Looks like damping off which is often caused by overly wet soil and stagnant air. Cool temperatures and low light conditions can also contribute to this disease. Damping off can be caused by fungus spores that are present in the soil and on unsanitized seed trays and plant pots.
It's hard to tell from pic if this would help but try mixing 1 tsp of hydrogen peroxide with 2 cups of water and use this solution to water your plants. The peroxide oxygenates the soil, which kills fungus spores instantly! It's a great way to fight damping off.
