Wow, that sucks, maybe nematodes, or some root rot fungus? A fungicide for root rot night be in order like Agri-Foss. Once Fungi are in soil they usually don't leave. You could try it. Agri-foss is a systemic organic fungicide. A rare thing. It is Phosphorous Acid, which I would not worry about myself.Pull the plant and inspect roots. If tumors on the roots it's nematodes, if they are just plain dead, it could be a fungus.
Bonide makes a product, and also another product called Organicide or something like that, it is easily available,

This post was edited by Drew51 on Tue, Aug 12, 14 at 16:48

You mention that your area is hot and dry; is it that way now and has been for the last while?

Reason I ask is I live in a fairly dry climate, and during dry spells most if not all my rhubarb goes dormant. Looks like it died, leaves all turn brown and dry up. But it always comes back the next spring, or if we get a sustained wet spell. I generally will water a bit during droughts, but not usually enough to bring the plants back, as I typically am not interested in harvesting rhubarb in the summer/fall.

Rhubarb is pretty hard to kill in my experience. There was one root we transplanted and forgot about when we were moving. Found it in the field 5 years later, it did fine without any water or care from us.

Are you sure your rhubarb is dead and not just dormant?

Have dug down roots have rotted other plants next to them seem healthy now but this is how it has started before I've been growing rhubarb for over 35 years.
I've been thinking if I loose my plants again I might set out plants in separate spots not all together ,I have one plant in a flower bed and has been their for 25 years with no problems.

I hope I am wrong, but it sounds like nematodes. Rhubarb is susceptible to garlic bloat nematode. If you are noticing deformed, twisted or stunted stems along with crown rot, I would strongly suspect nematodes. The plant just doesn't look right. One bad plant in the middle of a new bed also points towards nematodes. Clean stock in clean soil is the only practical solution. You need both. Good luck. Maybe post on the other side of the pond. The Brits may have more experience with rhubarb and bloat nematode.

David