Damping Off

Damping off is caused by a complex of soil-borne pathogens, primarily Pythium and Rhizoctonia species, that attack seedlings at or just below the soil line. Pythium is technically a water mold (oomycete) rather than a true fungus, but it behaves like one in practice and responds to the same cultural controls. Both pathogens are present in virtually all garden soil and persist indefinitely.

The disease takes two forms. Pre-emergence damping off kills seeds before they break the surface, producing empty cells in the flat with no apparent cause. Post-emergence damping off is more recognizable: seedlings emerge normally, then suddenly collapse with a telltale pinching and darkening at the base of the stem. The infection typically spreads outward from a single point, so a flat that looks fine on Monday can show widening casualties by Wednesday.

Conditions that favor rapid spread: consistently moist soil surfaces, low light, poor air movement, and the use of non-sterile growing media. Cool, damp conditions favor Pythium; Rhizoctonia tends to be more active at warmer soil temperatures. Overwatering is the single most controllable contributing factor, since Pythium thrives when water sits in prolonged contact with stem tissue.

No resistant varieties exist for damping off. The pathogen complex attacks seedlings of virtually all susceptible crops regardless of genetics. Cultural controls are the only reliable prevention. Fresh sterile seed-starting mix, dry-down intervals between waterings, bottom-watering rather than overhead watering, and a small fan aimed at seedlings to maintain air movement will stop the majority of losses before they start.