Short answer. Powdery mildew (PM) is a fungus that lives on the leaf surface and shows up as white, flour-like patches that wipe off and come back. In indoor and greenhouse cannabis it is mostly Golovinomyces ambrosiae [1][2]. It loves still air, moderate temperatures and humid microclimates inside dense canopies. Prevent it with airflow, steady humidity, clean starting material and resistant genetics. Treat it early in veg with contact options. Once it is in the flowers, the honest answer is: you can slow it, not clean it.
What's happening
PM fungi are obligate biotrophs: they feed on living cells through tiny feeding structures (haustoria) pushed into the leaf's outer layer, while the white mycelium and chains of spores (conidia) sit on top. Spores are airborne and land on new tissue, germinate without needing a film of water, and produce a new crop of spores in roughly a week under favourable conditions (5–10 days is typical for related PM fungi on other crops)being reviewed. That short cycle is why "a few spots" becomes a room-wide problem in two or three weeks.
Older literature and some growers' guides call the cannabis pathogen Golovinomyces cichoracearum or G. spadiceus; recent work and the PNW Handbook use G. ambrosiae [1][2]. The PNW Handbook also lists Podosphaera macularis (the hop PM fungus, bright white colonies) on hemp [2]. Practically, management is the same.
Is PM "systemic"? Contested. The fungus is mostly external, but growers report PM reappearing on clones taken from infected mothers even after the leaves looked clean. Latent infection of buds and stems, and persistent spores on the plant, are plausible explanations; a systemic, inside-the-plant phase has not been demonstratedbeing reviewed. Treat clones from infected mothers as infected.
How to recognise it
- First signs: small, round, white-to-grey powdery spots on the upper leaf surface, often on lower or middle fan leaves in the shadiest, least-ventilated part of the canopy. Sometimes on the underside first.
- Progression: spots merge into a felt-like coating; leaves yellow under the colonies, then brown and drop. Stems and petioles get coated. In flower, sugar leaves and bracts turn dusty white.
- The wipe test: rub with a damp finger. PM smears off and comes back in a few days in the same place. Trichomes do not wipe off; mineral residue from sprays does not regrow.
- Look-alikes: spray residue or hard-water spots (no regrowth, often on top surfaces only in droplet shapes), trichome-heavy sugar leaves (glandular heads under a 30–60x loupe), spider mite webbing (moving dots, stippling), downy mildew (rare on cannabis; grey-purple growth on the underside, yellow angular patches on top). See white powder on leaves.
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| Day air temperature | 24–28 °C (75–82 °F) veg; 22–26 °C (72–79 °F) flower | PM develops readily in moderate temperatures; temperature alone does not prevent itbeing reviewed |
| RH, veg | 55–70 % | Avoid wide swings; PM spores germinate in high humidity without free water |
| RH, mid to late flower | 40–50 % | Also limits botrytis |
| VPD | 0.8–1.2 kPa veg; 1.0–1.5 kPa flower | Very low VPD (< 0.6 kPa) and still air favour leaf-surface fungi (Grower practice) |
| Air movement | Visible leaf flutter throughout canopy, including lower third | Dead air pockets are where PM starts (Grower practice) |
| Day–night temperature drop | ≤ 5 °C (≤ 9 °F) | Large drops spike RH at lights-off |
| Silicon (root-applied) | Reduced PM severity in soilless trials | PNW Handbook reports reduced severity in upper and mid canopy [2]; rates are crop/medium-specific, follow product guidance |
| UV-C (night) | 70–200 J/m² in other crops | Suppressed PM in grape, strawberry and cucumber [6]; no published cannabis dose —being reviewed |
What to check first
- Confirm it's PM. Wipe test plus a loupe. Mycelium and spore chains look like tangled cotton threads and beaded chains; trichomes are stalks with round heads.
- Map where it is. Lower canopy and corners = airflow and microclimate problem. Everywhere at once = high spore load, probably came in on clones or mothers.
- Check the climate log for RH at lights-off and the hour after. A night-time spike to 70–80 % RH is common and is when spores germinate.
- Check canopy density. Leaves touching leaves, no light reaching the lower third, no air movement under the canopy.
- Trace the source. New clones in the last 2–4 weeks? Mother plants with a history? Outdoor air intake with no filtration?
- Rule out the root zone and nutrition for the plant's overall health, but don't expect them to explain PM. Stressed, soft, over-fed (high-nitrogen) plants are reported to be more susceptible (Grower practice).
How to fix it
Cultural and environmental (always, first)
- Airflow: oscillating fans above and below the canopy; no dead corners.
- Defoliate and thin inner and lower growth that never sees light. Bag infected leaves at the plant and remove them; don't shake them over the canopy.
- Stabilise RH, especially around lights-off: dehumidify on a schedule, ramp temperature down slowly.
- Space plants; don't overlap crops. The PNW Handbook notes epidemics build when young plants sit next to older infected plants [2].
- Remove infected mothers or retire them. Clones from an infected mother start infected.
Genetics
- Established: resistance exists. PM1, a single dominant gene, gave complete resistance to a Pacific Northwest isolate [3]; PM2 is a second locus that suppresses infection and sporulation [4]. Loss-of-function in susceptibility genes (CsMLO1, CsMLO4 candidates) is another route breeders are exploring [5]. Ask breeders for tested resistance, not "mould resistant" marketing.
Biological and least-toxic options (veg, early transition)
Registration and allowed uses for cannabis differ by country, state and province. Check local regulations and the product label before applying anything; many labels do not allow use on cannabis, and licensed producers have strict lists. - Bicarbonates (potassium bicarbonate): contact action; raises leaf-surface pH and dries colonies. Needs full coverage, repeated every 7–10 days [2]. - Horticultural and plant-derived oils: smother colonies; full coverage essential. Do not apply below 10 °C (50 °F), above 32 °C (90 °F), on stressed or wet plants, or close to sulfur applications (phytotoxic combination)being reviewed [2]. - Sulfur: protectant; works best before infection. Not for use under hot lights or close to oils. Never in flower (residue and smell). - Biofungicides: Bacillus spp. (e.g. B. subtilis, B. amyloliquefaciens strains) and plant-extract resistance inducers are listed for hemp PM in some jurisdictions [2]; efficacy varies and some trials showed no effect [2]. Supported in other crops, limited cannabis data. - Resistance inducers / silicon: root-applied silicon reduced severity in soilless hemp [2]. Supported. - UV-C at night: Supported in other crops (grape, strawberry, cucumber) [6]; no peer-reviewed cannabis dose found. Treat as experimental and test on a few plants for leaf injury first.
What not to do in flower
- No sulfur, no oils, no bicarbonate sprays on buds after the first 2–3 weeks of flower (grower consensus; label pre-harvest intervals vary). They leave residue, change flavour, and residue on flower can fail testing where testing is required.
- No home remedies on flowers (milk, vinegar, hydrogen peroxide). Wet buds plus PM is an invitation to botrytis.
- Don't spray to "clean up" a PM'd crop before harvest. Remove the worst plants or branches and accept the loss. PM-coated flower is not something to smoke or extract; mould spores and residues end up in the product.
- Don't raise humidity with foliar sprays in late flower. Ever.
How to confirm it worked
- No new colonies on new growth for 14 days, checked every 2–3 days with a loupe on the lower canopy.
- Old spots stay dry and grey rather than fresh white and expanding.
- RH logs show no night-time spikes above your targets.
- Next cycle: a sticky-tape lift from lower leaves under a 60x loupe shows no spore chains.
Common mistakes
- Treating only visible spots. Spores are already on neighbouring leaves; treat the whole plant and neighbours.
- Relying on one spray. Contact options need repeat coverage.
- Keeping an infected mother "because she's the keeper". She will re-seed every cycle.
- Huge night-time temperature drops in a sealed room.
- Mixing or rotating oil and sulfur too closely: burned leaves.
- Buying "PM resistant" seeds without any test data.
Evidence
- Established: G. ambrosiae is the main cannabis PM pathogen in North American indoor and greenhouse production [1][2]; dense canopies, stable humid microclimates and overlapping crop cycles promote epidemics [2].
- Established: single-gene resistance (PM1) and a second locus (PM2) exist in cannabis [3][4]; MLO genes are susceptibility candidates [5].
- Supported: root-applied silicon reduces severity [2]; bicarbonates and oils give contact suppression [2]. Cannabis efficacy data for many products are lacking [3].
- Supported in other crops, unknown in cannabis: night-time UV-C [6].
- Contested: whether PM persists "systemically" inside cannabis plants.
- Myth: "Milk spray cures PM." Some suppression is reported on cucurbits; it is not a cure, and has no place on flowers.
