Diagnosing and Preventing Seedling Damping-Off and Disease

Common Causes of Seedling Collapse

Your symptoms—multiple species failing simultaneously after starting in "organic Miracle-Gro" soil—point to two primary issues:

1. Damping-off (fungal): Caused by Pythium, Rhizoctonia, Fusarium. Thrives in overly wet, poorly ventilated conditions. Stems pinch at soil line; seedlings topple.
2. Overwatering: "Organic" potting mixes often retain more moisture. Seedlings need moist, not saturated, media.

Sanitation: The Critical Prevention Step

• Sterilize containers: Soak in 10% bleach solution (1:9 bleach:water) 30 minutes; rinse well.
• Use fresh, sterile seed-starting mix—not garden soil or reused potting soil.
• Clean tools and work surfaces with 70% alcohol or bleach solution.
• Wash hands before handling seeds/seedlings.

Environmental Controls

• Air circulation: Small fan on low, running 24/7 after germination.
• Bottom watering: Water from tray below; keep surface dry.
• Light: 14-16 hours/day of strong light (LED grow lights 2-4" above plants).
• Temperature: 65-75°F for most vegetables; use heat mat only for germination.

Restart Strategy

Discard affected seedlings and soil. Clean everything. Start fresh with sterile mix, proper sanitation, and environmental controls. See Chiots Run's Seed Starting 101 series for detailed guidance with photos.