Raspberry problems

Over the past few weeks a number of raspberry canes have died after appearing healthy. They all bore leaves & flowers then the leaves shrivelled so I've cut them down & dug out the roots. There does not appear to be any pest infection & I cannot see any fungal growth on roots or canes. However, the thin outer bark of the canes, just above soil level is split. I thought it may be Verticillium wilt but the canes do not show all the symptoms of this problem. I've taken 4 images on my 'phone so hope all who view this can see them.

 

Could it be lack of water?

 

That crossed my mind but the soil is fairly moisture retentive.