Short answer. Round, flour-like white patches on the upper leaf surface that smear off with a finger and come back within days are powdery mildew (PM) until you prove otherwise. Before treating, rule out the look-alikes: dried spray or hard-water residue, normal trichomes on sugar leaves, silvery thrips scarring, and spider-mite stippling. A 20–60× lens settles most cases in a minute.
What's happening
Cannabis powdery mildew is caused mainly by Golovinomyces species (reported as G. cichoracearum / G. ambrosiae) [1][2], and in the Pacific Northwest Podosphaera macularis has also been reported on hemp [1]. The fungus lives on the leaf surface and sends feeding structures into epidermal cells; its chains of spores give the powder look. Unlike most fungi, PM does not need free water on the leaf. It was the major foliar pathogen in a Canadian survey of commercial cannabis [2]. Resistance genes exist (PM1, PM2), so cultivar susceptibility varies a lot [3].
Look-alikes are not infections: mineral residue, trichomes (resin glands), feeding scars from thrips (silvery with black frass dots) [4], spider-mite stippling (pale dots, webbing) [5], and whitefly honeydew or waxy nymph cases on undersides [6].
How to recognise it
| Appearance | Where | Most likely | Confirm |
|---|---|---|---|
| Circular, fuzzy, flour-like patches, expanding and merging; smear off and return in 2–5 days | Upper surface first, older/shaded leaves, also stems and buds | Powdery mildew | Lens: cottony threads and chains of spores |
| Even, crusty or chalky film, drip-line pattern, does not grow | Only on sprayed surfaces | Spray or hard-water residue | Wipe with damp cloth; doesn't return |
| Glassy, mushroom-shaped glands, sparkle | Sugar leaves, buds, petioles | Trichomes (normal) | Lens: stalk and head |
| Silvery, papery patches with tiny black dots | Upper surface | Thrips feeding [4] | Shake onto paper; slender insects |
| Tiny pale dots, fine webbing, bronze cast | Upper surface; mites underneath | Spider mites [5] | Lens on underside |
| White waxy flecks, tiny white flying insects when shaken | Underside | Whiteflies [6] | Yellow sticky card |
| White crust on media surface | Medium, pot edges | Salt build-up | Runoff EC high |
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| Lens magnification | 20–60× | Generic |
| RH, veg | ~55–70% | Grower practice; PM favoured by humid canopies with dry leaf surfaces and moderate temperatures (being reviewed |
| RH, flower | ~40–55% | Grower practice |
| Canopy air movement | continuous, leaves visibly flutter | Grower practice |
| Day–night temperature swing | small; avoid dew-point events at lights-off | Grower practice |
What to check first
- Wipe test. Rub a patch with a fingertip. PM smears, leaves a faint mark, and regrows within days. Residue stays gone.
- Lens. Threads and spore chains (PM) vs glassy glands (trichomes) vs crystals (residue).
- Location. Only where you sprayed? On sugar leaves only? Leaf undersides?
- Insects. Shake a branch over white paper and check sticky cards.
- Spread. Mark a leaf and look again in 3 days.
- Climate log. Humid nights, stagnant corners, crowded canopy.
How to fix it
- Powdery mildew confirmed: remove and bag the worst leaves, increase airflow, lower RH, open the canopy, and follow powdery mildew for least-toxic treatment classes and what not to do in flower. Check product registration and labels in your jurisdiction.
- Residue: use softer water for foliar sprays, spray only in veg, and don't spray under strong light.
- Trichomes: nothing to fix; that's the crop.
- Thrips, spider mites, whiteflies: see thrips, spider mites, whiteflies.
- Salt crust: check runoff EC; if it is far above input, do a root-zone flush of the medium (see the root-zone flush) and see brown or burnt tips for runoff EC management.
How to confirm it worked
- PM: no new colonies on marked leaves for 10–14 days; new growth clean.
- Residue: new leaves clean once spraying stops.
- Pests: sticky-card and leaf counts fall over two weekly checks.
Common mistakes
- Spraying anything on buds in flower to "treat" what turns out to be trichomes.
- Wiping PM off leaves and spreading spores across the room.
- Treating PM only on the leaves you can see; it is usually further along than it looks.
- Raising RH to fix tip burn in a room with PM history.
