Why are my seedlings collapsing and dying?
Seedlings fail in two completely different ways, and the difference is visible in ten seconds. Either the stem failed where it meets the compost, or something removed the plant. One is a hygiene problem indoors, the other is a pest problem outdoors.
The one observation that decides it
Look for the missing plant. A seedling toppled over with a thin, pinched, water-soaked stem at the compost line, still attached, is damping-off, and it spreads through the tray from unclean water or reused compost. A seedling cut clean through at soil level and left lying beside its own stump is cutworm. A seedling gone entirely, with a slime trail, is slugs; peppered with tiny shot-holes but standing, it is flea beetle.
Start from the plant in front of you
6 things it could be
Ordered by how much they can cost you — if the first entry fits what you can see, rule it out before considering the milder ones.
Seeds rot before sprouting, or seedlings topple over with a thin, water-soaked, pinched stem at the soil line; roots are brown and mushy.
Conditions: 5–20°C · humidity ≥90% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysWilting despite obviously wet soil, yellowing of the lower leaves, and leaf drop. Stems may show soft brown rot at the base, and the soil smells sour or of rotten eggs. Growth stops entirely and the plant becomes an easy target for Phytophthora.
Conditions: humidity ≥90% · wet leaves / rainFirst move: Fix the cause, not the leafYoung plants cut clean through at soil level and left lying beside the stump. Later in the season, surface grazing on potato tubers and carrot shoulders. Scrape the soil within a few centimetres of a felled plant and a fat grey-brown caterpillar curls up in a C shape.
Conditions: 12–30°C · dry air ≤60%First move: Treat, then re-check in 5–7 daysSeen on: Sweetcorn
Dozens of small round holes peppering the leaves, heaviest on the youngest growth. Seedlings can be killed outright or checked so badly they never catch up. The beetles scatter with an audible flick when you lean over the row.
Conditions: 18–32°C · dry air ≤55%First move: Treat, then re-check in 5–7 daysIrregular ragged holes with smooth edges in leaves and fruit, seedlings eaten to the ground overnight, and tell-tale silvery slime trails on soil and foliage.
Conditions: 5–22°C · humidity ≥80% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysSeen on: Lettuce, Peas, Celeriac, Swiss chard, Rhubarb, Strawberries and 6 more
Narrow, clean holes bored straight into potato tubers, carrots and other roots, often with secondary rot following. Seedlings and young transplants wilt and die from stems chewed at soil level. The larvae are thin, tough, orange-brown and up to 25 mm long.
Conditions: 8–25°CFirst move: Fix the cause, not the leafSeen on: Potatoes