Triple crown blackberry has almost stopped bearing
I planted a triple crown about three years ago. The second year after planting I put away 10 jars of jam, the next year 1 and this year there are not even enough berries to fill a bowl (there were fewer blossoms as well). The plant has some leaves that look sort of burnt and frilly / crenellated, but otherwise the plant looks fine. Is this a virus? Is it not getting enought light/ nutrients (I compost it every spring). I can't be certain, but the plants were from a reputable mail order nursery in the North West and were labeled virus free.
What do I do now? I really enjoyed the berries the couple of years the plant bore them.