# 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 scarring and mite stippling 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](https://genetixbay.com/knowledge/glossary/petiole.md), 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, 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, 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 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](https://genetixbay.com/knowledge/glossary/whitefly.md) 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 [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