Onions rotting from the inside
I grew Candy, Copra, and Redwing onions this year. The Candy and Redwing each had a relatively short shelf life, as expected. The Copra, however, are supposed to be better keepers. What we have seem to be holding up relatively well--little sprouting and fairly solid. However, when you cut into them the insides are rotting with usually only a layer or two, on the outside, that is any good.
Any ideas as to what is causing the problem? Any way to prevent next year?