflow and defoliate to open the canopy
- Clean the tent — wipe interior surfaces with dilute hydrogen peroxide (1:10 ratio with water)
Flower treatment (limited options)
In flower, your options are limited because anything you spray ends up on buds that will be consumed:
- Weeks 1–3: potassium bicarbonate spray is still acceptable (low residue, food-safe). Remove affected leaves
- Weeks 4+: avoid spraying buds directly. Remove affected fan leaves, increase airflow, and hope to outrun the infection to harvest
- Biological control: Bacillus subtilis (Serenade/Cease) is a biological fungicide considered safer for flower. Spray on leaves, not buds
- Hydrogen peroxide bud wash after harvest removes surface PM from harvested buds. This doesn't fix systemic PM but cleans the exterior (see bud washing section below)
Bud washing after harvest
If you harvested from a PM-affected plant, bud washing removes surface mold and spores:
- Fill three buckets:
- Bucket 1: lukewarm water + 1 cup baking soda + 1 cup lemon juice per 5 gallons
- Bucket 2: plain lukewarm water (rinse)
- Bucket 3: plain cool water (final rinse)
- Dunk cut branches in bucket 1, swirl gently for 30 seconds
- Transfer to bucket 2, swirl for 30 seconds
- Final rinse in bucket 3
- Hang to dry as normal
Bud washing is controversial but widely practiced. It visibly cleans the buds (you'll see the rinse water turn cloudy) and doesn't noticeably affect trichome retention or potency.
When it's too late
PM on buds
If powdery mildew has reached the buds themselves — not just fan leaves — the contamination is severe:
- Visible PM on buds cannot be fully removed. Bud washing removes surface spores but PM mycelium penetrates into the calyx tissue
- Consuming PM-contaminated buds carries health risks, especially for immunocompromised individuals (respiratory issues, allergic reactions)
- Discarding heavily infected buds is the safest choice
Common mistakes
- Ignoring the first spot — "it's just one leaf" becomes "it's on every leaf" within a week. PM spreads exponentially. Treat the first spot aggressively
- Spraying buds with neem oil in flower — neem residue on buds produces harsh, chemical-tasting smoke. Use potassium bicarbonate or biological fungicides in flower, never neem
- Not treating the environment — treating the plant without fixing the conditions (stagnant air, crowded canopy) guarantees reinfection
- Introducing clones without quarantine — this is how PM enters most indoor grows. Quarantine is not optional if you're bringing in plant material from external sources
- Assuming PM is gone because you can't see it — PM is systemic. A plant that tested positive carries PM in its tissue. Even after successful treatment, maintain preventive sprays and monitoring
For bud rot (a different fungal threat), see our Botrytis guide. For integrated pest and disease management, see our IPM guide. For environmental controls, see our VPD guide.