Answer: Have the plants been receiving adequate water? If you have had a very dry spring and early summer, as we have had up in the north, then your plants could simply be suffering from drought, especially since they are quite young. On the other hand, if they are sitting in saturated soil, that could cause similar symptoms.
If you are sure water isn't a problem, then I would examine the leaves carefully. Look for spots, powdery patches, bright orange dots, or fine webbing. These signs would help us identify the culprit. Also look for small holes near the base of the cane--evidence of a borer infestation. Keep the plants pruned so they get good air circulation; this can help reduce disease problems.
If only some of the canes are affected, I would prune these back right to the ground and destroy them. That way, whether it is a borer or a disease, you'll be removing at least some of the culprits.
I hope this helps.
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