[Nobbie]

Went lottie today and the onion sets which were looking in full health last week are looking quite I'll. When I looked closely they seemed to be covered in a fine grey/black dust. Any ideas what this is? Nearby garlic had serious rust, but this was orange.

[DD.]

Could it be Aspergillus Niger?

http://en.wikipedia.org/wiki/Aspergillus_niger

[Salmo]

Without seeing the onions it could downy mildew which is the result of damp weather. I have seen a number of crops badly affected. It may be a variety thing as some crops affected and others near to them not affected.

[Nobbie]

I'll try and get a photo next time I'm up the plot, luckily they're a decent size already.