Damping-Off
A seedling disease caused by several soilborne pathogens - primarily Pythium, Rhizoctonia, and Fusarium species - that rots stems at or below the soil line, causing seedlings to collapse shortly after emergence.
Damping-off is the sudden collapse and death of seedlings at or just below the soil line. One day the seedlings look fine; the next, a portion of them are lying flat with pinched, water-soaked stems at the soil surface. It kills fast and is always worse than it looks the first morning you notice it.
The cause is not a single pathogen. Several soilborne fungi and water molds - primarily Pythium spp., Rhizoctonia solani, Fusarium spp., and Phytophthora spp. - cause damping-off individually or in combination. They are present in virtually all non-sterile soils and organic matter; the question is whether conditions favor their proliferation to damaging levels.
Conditions That Promote Damping-Off
Excess moisture. Damping-off pathogens thrive in saturated or persistently wet soil. Overwatering is the primary management failure. Pythium and Phytophthora are water molds that move through soil moisture; waterlogged conditions allow them to spread rapidly through a seedling flat.
Cool soil temperatures. Seedling growth slows at low temperatures, extending the time the emerging seedling is vulnerable. Meanwhile, Pythium and Rhizoctonia remain active at lower temperatures than many competing organisms. Starting seeds in cold (below 60°F) growing medium increases damping-off risk.
Poor air circulation. Humid, stagnant air keeps surface soil and seedling stems persistently wet. Damping-off progresses faster when air movement is minimal.
Dense seeding. Too many seedlings per cell or per flat creates overcrowding that limits air circulation at the soil surface and concentrates susceptible tissue.
Non-sterile growing medium. Garden soil, partially finished compost, or reused potting mix can harbor high pathogen loads. Sterile commercial seed-starting mix or new potting mix significantly reduces pathogen pressure at germination.
Prevention
Use sterile seed-starting mix. Avoid using garden soil or homemade compost for seed starting. Commercial mixes based on peat, coir, and perlite are low-pathogen environments for germination.
Bottom water. Watering from below (setting trays in shallow water and allowing the mix to absorb moisture) keeps the surface relatively dry. This directly counters Pythium and Rhizoctonia favorability.
Water sparingly and let the surface dry slightly between waterings. The mix should stay moist several inches down; the surface should be allowed to visually dry before watering again.
Provide air circulation. A small fan running on low near seedlings reduces surface humidity and strengthens stems.
Don’t seed too densely. Thin to appropriate spacing at cotyledon stage.
Use containers with drainage holes. Never allow seedlings to sit in standing water.
Treatment and Response
Once damping-off begins, it spreads quickly through a flat. Remove and discard affected seedlings immediately. Reduce watering, increase air circulation, and consider applying a thin dusting of perlite or sand to the soil surface to promote drying at the stem base.
Biological fungicide products based on Bacillus subtilis (such as Serenade) or Trichoderma species have some efficacy against damping-off pathogens as preventive treatments - they don’t reverse established infections but can reduce spread. Conventional fungicides (captan, thiram) are used in commercial seed production but are generally not practical for home seed-starting.
If a specific seed lot or flat is consistently experiencing damping-off, the problem is usually cultural (too wet, too cool, too dense) rather than needing treatment.