The fungal diseases infecting onion seedlings are
Basal rot (Fusarium oxysporum f.sp cepar)
Damping off (Phythium sp. Fusarium sp. Rhizoctonia solani)
Phytopthora neck rot ( Phytopthora nicotianae)
Pink root ( Phoma terrestris)
Smut (Urocystis colchici, Urocystis cepulae)
Southern Blight ( Sclerotium rolfsii)

Though there are many bacterial disease infect onion, very few occurs during seedling stage.

Description for many diseases can be found in our onion entry.
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