If you want to keep a close eye on your onions, this is what I did before I quit growing them.
When the bulbs are starting to become usuable size and you see a tilting one, give it a little tug, if it comes out too easily, you may notice that there are not enough roots to keep it upright. On close inspection, you may see white powdry stuff around the base of the bulb.............panic time!
After you have collected yourself, you may want to gently tug at all the onion tops, harvest the ones that come out too easily and leave the rest. You can use the affected ones but like it was said before, they don't keep very long.
The bulb may look fine as Tim has demonstrated but when there are hardly any roots, something is wrong. I guess the white rot gets the roots first and then goes up into the bulb, sometimes the rot is not even visible on the outside of the bulb.
