I hope you get a reply. Mine act and look just like yours(Heritage Raspberry) My soil is 5.5 and My Acidified water is aprox. 6.0.
I have another the same age and height in a 5 gallon pot right next to it that looks fine and both pots have canes that are about 3ft high.
I have been giving them Miracid for about 2 weeks and the growth is great. They are both in a greenhouse that isn't exactly insect proof. I was giving them Ammonium Sulfate and the PH was getting too low, hence the change to Miracid.
Have not noticed spider mites, thirps or any other insects and I go out every night to check
I have had mine for 5 years and this is the first time this has happened.

I haven't been checking soil ph or anything like that. The younger plants right next to them are fine (for now), but it's spreading so I don't know how long they have.

I've been checking for bugs and haven't been able to see anything.

These canes are in their third year in this soil.

I was of the understanding that lime sulfur should only be used on dormant raspberry plants. If you sprayed your plants with lime sulfur recently, you may have caused further damage to the plant.

It sounds like your original problem may be Phytophthora Root Rot, especially if you have had a lot of wet weather recently. Heritage raspberries are not good with wet soil, at least mine did not live long in it. I moved all of my raspberries into raised beds and they have been doing a lot better. There are also different raspberry varieties that tolerate wet soil better too.

Link below.

Here is a link that might be useful: Phytophthora Root Rot

Ours are all Himbo-Top raspberries so they are supposed to be resistant to root rot, but that fact sheet describes to a T the behavior.

We're also cursed with really heavy soil here so that is sounding like the issue. I really appreciate the information.

Do you think it would be possible to dig them up and dig out the the whole bed and replace it with loose potting soil as a fix?

Or are my plants now infected and should they be discarded?

Unfortunately "resistant" isn't immunity. There are fungicides that can control Phytophthora root rot but if your plants are already dying, they may be beyond the point of saving. I'm including another link with a list of fungicides that may help control it. Ideally, I would suggest checking with a local nursery or your local agriculture extension office for advice that is relevant to your locale.

I have heavy clay soil which drains poorly, so I had to build raised beds that are 6-8" above the surface and use soil amendments, like peat moss, that allow better drainage. I grow 8 different types of raspberries successfully now by growing them in raised beds.

Let me know what you do.

Here is a link that might be useful: Control of Phytophthora

If you have another spot in your yard to restart your berries with new plats that would be best. I would not try to fix the soil. Your new softer soil will hold water like a bowl as the surrounding dirt is slow to absorb. Can you mound up the new spot for them? Not a lot, 6" or so? I realize you don't have dirt just sitting around, but may be able to get some from a construction site or whatever. Maybe a friend with a farm? Or redo part of your yard to create better drainage. Or you can do a raised bed with the whole potting soil mix. I'm going to be digging out a bunch of Heratage that have overgrown their area. could send you some in March or April