Posted: 10/02/2013 at 12:28
If faced with this problem I would plant new canes in a clean plot. You will never get rid of the couch grass in one season and if you are re-planting raspberry canes you will have to cut them to the ground anyway - or you should - unless they are autumn-fruiting, when they would be cut down without missing a season of cropping.
If you have just taken over the plot you will not know the variety, how old they are or whether they have virus. They may have also exhausted the place where they are.