Damping-Off
Definition: Damping-off is a disease that kills young seedlings, usually caused by soilborne water molds and fungi such as Pythium, Rhizoctonia and Fusarium. Infected cannabis seedlings develop a thin, dark, water-soaked stem at the soil line, then topple over and die. Cool, wet, poorly drained conditions are the main trigger.
How Damping-Off works
The pathogens behind damping-off live in many soils and in unclean trays or water. They attack soft, young stem tissue that has not yet hardened. Once the stem base pinches and collapses, the seedling cannot move water upward, and there is no saving it. Damping-off can also strike before emergence, rotting seeds in the ground, which growers often mistake for bad genetics.
Warning signs
- A seedling that looks fine in the morning and lies flat by evening.
- A narrow, brown or gray constriction right at the surface of the medium.
- White, cottony growth on the soil surface in very humid setups.
Prevention
- Clean start: use fresh, bagged seedling mix or plugs, and wash reused trays and domes with a mild bleach or hydrogen peroxide solution.
- Careful watering: let the surface dry slightly between waterings, and avoid cold water.
- Warmth and air: keep media around 72 to 78°F (22 to 26°C) and run a gentle fan once the dome comes off.
- Biologicals: beneficial microbes such as Trichoderma and Bacillus species can compete with the pathogens.
Remove infected seedlings and their media at once, then cut back watering for the rest of the tray. The same organisms cause root rot in older plants, especially where overwatering is routine.