Answer: Your onions probably had bacterial neck rot. The bacteria enter the neck of the onions through wounds made during growing, harvesting and curing. Unfortunately, there isn't much you can do once your onions have the disease, but you can prevent neck rot in future crops.
Avoid using mulch and take care not to wound the tops while cultivating. When selecting varieties avoid white and thick-necked varieties such as 'Sweet Spanish', which are more susceptible to the disease.
To harvest and cure onions properly, wait until 75% of the tops have fallen over naturally, gently dig/pull all the onions, and store them in a dry, shady place with good ventilation, such as an outdoor shed or barn, for 10 days to two weeks. After the onions have cured, put them in slatted crates or mesh bags and store them indoors in a cellar with low humidity and temperatures between 33F and 45F.
Q&A Library Searching Tips