spuds,whats happened?

Can anyone shed any light on my query,i have one row of Charlotte,one row of Pentland Javelin,both rows doing exceptionally well.However i also have four rows of Estima,most of which are fine at the moment,however i have had four losses,none in the same rows,the problem begins with limp foliage and progresses to such an extent that the foliage dies back to ground level.This evening i lifted them to see what the problem may be and there was not a single spud below,only the rotting seed potato,these have been in the ground since the third week of March.It is not blight,i know this because i have had it before.

Have alook here at potato diseases database
http://www.potato.org.uk/department/knowledge_transfer/pests_and_diseases/viewer.html

Hi, seems like something burrowing.