Spotted Snake Millipedes

are there any white (cotton wool looking) growths on the bulbs? or possibly large areas of small black seed-like growths? if so then you have allium white rot (which has just destroyed my crop of red, white and shallots)

if not, i guess it could be the millipede's. have you check the RHS website?

Your original post doesn't quite make it clear (to me!) whether your whole allotment site has this problem?

Don't mix up millipedes (of which the spotted snake probably causes the most problems in this country) and centipedes, which are generally carnivorous predators so are usually regarded as "good guys."The spotted snake millipede is considered to be a pest of potatoes but opinions seem to vary as to whether it attacks undamaged roots and tubers or whether plants have to be damaged by something else first (often slugs in the case of potatoes.)

They can't actually cause allium white rot though, so if you've found evidence of that, I'm afraid it's probably that which is your main problem, although I would have expected to see visible evidence on all your affected plants.