When your seedlings die suddenly, the most common cause is a fungal problem called damping-off. This disease attacks young stems at the soil line and can wipe out a whole tray of plants in just one or two days. One morning your seedlings look fine. The next morning they lie flat on the soil with pinched stems.
When I first started growing vegetables from seed, I lost dozens of tomato seedlings to this exact problem. They came up strong and green looking. A week later they all fell over like someone had cut them down with scissors. I found thin brown spots at the base of each stem where the fungus had rotted through the plant tissue.
Damping-off disease comes from fungal pathogens that live in soil and water all around us. The main culprits are Pythium and Rhizoctonia, which thrive in wet and cool conditions. These fungi attack soft stem tissue right at the soil surface where your seedlings are weakest. Once they get inside the stem, they spread fast and kill the plant.
The fungi need certain conditions to take hold and spread through your seed trays. Too much water keeps the soil soggy and gives fungi the moisture they need to grow well. Poor air flow lets humidity build up around your young stems. Cool temps below 65°F (18°C) slow plant growth while letting fungi spread faster. Any of these problems raises your risk of losing plants.
Beyond fungi, several other seedling failure causes can kill your young plants just as fast. Cold drafts from nearby windows can shock warm seedlings and damage their tender cells. Too much water drowns roots and rots them before you see any above-ground symptoms. Rough handling during transplant can snap fragile stems or tear tiny root hairs from the soil.
I tested this myself and found that overwatering ranks second after damping-off as a seedling killer. New growers tend to water too often because they worry about drought hitting their plants. But seedlings in small cells or trays can stay wet for days after a single watering. That constant moisture invites all sorts of root problems for your young plants.
You can prevent most sudden seedling deaths by controlling your growing conditions from the start. Use fresh sterile seed-starting mix from a sealed bag rather than garden soil from outside. Garden soil contains fungal spores and other pathogens that attack tender seedlings right away. Sterile mix gives your plants a clean start free from these threats.
Keep air moving around your seedlings with a small fan running on low speed nearby. This dries the soil surface and strengthens stems through gentle stress. Water from below by setting trays in a pan of water about one inch deep. Let soil wick up what it needs. Remove the tray after 10-15 minutes so it does not stay soaked all day.
Give your seedlings room to breathe by not sowing seeds too thick in your trays. Crowded seedlings trap humid air between their leaves where fungi love to grow fast. Keep temps above 65°F (18°C) using a heat mat under your trays if your room runs cold. Warm soil and moving air make it very hard for damping-off to take hold in your seed starting setup.
Read the full article: 6 Plant Growth Stages Explained Simply
