Problem with growing onions

Hi all,
48 hours ago we had a storm here - very heavy rain sheeting/lashing down.
Today I visited my allotment to find that some of  the leaves of my well-established onions have turned black and have become very limp.  They have also become very grainy and seem to have been sprayed with soil.
My first thought was that the leaves are suffering some sort of fungal foliar disease.  But this has happened very quickly - almost overnight.
However, I am also thinking that the problem might have been caused by the very heavy rain splattering down, whipping up the soil to cover the leaves and causing them to decay.
My question is whether this problem is due to some disease or is heavy rain is the culprit.  What do you think?  In either case, what action should I now take?

Thank you in advance,

Liam

did you use fresh manure before planting

Thank you for your advice.  Here are some photos.

Hi All
Thank you for comments and advice.  Here are some more pictures of the onions.  For the first time I prepared the soil by using green manure, a mixture of rye and vetch.  The onions were doing very well until we had the storm, so probably that's the cause.  But the leaves of many turned black and withered within a day or so.  I have removed most of the damaged leaves and hope the onions will recover.  But in the photos you can see the amount of soil which has been splattered on to the onions.  Also, the last picture shows one of the damaged leaves.

Thanks again for all the help.. Liam