This is my second year to grown onions they have grown very well, I lifted them in September and but them in a shed with doors now they are all rotting from the stem down. What can I do or what am I doing wrong?
It is most severe when onions have been grown on ground that is too rich with too much organic material that tends to make them soft.
Apply some potash to your soil and try to grow in well-drained, less rish soil and in full sunshine, with good air movement. If the soil is heavy, grow on drills to improve the drainage.
This is mainly a seed-borne disease, the spores being on the seeds and sets. You could try a different source or a seed dressing.