Damping-Off: Preventing Seed & Seedling Losses in the Greenhouse

Damping-off—the rotting of seeds pre-germination or death of newly emerged seedlings—is caused primarily by Pythium, with Fusarium and Rhizoctonia also involved. These fungi are ubiquitous in soil and cannot be eliminated from the greenhouse environment. The solution: exclude them from the propagation zone entirely.

Conditions Favoring Damping-Off

• Excessive soil moisture and overhead misting
• Low soil temps pre-germination (<68°F / 20°C)
• High soil temps post-emergence (>77°F / 25°C)
• Overcrowded flats/seedbeds

Greenhouse Control Program

1. Start clean: Pasteurized soil or soilless mix in sterile flats/pots. Heat to 180°F, hold 30 min. Most commercial soilless mixes need no treatment.
2. Prevent recontamination: Clean/disinfest tools, benches, surfaces. No dirty floors, benches, tools.
3. Bottom heat: Maintain 70–75°F (22–24°C) soil temp for rapid germination/growth.
4. Even moisture: Avoid extremes during germination and seedling development.
5. Treated seed: Buy fungicide-treated (captan, thiram) or treat yourself. Test on portion first—some fungicides inhibit rooting.

When damping-off appears: discard the entire flat—soil and plants. Sterilize the flat, refill with clean mix using clean tools, reseed. Fungicide drenches don't cure established rot; surviving fungi rebound. Infected seedlings never fully recover or perform well in the garden.

Recurrent problems? Use fungicide-treated seed. Established plants resist damping-off.