# Powdery mildew

> A fungal disease, on cannabis caused mainly by Golovinomyces ambrosiae, that grows as white powdery patches on leaves and buds and thrives in moderate humidity.

- Canonical: https://genetixbay.com/knowledge/glossary/powdery-mildew.html
- Also written: PM, WPM, white powdery mildew, Golovinomyces ambrosiae, Golovinomyces cichoracearum, Erysiphales
- Glossary category: Pests and diseases · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Powdery mildew is a fungal disease caused by obligate parasites in the order Erysiphales that live on the plant surface. On cannabis the main species is *Golovinomyces ambrosiae*, which was previously reported as *G. cichoracearum*. It forms white, flour-like patches of mycelium and spores on leaves, stems and, later, buds. Infected tissue yellows, [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) drops and buds become unsellable.

## In practice

Unlike most fungi, powdery mildew does not need free water on the leaf, and moderate humidity, still air and low light within the canopy favour it. It is spread by airborne spores and on clones; the fungus has also been reported to persist inside some infected plant material. [Prevention](https://genetixbay.com/knowledge/glossary/cultural-control.md) is airflow, sensible humidity, [plant spacing](https://genetixbay.com/knowledge/glossary/plant-density.md) and clean starting material. Resistance [genes](https://genetixbay.com/knowledge/glossary/gene.md) such as *PM1* have been mapped, so cultivar choice matters. Once it reaches flowers, options are very limited; check what is registered for cannabis locally.

## Don't confuse with

- **Trichomes**: [resin glands](https://genetixbay.com/knowledge/glossary/trichome.md) on buds and [sugar leaves](https://genetixbay.com/knowledge/glossary/sugar-leaf.md); mildew spreads across [fan leaves](https://genetixbay.com/knowledge/glossary/fan-leaf.md) too and wipes off as dust.
- **Downy mildew**: a water mould with grey-purple growth under leaves and yellow patches above.
- **Spray residue or hard-water spots**: no spreading growth.

## Related terms

[Botrytis](https://genetixbay.com/knowledge/glossary/botrytis.md), [Sulfur burner](https://genetixbay.com/knowledge/glossary/sulfur-burner.md), [Action threshold](https://genetixbay.com/knowledge/glossary/action-threshold.md), [Sanitation](https://genetixbay.com/knowledge/glossary/sanitation.md), [Downy mildew](https://genetixbay.com/knowledge/glossary/downy-mildew.md)

## Read more

- [Powdery mildew in cannabis](https://genetixbay.com/knowledge/diseases/powdery-mildew.md)
- [White powder or white spots on cannabis leaves: a differential](https://genetixbay.com/knowledge/diagnosis/white-powder-on-leaves.md)

## Sources

1. PNW Plant Disease Management Handbook. Hemp (Cannabis sativa) - Powdery Mildew. https://pnwhandbooks.org/plantdisease/host-disease/hemp-cannabis-sativa-powdery-mildew
2. Punja ZK, Collyer D, Scott C, et al., 2019. Pathogens and molds affecting production and quality of Cannabis sativa L. Frontiers in Plant Science 10:1120. https://www.frontiersin.org/articles/10.3389/fpls.2019.01120/full
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://www.frontiersin.org/articles/10.3389/fagro.2021.720215/full