It appears that you could be dealing with one of several diseases that affect cane plants. The only true way to diagnose the disease is a lab pathology work up. I have put together two possibilities with a reading reference. Keep in mind that we have had a moderately cool, wet spring followed by a hot, dry early summer. Perfect growing conditions for blackberries and unfortunately, fungal diseases. - Botryoshaeria Cane Canker or “Bot Rot” - Cane Blight is a fungal infection which can cause cane canker. It is important to clean your trimming shears with alcohol after each use. The fungus can be spread on garden tools. https://hort.extension.wisc.edu/articles/cane-blight/