My row of Natchez blackberry is experiencing some mysterious die back. These are primocanes, the floricanes have been pruned out for some time. About 6 weeks ago the leaves begin to turn purple, then a red-yellow, and eventually turning brown and dying off. This condition has been systematically moving up the main trunk of the plant from the base. Surprisingly, the laterals seem unaffected. We haven't had any cold weather to speak of. It has been hot and dry, but I've been trying to give them a good drink on the weekends. I also did a soil test and found that my pH shot up to about 7. I am thinking it could be Nitrogen deficiency because the old growth being affected most or Iron deficiency due to the high pH. I put some sulfur down and watered with about a gallon of water with a couple tbsp of vinegar, and I also put down some 10-10-10 fertilizer. Has any one experienced this before or notice any tell-tale signs in the attached photo?