Damping off: seedlings collapsing at the base
Seedlings that come up fine and then flop over, with a pinched, water-soaked stem at soil level. It is a fungal problem and it spreads fast.
What you see
Seedlings germinate and look healthy for a few days, then collapse. The stem at the base is thin, dark and water-soaked — a pinch point. The seedling falls over while still green. Sometimes the seed rots before it even emerges.
It does not affect older plants. It is a seedling disease almost exclusively, and it appears in patches that spread outward from the first affected cell.
Why it happens
Too much water
The overriding cause. Damping-off fungi need saturated conditions. A medium that stays soaked with no air in it is the ideal environment, and it is what most beginners provide.
No airflow
Still, warm, humid air around the seedlings lets the fungus establish and spread. A closed propagator with condensation on the lid is the classic scenario.
Dirty equipment
Reused trays, pots and tools carry the pathogen between cycles. So does reusing medium.
Sowing too densely
Seedlings touching each other transfer the infection directly. Microgreens and densely sown trays are especially prone.
Cool, wet medium with warm air
The temperature combination matters. Cold wet roots plus warm humid air above is the worst case.
What to do
Discard the affected seedlings immediately. Do not try to save a collapsed seedling — it will not recover, and leaving it spreads the fungus to its neighbours.
- Water from below. Stand the pot in a shallow tray of water rather than pouring over the top. The surface stays drier where the fungus works.
- Increase airflow immediately. A small fan moving air across the seedlings is the single most effective change.
- Let the medium dry slightly between waterings. Roots need oxygen as much as water.
- Sow less densely so air moves between seedlings.
- Clean everything between cycles — trays, net pots, tools. Hot soapy water is enough.
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All problems
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