Damping-off
Pythium spp., Rhizoctonia solani (parfois Fusarium)
Damping off: the young seedling collapses, its stem pinched and browned at the neck, here on a seed tray, and the symptom is the same in the kitchen garden.
Photo: INAKAvillage211 · CC BY-SA 3.0 · source
Role in the garden
A complex of soil fungi that makes young seedlings collapse, the stem pinched and thinned right at the surface of the substrate, often within a day or two of emergence.
Practical guide
Where to find them
On the young seedlings of most vegetables and flowers, especially in pots, seed pans or trays, from sowing until the first true leaves. The fungi responsible live in almost all soils and substrates.
Prevention
Sow thinly: this is the first protection, because overcrowded seedlings hold a dampness that favours the disease. Use a clean, light, free-draining substrate and cleaned containers. Water sparingly, preferably by misting or from below, and let the surface dry out between waterings. Give the seedlings air and avoid stagnant conditions. Sow at the right time, once the substrate is no longer cold, because a cold, waterlogged medium slows emergence and leaves the field clear for the fungi. Favour healthy seed.
Early signs
Shortly after emergence, seedlings that were vigorous until then suddenly topple and collapse, often in small patches that spread. Looking at the base of the stem, you see a pinch: there the stem turns thin, translucent as if water-soaked, then browns. Some seeds may also rot before they even emerge. A fine whitish veil sometimes appears on the substrate.
Action / control
Once a seedling has collapsed, it can't be saved: better to act quickly to limit the spread to its neighbours. Remove the affected seedlings and the substrate around them without delay, space out or prick out the survivors, stop watering while the surface dries, and markedly improve airflow. If you lose them, re-sow in fresh substrate and a clean container rather than treating. Prevention (sowing thinly, free-draining substrate, measured watering) remains by far the best lever.
Sources
- Ephytia, INRAE (Tropilég) : Fontes de semis
- Jardiner Autrement (SNHF / Ecophyto) : fiche « Fonte des semis »
- University of Connecticut, CAHNR Integrated Pest Management : Damping-off of Ornamental and Vegetable Seedlings