Any ideas why the plant is the linked picture seems to have been nuked? All the others are fine. My only idea is that it could have been a mole - their hills are very close to this raised bed. Or could it be something else?
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Thanks in advance for your help,
I'd dig it up and have a look at the tuber and the base of the stems.
As I wrote this post I was thinking to myself, "I should just dig it up and take a look".
It does look indeed as if a parasite has had a go at it - the surrounding plants look so fresh and strong - get it before it spreads.
Well I dug it up and there were two or three small potatoes that were rotting mush. Found a leatherjacket - could that have been the culprit?