Most serious disease of vegetable seedling is

The most serious disease of the vegetable seedling is Damping off.
It is a soil-borne fungal disease caused by Rhizoctonia, Fusarium or phytophthora, etc. It negatively affects the vegetables and flowers in your garden, primarily targeting seeds and new seedlings.
Damping-off refers to the rotting of stem and root tissues at and below the soil line. In most cases, infected plants will germinate and come up fine, but within a few days, they become water-soaked, mushy, collapse at the base and die.