# White powder or white spots on cannabis leaves: a differential

> Flour-like circular white patches that smear off and come back are powdery mildew until proven otherwise. Rule out spray or hard-water residue (on the surfaces you sprayed, dries in drip lines), trichomes (on sugar leaves and buds, glassy under a lens), silvery thrips scarring and mite stippling. Confirm with a 20-60x lens before treating.

- Canonical: https://genetixbay.com/knowledge/diagnosis/white-powder-on-leaves.html
- Category: Diagnosis · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Flower · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What is the white powder on my cannabis leaves?** White powder on cannabis leaves that forms round, flour-like patches on the upper surface, smears off with a finger and returns within 2 to 5 days is [powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md) until proven otherwise. It is caused mainly by Golovinomyces fungi and does not need free water on the leaf. Rule out spray or hard-water residue, [trichomes](https://genetixbay.com/knowledge/glossary/trichome.md), [thrips](https://genetixbay.com/knowledge/glossary/thrips.md) scarring and mite [stippling](https://genetixbay.com/knowledge/glossary/stippling.md) with a 20 to 60x lens.

**Is it powdery mildew or trichomes on my cannabis?** Powdery mildew and trichomes look very different under a 20 to 60x lens. Trichomes are glassy, mushroom-shaped resin glands with a stalk and head, found on [sugar leaves](https://genetixbay.com/knowledge/glossary/sugar-leaf.md), buds and petioles, and they sparkle. Powdery mildew shows cottony threads and chains of spores, forms fuzzy patches that spread across fan leaves and stems, and smears when rubbed. Trichomes need no treatment.

**Why are there white spots on cannabis leaves after spraying?** White spots on cannabis leaves after spraying are usually dried spray or hard-water mineral residue: an even, crusty or chalky film in a drip-line pattern, only on sprayed surfaces, that does not grow. Wipe it with a damp cloth; residue stays gone, while powdery mildew returns. Use softer water for [foliar sprays](https://genetixbay.com/knowledge/glossary/foliar-feeding.md), spray only in veg and not under strong light.

**What causes silver streaks on cannabis leaves?** Silver streaks on cannabis leaves are usually thrips feeding damage: silvery, papery patches on the upper surface dotted with tiny black specks of [frass](https://genetixbay.com/knowledge/glossary/frass.md), according to extension guidance. Confirm by shaking a branch over white paper and looking for slender insects, and check [sticky cards](https://genetixbay.com/knowledge/glossary/sticky-trap.md). [Spider mites](https://genetixbay.com/knowledge/glossary/two-spotted-spider-mite.md) cause tiny pale dots and fine webbing instead, with mites on the underside.

**How do I confirm powdery mildew on cannabis?** Confirm powdery mildew on cannabis with a wipe test and a lens. Rub a patch: mildew smears, leaves a faint mark and regrows within days, while residue stays gone. Under 20 to 60x magnification, mildew shows cottony threads and spore chains. Mark an affected leaf and recheck in 3 days for spread, and avoid wiping patches, which spreads spores.

**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](https://genetixbay.com/knowledge/glossary/salt-build-up.md) | [Runoff EC](https://genetixbay.com/knowledge/glossary/runoff-ec.md) high |

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Lens magnification | 20–60× | Generic |
| [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md), veg | ~55–70% | Grower practice; PM favoured by humid canopies with dry leaf surfaces and moderate temperatures ( |
| 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

1. **Wipe test.** Rub a patch with a fingertip. PM smears, leaves a faint mark, and regrows within days. Residue stays gone.
2. **Lens.** Threads and spore chains (PM) vs glassy glands (trichomes) vs crystals (residue).
3. **Location.** Only where you sprayed? On sugar leaves only? Leaf undersides?
4. **Insects.** Shake a branch over white paper and check sticky cards.
5. **Spread.** Mark a leaf and look again in 3 days.
6. **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](https://genetixbay.com/knowledge/diseases/powdery-mildew.md) 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](https://genetixbay.com/knowledge/pests/thrips.md), [spider mites](https://genetixbay.com/knowledge/pests/spider-mites.md), [whiteflies](https://genetixbay.com/knowledge/pests/whiteflies.md).
- **Salt crust:** check [runoff](https://genetixbay.com/knowledge/glossary/runoff.md) [EC](https://genetixbay.com/knowledge/glossary/ec.md); if it is far above input, do a [root-zone flush](https://genetixbay.com/knowledge/glossary/root-zone-flush.md) of the medium (see [the root-zone flush](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md)) and see [brown or burnt tips](https://genetixbay.com/knowledge/diagnosis/brown-burnt-tips.md) 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](https://genetixbay.com/knowledge/glossary/nutrient-burn.md) in a room with PM history.

## Evidence

- PM pathogens on cannabis and its importance: *Established* [1][2].
- Genetic resistance: *Established* for specific loci [3].
- Look-alike distinctions: *Established* extension guidance [4][5][6].
- Cannabis-specific RH/temperature thresholds for PM: *Grower practice*.

## Sources

1. Pacific Northwest Plant Disease Management Handbook. Hemp (Cannabis sativa)-Powdery Mildew. https://pnwhandbooks.org/plantdisease/host-disease/hemp-cannabis-sativa-powdery-mildew
2. Punja ZK, et al., 2019. Pathogens and Molds Affecting Production and Quality of Cannabis sativa L. Frontiers in Plant Science 10:1120. https://doi.org/10.3389/fpls.2019.01120
3. Mihalyov PD, Garfinkel AR., 2021. Discovery and Genetic Mapping of PM1, a Powdery Mildew Resistance Gene in Cannabis sativa L. Frontiers in Agronomy 3:720215. https://doi.org/10.3389/fagro.2021.720215
4. UC IPM Pest Notes: Thrips (Pest Note 7429). https://ipm.ucanr.edu/PMG/PESTNOTES/pn7429.html
5. Utah State University Extension. Two-Spotted Spider Mite (hemp). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-spider-mites.php
6. UMass Extension. Whiteflies on Greenhouse Crops. https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/fact-sheets/whiteflies-on-greenhouse-crops