I have also noticed this - (my potatoes are Gourmande). My neighbour thinks it is to do with the gross humidity. It does look like salt & rubs off ok - I am wondering if it is salt!
Judi.

Thanks for your advice Primrose & Geoff - I am going to give it a trim & hope for the best. Its not a fancy one - just very old.

I know this is probably not the right place for this question - but I am having trouble finding it! Anyway, a lilac tree of quite a good age blew down earlier this week & we have dug a large hole, added rotted manure & replanted it in a different place. As the roots were severed at about 30c...

I planted summer onions next to broad beans last year, & they got a sort of rust type thing on the leaves &some of the pods went rotten! I have no idea if this was to do with the onions or the fact that the seeds were from the year before. The onions were fine.