Damping-off

In nurseries and seed beds, damping-off kills seedlings either before they emerge from soil (preemergence damping-off) or shortly after sprouting when they suddenly collapse and wilt. Seedlings appear healthy when they're underground, but wilt dramatically once they break the soil surface. The problem is most severe in cool, wet conditions when soil moisture stays high, and it can devastate entire seed trays or nursery beds, especially in Douglas-fir and other conifer seedlings common in Pacific Northwest nurseries.

Damping-off is caused by multiple soilborne fungi, most commonly Fusarium and Pythium, and it thrives in wet, poorly drained soil. Prevention depends on controlling soil conditions and pathogen contamination from the start: use properly conditioned seeds, avoid waterlogged soil, and rotate planting beds with nonsusceptible crops like small grains to reduce fungal populations. Never reuse containers without disinfecting them; if you reuse styroblocks, sanitize them with heat or approved biocides. These practices interrupt the disease cycle and protect your seedlings through their most vulnerable stage.