Damping-off

Patchy, thin emergence — seed that never comes up, or bare blank spots in a tray or seedbed where the rest of the row came up fine. That missing seed has already rotted underground before breaking the surface.

Look right at the soil line — the stem there is thin, pinched-in and water-soaked, like it has been squeezed wet between two fingers. It turns brown or black and the seedling falls flat while its leaves may still look green, as if pushed over.

Start with one fallen seedling and each day the bare patch grows wider into the neighbours, because the rot travels through the wet soil. In a very damp nursery you may also see a fine, cobweb-like or cottony white or grey growth on the soil surface or on the rotting stems.

with cutworm damage (the stem is cut cleanly through and the seedling lies cut off, not pinched and rotted — and you'll find a fat grey grub in the soil), with drought wilt (the whole seedbed droops evenly and perks up after watering, with firm stems), or with fertiliser/salt burn (leaf tips scorch brown but the stem base stays firm). Damping-off is always a soft, wet, pinched stem at the soil line in a spreading patch.

this disease only attacks at the seedling stage, so watch hardest from the day you sow until your seedlings are hardened off and ready to transplant — that is the whole danger window.

check every nursery bed and tray once a day, every day during that window, especially in cool, cloudy, wet spells after heavy watering or rain — the weather this disease loves.

don't just glance from the path. Kneel and look at the soil line of the seedlings. Walk the bed in a "W" so you cover both edges and the middle, and stop to check the wettest, most crowded, shadiest, poorest-draining corners first — that is where it always starts.

blank gaps in emergence, seedlings toppled over, and stems that are thin, brown and water-soaked where they meet the soil. Gently squeeze a suspect stem — if it is soft and mushy, not firm, that is damping-off.

The moment you find even 2 or 3 seedlings collapsed together at the soil line, treat it as an outbreak: act that same day. Do not wait for a count or a percentage — by the time many are down, the patch is already spreading.

Sow thinly so seedlings aren't crowded, in raised, well-drained beds or clean sterile media (fresh potting mix or well-cured compost, not old garden soil). Don't sow too deep. Start with clean or treated seed.

Over-watering is the single biggest cause. Water in the morning so the surface dries by evening, water less rather than more, and never let the bed stay soggy. Open up the nursery for good airflow and light — lift shade cloth in the day and space the trays so air moves between them.

Mix a biofungicide based on beneficial fungi or bacteria (a Trichoderma or Bacillus product — these are living helpers that crowd out and attack the rot) into your seedbed media, or blend in well-cured compost, which carries the same natural protectors. This shields the seedlings that are still standing.

stop watering that bed, remove and destroy the affected trays or the seedbed section together with a margin of soil around them, and let the area dry out and get more air. Removing the wet, infected patch is often what stops the spread.

If prevention fails and it keeps moving, use treated seed at sowing or drench the seedbed with an approved fungicide effective against water-moulds (for example a metalaxyl-type product for Pythium). Safety: cover your skin, eyes, nose and mouth, mix and spray in the cool of the day with the wind at your back, keep it away from food, drinking water, children and bees, wash yourself and your equipment afterwards, and respect the pre-harvest interval (the number of days the label says you must wait between treating and eating the crop). To stop the fungus becoming immune, rotate to a different chemical group rather than using the same one every time.

the media or seed is likely contaminated — throw out that batch of seedlings, do not re-sow into the same trays or bed without washing and drying them fully (sun-baking the soil helps), switch to fresh sterile media, and start again with new clean seed. Better to lose one tray now than the whole nursery. When a bed is already mostly collapsed, cut your losses: clear it, dry it out, and resow fresh rather than nursing the survivors.