I'm not an expert, but I would look around the edges for new offshoots that you can dig up and move to a new spot. Now is not likely to be a good time for that, but you could start work on a couple of new spots, preparing them for the raspberry transplants going in this autumn.
If the problem is not a disease, it may just be that they have depleted some nutrient that they love. This is one of the places where polyculture is a plant's friend: different plants have different needs, and they share with each other. Think about how you might guild your raspberries, both this patch and your new (smaller) patches.
If you have a brush mower, you could try going right down the middle, clearing that out and then piling on some
compost in the alley you made. This might reinvigorate the younger canes on either side.
Monoculture tends to lead to problems, I recommend diversifying your plantings.