I put a couple of rhubarb roots in a year ago and they looked pretty healthy. Towards the end of last summer one of the roots started to look a bit less healthy. As new stems appeared the old ones turned brown and died. I thought maybe they weren't getting enough light or air circulation so I moved both roots to a full-sun, good circulation spot. Whatever the problem is, it's back again. Both roots are affected, although one much more so than the other. I've grown rhubarb for many years and never had any problems. Usually it's a very vigorous plant. Any ideas what the problem might be?