# Downy mildew

> An oomycete leaf disease of cannabis, historically attributed to Pseudoperonospora cannabina, causing yellow patches above and grey-purple fuzz under leaves in cool, wet weather.

- Canonical: https://genetixbay.com/knowledge/glossary/downy-mildew.html
- Also written: Pseudoperonospora cannabina, Pseudoperonospora humuli, downy mold, DM
- Glossary category: Pests and diseases · Evidence: Supported
- 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"

Downy mildew is a disease caused by oomycetes, water moulds rather than true fungi. On cannabis it has historically been attributed to *Pseudoperonospora cannabina*; the hop downy mildew pathogen, *Pseudoperonospora humuli*, has also been reported on [hemp](https://genetixbay.com/knowledge/glossary/hemp.md). Infected leaves show pale yellow, angular patches on the upper surface and greyish to purplish downy growth underneath, where spores form.

## In practice

Downy mildew needs free water and cool temperatures, so it is chiefly an outdoor and greenhouse problem in wet spells, not a common indoor one. Spores spread by wind and splash, and infected leaves curl, brown and drop. Growers manage it with airflow, wider spacing, watering at the base, removing infected leaves and keeping hop yards and cannabis apart where both are grown. Diagnosis is easier with a hand lens on the leaf underside early in the morning, when sporulation is visible. Check what is registered for cannabis locally.

## Don't confuse with

- **Powdery mildew**: white powder on upper surfaces, grows without leaf wetness.
- **Septoria leaf spot**: tan lesions with dark margins and pycnidia.
- **Magnesium deficiency**: [interveinal yellowing](https://genetixbay.com/knowledge/glossary/interveinal-chlorosis.md) with no growth underneath.

## Related terms

[Powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md), [Pythium](https://genetixbay.com/knowledge/glossary/pythium.md), [Leaf spot](https://genetixbay.com/knowledge/glossary/leaf-spot.md), [Sanitation](https://genetixbay.com/knowledge/glossary/sanitation.md)

## Read more

- [Powdery mildew in cannabis](https://genetixbay.com/knowledge/diseases/powdery-mildew.md)

## Sources

1. 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
2. BC Ministry of Agriculture. Diseases of cannabis in British Columbia (factsheet). https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/animal-and-crops/plant-health/disease-factsheets/specialty-crops/diseases_of_cannabis_in_british_columbia.pdf