We have been experiencing a growing problem of onions. They get rotting during bulb formulation.

Question

Onion growing in Zambia, Mpika in particular, is becoming a problem.

Answer 1

It may be Fusarium basal plate rot. This article should help:

https://www.gardeningknowhow.com/edible/vegetables/onion/treating-onion-fusarium-rot.htm