Gray Mold Or Damping Off

Because we can’t clearly see the problem in your picture, we can’t tell exactly what is going on, so we’ll go with the two most likely cullprits. The first is gray mold (Botrytis), which usually takes new seedlings a leaf at a time. It is most active in cool, humid environments, so you can counteract it by warming the temperature, reducing humidity (which warming alone will do), and increasing air circulation. There are also fungicides made to treat infected plants. The second possibility is damping off disease, which includes a complex of fungi that kill the seedling by attacking its stem. The up-front prevention is to use absolutely sterile soil mix, with no reused soil from last year or garden compost—nothing that contains living organisms. Once damping off starts, you can’t stop it.