Our rhubarb seems to be dying. This has been occurring over the past 3-4 years. It comes up as normal in the spring, but the petiole develops slowly, and is short and spindly. The stem is stringy and tough. The leaves yellow, wilt and die, as if it was in a severe drought. I have harvested only a few stems over these past years. (I am enclosing a photo, taken yesterday). It is an old variety, grown in Washington County by my husband’s grandmother. I transplanted it to its current location 8-10 years ago. The soil is sandy loam, and is in full sun from about 11:30 am on. I water it only when it is very dry, fertilize it with 10-10-10 fertilizer in mid-summer, after harvest, and side dress with finished compost at that time. I did transplant some to another location in early spring, and it is better, but does not seem a vigorous as other transplants in the past. (See second photo.) There is a mature black walnut tree not far away. The rhubarb is not in the drip line, but perhaps 10 feet from the drip line. What do you think is the problem?