Cane blight was resulting in poor growth and dieback of this blackberry.
OSU Plant Clinic image, 2013.
See:
Raspberry (Rubus spp.) - Cane Blight
Note: 'Boysenberry', 'Loganberry', and 'Youngberry' canes are seldom infected with cane blight unless canes first are frozen, pruned, or mechanically damaged.