Short answer. If a healthy-looking seedling suddenly topples over, with the stem thin, water-soaked or brown right at the soil line, it's damping-off. It's caused by soil-borne pathogens, most often Pythium and Rhizoctonia, also Fusarium and Phytophthora [1][2][3]; in cannabis, Pythium and Fusarium are among the documented root and seedling pathogens [5]. A collapsed seedling can't be saved. Prevention: clean containers, fresh mix, no soggy medium, warm roots, strong light, moving air.
How to recognise it
- Pre-emergence: seed rots in the medium; nothing comes up; dig and find a soft, brown seed or radicle.
- Post-emergence: stem pinched or constricted at the soil line, often darker or translucent; seedling falls over while leaves are still green.
- Pattern: often patches of a tray, spreading along wet zones.
Differential diagnosis
| Looks like | Distinguish by |
|---|---|
| Stretch / weak stem from low light | Stem uniformly thin and pale, not pinched or brown at the base |
| Overwatering droop | Whole seedling droops but stem base is firm |
| Heat or dry-out | Leaves crisp; stem dry, not water-soaked |
| Fungus gnat larvae damage | Larvae visible in the top 1–2 cm; roots chewed |
What drives it
- Free water and saturated media: Pythium needs free water to spread [4].
- Temperature: Pythium favours wet soils; some species cool, some warm; Rhizoctonia acts in warm to cool and even drier conditions [2][3].
- Slow seedlings: low light, cold medium, excess fertiliser salts all lengthen the vulnerable phase [1].
- Contamination: dirty pots, reused mix, garden soil, hose ends, hands and tools [1].
Prevention checklist
- New, sterile seedling mix; never garden soil [1].
- Clean trays and pots: soak 30 min in 10 % household bleach, rinse [1].
- Water only when the plug lightens; let the surface dry slightly between waterings [4].
- Use clean, room-temperature water; cold water slows growth [1].
- Medium at 21–25 °C (70–77 °F); light at seedling levels (seedling care).
- Vent domes daily; gentle air movement.
- No fertiliser until true leaves form [1].
- Fungicides only if damping-off is a known recurring problem, and late as possible, since they can stunt seedlings [2]; in many places no products are registered for cannabis, so check local rules.
If it happens
Remove affected seedlings and the medium around them, let the tray dry slightly, improve airflow, and sanitise before the next sowing. Beneficial biological root inoculants (e.g. Trichoderma, Bacillus) are used preventively by many growers (Grower practice; evidence in cannabis is limited).
Common mistakes
- Overwatering "because seedlings are thirsty".
- Reusing last run's mix for seedlings.
- Cinnamon "cures" (Myth as a reliable control; no supporting data).
Safety
Bleach: dilute in a ventilated space, gloves on, never mix with acids or ammonia.
Evidence
Causes, conditions and prevention are Established extension consensus [1][2][3][4]. Cannabis pathogen reports are Established [5].
