I dug my yellow semp up today and its definitely root rot. We've had an awful lot of rain this summer and I guess some are fairing better than others. It could be that some of the roots were damaged when I transplanted them into the bed because they were all cramped up in a small pot and the roots were all tangled together. I've cut the root out as far as I can and have left it to dry out before re siting it, but I don't know if its too little too late.
