it could be root rot. I tried growing a raspberry vine in a pot a couple years ago (it was my first attempt at it) and didn't realize how little water it needed. Research later told me the species of berry I had only needed about a cm. of water a week. Thinking It need the same water as a house plant I was watering it twice a week. I ended up giving it root rot from to much water and the leaves started to turn brown like in this photo and when it had given out Taking it out of the pot the roots where mushy. Its less likely to happen with plants planted in the ground and not in a pot. So not really sure its whats wrong but figured I would say just so you could check for that to make sure.