leaves on onions

[cheshire61]

i noticed today that the leaves on my maincrop onions have like a black sooty stuff on them its not from fires as our site don't have bonfires
i have sprayed them with deris dust spray as do not know what it can be i noticed other plotholders are being affected as well as mine
the onions are not ready to lift yet.

any ideas would be most welcome

[Paulottie]

Well, maybe some sort of sooty mould in this weather...or... the worst case scenario is onion smut. One of those burn em and don't grow onions for ages diseases. fingers X'd.