Pythium Damping Off — Permaculture Database

Overview

Pythium damping-off is a seedling disease complex caused by Pythium and related oomycetes that attack seeds and young stems at the soil line. Cool, wet, poorly aerated media and dense seeding favor outbreaks. Both pre-emergence seed rot and post-emergence collapse occur. Greenhouse trays, outdoor seedbeds, and direct-sown garden rows can be hit when conditions stay wet. Fast-growing, soft seedlings are especially vulnerable until true leaves expand and stems toughen.

Symptoms

Seeds fail to emerge, or seedlings topple overnight with pinched, water-soaked stems at the soil line. Roots may be stubby, gray-brown, and lacking fine hairs. Patches of a tray or row die while neighboring seedlings in drier cells survive. A fuzzy fungal look is less common than with some true fungi; the hallmark is soft collapse at the crown in wet media. Surviving plants can remain stunted if root tips were pruned by infection.

Environmental Conditions

Humid

Prevention

Use pasteurized or high-quality sterile seed-starting mix, clean trays, and potable water. Sow thinly and provide strong light so seedlings are sturdy. Water from below when practical and empty saucers. Harden plants before transplanting into cold, wet soils. Improve field drainage for direct seeding. Do not overfertilize seedlings with nitrogen that keeps tissues soft. Rotate seedling areas and scrub benches between crops.

Management

Discard severely affected trays rather than nursing a few survivors into clean beds. Re-sow in fresh, well-drained media with lower density. Reduce watering and increase airflow and light. Slightly warmer germination temperatures within crop limits help many species outgrow Pythium. Biological seed treatments and beneficial microbes can help in some systems. Avoid reuse of unsterilized, disease-ridden media for successive seedling cycles without rejuvenation and sanitation.