Causes of this type of damage include winter injury, wind damage, crown borers, and rodent damage.
Winter injury damages floricanes but not primocanes. Varietal differences are apparent. Often injury occurs in March when temperatures fluctuate, rather than in mid-winter. Occasionally, winter injury is not apparent until fruiting laterals begin to grow. With the onset of warm temperatures, the injured vascular connections cannot supply the laterals with water, so the laterals collapse.
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High winds can break floricanes at their base, causing them to wilt and die. Usually affected canes can be pulled from the crown with a quick jerk.
Crown borers damage floricanes at their base, causing them to wilt and die. Look for burrows at the base of canes and in the crown.
Adult crown borers are clear-winged moths that resemble yellow jackets.
Crown borer eggs
In this case, rabbits ate the bark of these thornless blackberries during winter, girdling them.
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This is a part of The Berry Diagnostic Tool - a companion to the NRAES Production Guides for Strawberries, Raspberries and Blueberries