By Angusross
United Kingdom 
ONIONS GROW SMALL THEN GO MOULD IN POLYTUNNEL AND OUTDOORS WHAT IS PROBLEM
WHEN PLANTED THEY GROW TO ABOUT 2"THEN MOULD GROWS ON THEM THE SAME THING HAPPENS OUTDOORS ALL OTHER VEG GROWS OK I HAVE TRIED DIFFERENT VERIETES THEY ALL GO SAME WAY
25 Oct, 2012
Answers
white rot.....
25 Oct, 2012
If they go really squidgy and rotten, as Scrumpygrat says it is likely it is white rot. If so, you will have problems with all members of the onion family. Try replacing a whole area of the polytunnel bed with new soil (topsoil bought in) and see if that produces different results... but the fungus will eventually spread to that too.
26 Oct, 2012
or
http://www.armillatox.com/
26 Oct, 2012
Probably poor air circulation - could you expand on the problem
25 Oct, 2012