The stem becomes too weak to support the rest of the plant, and it falls over. It quickly withers and dies. Either the fungus Pythium or Rhizoctonia .
Prevention is the key as once symptoms are apparent the plant quickly dies.
Use sterile potting media. Keep benches, pots, garden tools, and hoses clean. Fungus spreads readily via garden tools. Select planting date to promote fast germination, wait for soils to warm prior to planting spring crops. Use seeds that have been pre-treated with a fungicide.