# Root rot and Pythium in cannabis

> Pythium root rot is a water mould that thrives in warm, low-oxygen root zones and swims between plants in recirculating water. Brown, slimy roots whose outer layer slides off, plus wilting with a wet root zone, are the signature. Fix the root-zone conditions first: solution at about 18–21 °C (65–70 °F), high dissolved oxygen, clean water and no overwatering; then remove dead roots and re-establish beneficial microbes.

- Canonical: https://genetixbay.com/knowledge/diseases/root-rot-pythium.html
- Category: Diseases · Level: Intermediate
- Evidence: Established · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Flower · Systems: DWC, RDWC, NFT, Ebb and flow, Drip, Aeroponics, Coco, Rockwool, Soil
- 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

**Why are my [DWC](https://genetixbay.com/knowledge/glossary/dwc.md) cannabis roots brown and slimy?** Brown, slimy DWC cannabis roots with a foul smell and an outer sheath that slides off are usually Pythium root rot, a water mould rather than a true fungus. Research identified Pythium dissotocum, P. myriotylum and P. aphanidermatum in hydroponic cannabis. It wins when the solution is warm and short of oxygen, most commonly above 22 °C (72 °F).

**What water temperature prevents Pythium root rot?** Grower practice keeps DWC, [RDWC](https://genetixbay.com/knowledge/glossary/rdwc.md) and [NFT](https://genetixbay.com/knowledge/glossary/nft.md) solutions at 18 to 21 °C (65 to 70 °F), and below 22 °C (72 °F), to prevent Pythium root rot. Cooler water holds more oxygen: saturation is about 9.1 mg/L at 20 °C versus 8.3 mg/L at 25 °C. Research in hydroponic spinach found the aggressive warm-water species took over above about 23 °C.

**How do I treat Pythium in cannabis hydroponics?** Treat Pythium in cannabis hydroponics by fixing [root-zone conditions](https://genetixbay.com/knowledge/glossary/root-zone.md) first: cool the solution to 18 to 21 °C, maximise aeration, change the solution completely and clean the reservoir and lines. Trim the worst rotten roots with sanitised scissors, rinse gently and black out any light reaching the water. Then choose either a beneficial-microbe or a fully sterile approach, never both.

**Is brown root colour from nutrients or root rot?** Brown cannabis roots that stay firm, with intact tips and no smell, are often just tea-coloured from additives, dyes or organics, according to grower practice. Root rot is soft and slimy, smells rotten, and its outer layer pulls off between the fingers leaving a thin white strand. The belief that brown roots always mean Pythium is a myth.

**Can I use hydrogen peroxide for cannabis root rot?** Hydrogen peroxide is one of the oxidiser classes used in hydroponics and can reduce free-swimming Pythium spores, but it does not cure infected roots, may alter pH and nutrient availability, and kills beneficial microbes. Never shock-dose living roots at high concentration, since that burns them too. Use oxidisers only as part of a fully committed sterile approach.

**Should a cannabis hydro reservoir use beneficial microbes or be sterile?** GenetixBay prefers a beneficial-microbe reservoir, because bacteria will turn up in any reservoir regardless, so it may as well host good ones such as Bacillus, Trichoderma, Pseudomonas and Streptomyces. A sterile reservoir is valid only if fully committed, with continuous oxidiser or UV and ozone and no organics. Mixing the two fails, because oxidisers kill the microbes.

**Why is my cannabis plant wilting when the soil is wet?** A cannabis plant wilting in wet soil is [drowning](https://genetixbay.com/knowledge/glossary/overwatering.md), not thirsty: chronic overwatering, poor drainage and compacted media starve roots of oxygen and allow root rot, and [fungus gnat](https://genetixbay.com/knowledge/glossary/fungus-gnat.md) larvae wound the roots further. Do not top up the plant. Let the media dry back, clear drainage, empty full saucers, manage gnats and consider preventive root-zone biologicals.

**Short answer.** In hydroponic cannabis, "root rot" is usually *Pythium* (a water mould, not a true fungus), sometimes with *Fusarium* alongside [1][2]. It wins when the root zone is warm and short of oxygen, and in [recirculating systems](https://genetixbay.com/knowledge/glossary/recirculating-system.md) its swimming spores reach every plant. Brown, slimy roots with an outer sheath that slides off, a foul smell, and a plant that wilts while sitting in water are the signature. Fix the conditions first, keep the solution near 18–21 °C (65–70 °F) with dissolved oxygen high, then clean up roots and re-establish beneficial microbes.

## What's happening

*Pythium* species (in cannabis: *P. dissotocum*, *P. myriotylum*, *P. aphanidermatum*) infect root tips and the outer root tissue (cortex) [1][2]. The cortex rots and sloughs, leaving the inner thread of the root. Water and nutrient uptake collapse, so the plant wilts and shows deficiency-like yellowing even when the solution is perfect. Spores called zoospores swim through water and are attracted to root exudates, which is why one sick bucket in an RDWC loop becomes a whole system [3].

Warm water holds less oxygen and speeds pathogen growth; stressed, oxygen-starved roots leak more exudate and resist less. In hydroponic spinach, 23 °C was the transition at which the aggressive warm-water species *P. aphanidermatum* came to dominate [4]. The same logic applies to cannabis (*Supported*, extrapolated from other crops). In soil and coco, root rot follows chronic overwatering, poor drainage and compacted media; fungus gnat larvae wound roots and carry spores [6].

## How to recognise it

- **Roots:** healthy hydro roots are white to cream, firm, with fuzzy fine hairs. Pythium roots are tan to brown, slimy, and the outer layer pulls off between your fingers leaving a thin white strand. Smell: rotten or sewer-like.
- **Plant:** wilting during lights-on that doesn't recover with water; slow growth; lower-leaf yellowing; then rapid collapse in warm weather. [Seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) and fresh clones may damp off (stem pinches at the base).
- **Solution:** cloudy, slimy biofilm on surfaces, pH drifting, foaming.
- **Stain vs rot:** brown roots that are **firm** with intact tips and no smell are often tea-coloured from additives, dyes or organics (*Grower practice*). Rot is soft and slides off. See [root browning in hydro](https://genetixbay.com/knowledge/diagnosis/root-browning-hydro.md).

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Solution temperature (DWC/RDWC/NFT) | 18–21 °C (65–70 °F); keep below 22 °C (72 °F) | Cooler water slows Pythium and holds more oxygen [3]; above ~23 °C warm-water species dominate [4] |
| Dissolved oxygen (DO) | ≥ 6 mg/L, ideally near saturation | Saturation is about 9.1 mg/L at 20 °C and 8.3 mg/L at 25 °C (fresh water); high DO reduces zoospore survival [3] |
| Solution change interval | 7–14 days, or top-off by EC | Grower practice; also restart after any outbreak |
| pH (hydro) | 5.6–6.2 | Out-of-range pH stresses roots; not itself protective (*Supported*) |
| Media moisture (soil/coco) | Allow partial dry-back; 10–20 % runoff in coco | Saturated, airless media favour root rot (*Grower practice*) |
| Room temperature effect | Every 1 °C on the air heats reservoirs | Chillers or insulated, opaque reservoirs in warm rooms |
| Light on solution | Zero | Light feeds algae, which consume oxygen at night and add organic load |

## What to check first

1. **Water temperature right now and at peak** (late in the lights-on period). Above 22 °C (72 °F) is the most common root cause.
2. **Aeration:** are [air stones](https://genetixbay.com/knowledge/glossary/air-stone.md) clogged, pumps undersized, return lines delivering? Measure DO if you can.
3. **Pull a root sample:** white and firm vs brown and slimy. Rinse; firm brown = stain, soft = rot.
4. **pH and EC:** rule out salt burn (very high EC also browns root tips) and pH crashes.
5. **[Light leaks](https://genetixbay.com/knowledge/glossary/light-leak.md)** into reservoirs or buckets.
6. **In soil/coco:** pot weight before watering, drainage holes, saucers sitting full, fungus gnats.
7. **History:** new clones, contaminated water source, shared tools or pots.
8. **If roots look clean but plants stunt:** consider [hop latent viroid](https://genetixbay.com/knowledge/diseases/hop-latent-viroid-and-viruses.md), [root aphids](https://genetixbay.com/knowledge/pests/root-aphids.md) or [fusarium](https://genetixbay.com/knowledge/diseases/fusarium.md).

## How to fix it

### Root-zone conditions (first, always)
1. **Cool the solution** to 18–21 °C (65–70 °F): chiller, insulated reservoir, move reservoir off a warm floor, reduce room heat.
2. **Max out oxygen:** larger air pump, clean stones, waterfall/return turbulence. In soil/coco: water less often, increase drainage, bigger air-porosity media.
3. **Change the solution** entirely; clean the reservoir and lines (see below).
4. **Trim the worst rotten root mass** with sanitised scissors in DWC; rinse gently with fresh, cool, pH-adjusted water.
5. **Black out** any light reaching the solution.

### Sanitation and water treatment (choose one approach, not a mix)

> **House position: living reservoir preferred.** Bacteria will turn up in any reservoir whatever you do, so you may as well have the good ones: GenetixBay prefers a beneficial-microbe reservoir. A sterile reservoir is a valid choice, but only if you commit to it fully (continuous oxidiser or UV/ozone, clean top-ups, no organics) and never mix the two. It's a personal choice; the half-way house is what fails.

Water-treatment classes used in hydroponics include chlorination, ozonation, copper ionisation, UV and oxidisers such as hydrogen peroxide [3]. They can change pH and nutrient availability, and most also kill beneficial microbes [3]. Follow manufacturer guidance and local rules. Wear gloves and eye protection when handling concentrated oxidisers; never mix chlorine products with acids or ammonia.
- **Biological approach first (house preference)**, see below. **Sterile approach:** oxidiser or UV/ozone kept running continuously in recirculating systems. Effective at reducing free-swimming spores; does not cure infected roots.
- **Biological approach:** beneficial microbes (*Bacillus* spp., *Trichoderma* spp., *Pseudomonas* spp., *Streptomyces* spp.) colonise roots and compete with or suppress pathogens. *Supported in other hydroponic crops*; cannabis-specific trial data are limited. Do not add them to an oxidiser-treated solution; the oxidiser kills them.
- **After an outbreak:** drain, scrub, disinfect, rinse, restart with fresh solution, then reintroduce biologicals if you use them.

### In soil and coco
- Let media dry back; don't top up a wilting plant in wet media (it is drowning, not thirsty — see [drooping](https://genetixbay.com/knowledge/diagnosis/drooping-overwatering-vs-underwatering.md)).
- Manage [fungus gnats](https://genetixbay.com/knowledge/pests/fungus-gnats.md), whose larvae wound roots.
- Root-zone biologicals (*Trichoderma*, *Bacillus*) are commonly used preventively (*Grower practice*, *Supported in other crops*).

### What not to do in flower
- **Don't keep heavily infected plants in a shared recirculating system.** Isolate or remove them; the rest of the loop pays otherwise.
- **Don't shock-dose oxidisers** at high concentrations on living roots hoping to "burn it out". You burn the roots too.
- **Don't add fungicides of unknown registration** to the reservoir. Residues can end up in flower, and most are not legal for cannabis.
- **Don't push heavy feeding** to "fix" the yellowing; damaged roots can't take it up and EC climbs.

## How to confirm it worked

- New root tips appear white and firm within 5–10 days.
- Plants stop wilting during lights-on; turgor returns.
- Solution stays clear; pH drifts slowly and predictably.
- Water temperature log stays below 22 °C (72 °F) at peak.

## Common mistakes

- Treating the symptom with additives while the water sits at 25 °C (77 °F).
- Mixing oxidisers and beneficial microbes in the same reservoir.
- Translucent reservoirs and light leaks.
- Topping off forever without ever changing or cleaning.
- Assuming all brown roots are rot (and all white roots are healthy — [HLVd](https://genetixbay.com/knowledge/glossary/hop-latent-viroid.md) and [root aphids](https://genetixbay.com/knowledge/glossary/root-aphid.md) can hide in white roots).
- Overwatering soil and coco "because it's flowering".

## Evidence

- *Established:* *P. dissotocum*, *P. myriotylum* and *P. aphanidermatum* cause root rot, stunting, yellowing and wilt in hydroponic cannabis in Canada [1][2].
- *Established in other crops:* cooler, oxygen-saturated solutions reduce Pythium [3][4].
- *Supported:* water-treatment classes (chlorination, ozone, UV, copper, oxidisers) reduce pathogens in recirculating systems but have side effects [3].
- *Supported in other crops*, limited cannabis data: beneficial microbes for suppression. Preferring a living reservoir: house position; sterile is valid if fully committed.
- *Grower practice:* exact solution temperature targets of 18–21 °C, change intervals.
- *Myth:* "Brown roots always mean Pythium." Stains from additives are common.

## 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. Punja ZK 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. University of Kentucky Cooperative Extension. Pythium root rot in hydroponic leafy greens and its management (E-706). https://greenhousehort.ca.uky.edu/sites/greenhousehort.ca.uky.edu/files/E706.pdf
4. Phytopathology 75:333, 1985 (abstract). Temperature and predominance of Pythium aphanidermatum vs P. dissotocum on hydroponic spinach. https://www.apsnet.org/publications/phytopathology/backissues/Documents/1985Abstracts/Phyto75_333.htm
5. PNW Plant Disease Management Handbook. Hemp (Cannabis sativa) - Root and Crown Rot. https://pnwhandbooks.org/node/12931
6. UConn Extension IPM. Managing fungus gnats in the greenhouse. https://ipm.cahnr.uconn.edu/managing-fungus-gnats-in-the-greenhouse/