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 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 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.
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
- Water temperature right now and at peak (late in the lights-on period). Above 22 °C (72 °F) is the most common root cause.
- Aeration: are air stones clogged, pumps undersized, return lines delivering? Measure DO if you can.
- Pull a root sample: white and firm vs brown and slimy. Rinse; firm brown = stain, soft = rot.
- pH and EC: rule out salt burn (very high EC also browns root tips) and pH crashes.
- Light leaks into reservoirs or buckets.
- In soil/coco: pot weight before watering, drainage holes, saucers sitting full, fungus gnats.
- History: new clones, contaminated water source, shared tools or pots.
- If roots look clean but plants stunt: consider hop latent viroid, root aphids or fusarium.
How to fix it
Root-zone conditions (first, always)
- Cool the solution to 18–21 °C (65–70 °F): chiller, insulated reservoir, move reservoir off a warm floor, reduce room heat.
- Max out oxygen: larger air pump, clean stones, waterfall/return turbulence. In soil/coco: water less often, increase drainage, bigger air-porosity media.
- Change the solution entirely; clean the reservoir and lines (see below).
- Trim the worst rotten root mass with sanitised scissors in DWC; rinse gently with fresh, cool, pH-adjusted water.
- 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 limitedbeing reviewed. 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).
- Manage fungus gnats, 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 and root aphids 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.
