# Pythium

> A genus of oomycete water moulds, such as Pythium myriotylum and P. aphanidermatum, that cause brown, slimy root rot, especially in warm hydroponic systems.

- Canonical: https://genetixbay.com/knowledge/glossary/pythium.html
- Also written: Pythium root rot, Pythium spp., pythium rot, Pythium myriotylum, Pythium dissotocum, Pythium aphanidermatum
- 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"

*Pythium* is a genus of oomycetes, water moulds that look like fungi but are more closely related to brown algae. Several species infect cannabis roots, including *P. myriotylum*, *P. dissotocum* and *P. aphanidermatum*. They spread by swimming zoospores, so they thrive wherever roots sit in water: [deep water culture](https://genetixbay.com/knowledge/glossary/dwc.md), [recirculating systems](https://genetixbay.com/knowledge/glossary/recirculating-system.md) and [overwatered](https://genetixbay.com/knowledge/glossary/overwatering.md) pots. They also cause [damping-off](https://genetixbay.com/knowledge/glossary/damping-off.md) of [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) and cuttings.

## In practice

Infected roots turn brown, soft and slimy, and the outer root tissue slips off, leaving the inner strand. Above ground the plant wilts in the heat of the day, yellows and stalls, despite a full reservoir. Warm nutrient solution above roughly 24-26 °C (75-79 °F), low dissolved oxygen and stagnant water favour it. [Prevention](https://genetixbay.com/knowledge/glossary/cultural-control.md) is cool, well-oxygenated solution, clean equipment, and either a dedicated [beneficial-microbe reservoir](https://genetixbay.com/knowledge/glossary/beneficial-microbe-reservoir.md) or a fully committed sterile one. [Fungus gnat](https://genetixbay.com/knowledge/glossary/fungus-gnat.md) larvae can carry it in media.

## Don't confuse with

- **Fusarium**: a true fungus that often causes wilt and brown vascular streaks in the stem.
- **Tea-stained roots**: brown from additives but firm, with white tips.
- **Overwatering in soil**: low oxygen without slime or root sloughing.

## Related terms

[Root rot](https://genetixbay.com/knowledge/glossary/root-rot.md), [Damping-off](https://genetixbay.com/knowledge/glossary/damping-off.md), [Fusarium wilt](https://genetixbay.com/knowledge/glossary/fusarium-wilt.md), [Biofilm](https://genetixbay.com/knowledge/glossary/biofilm.md), [Fungus gnat](https://genetixbay.com/knowledge/glossary/fungus-gnat.md)

## Read more

- [Root rot and Pythium in cannabis](https://genetixbay.com/knowledge/diseases/root-rot-pythium.md)
- [Brown roots in hydroponics (DWC, RDWC, NFT): a differential](https://genetixbay.com/knowledge/diagnosis/root-browning-hydro.md)

## Sources

1. Punja ZK, Rodriguez G, 2018. Fusarium and Pythium species infecting roots of hydroponically grown marijuana (Cannabis sativa L.) plants. Canadian Journal of Plant Pathology 40(4):498–513. https://doi.org/10.1080/07060661.2018.1535466
2. University of Kentucky Cooperative Extension. Pythium root rot in hydroponic production (E-706). https://greenhousehort.ca.uky.edu/sites/greenhousehort.ca.uky.edu/files/E706.pdf
3. PNW Plant Disease Management Handbook. Hemp (Cannabis sativa) - Root and Crown Rot. https://pnwhandbooks.org/node/12931