Hampshire,
United Kingdom 
this year has been very dry in the New Forest area were i have an allotment, the onion sets that were planted (4) varieties all have stems that look as if they are rusty, and about half the size as normal.
21 Jul, 2010
Can you post a pic for a better idea of what is happening. Do you water?
If it is Onion/Garlic rust, there will be small yellow-white dots all over the leaves and top of the onions.
22 Jul, 2010