From the base of my blackberry vines grew a thick stalk 5 times the size of the other vines, more like a sunflower size. The stalk has similar leaves but much bigger than the leaves on the vine. There have been no flowers for fruit just a thick stalk. Do you know what this might be?
Certified GKH Gardening Expert
It sounds like you may have witches broom. This article will explain more about the condition: