Perhaps crown root rot? If so your plant should be discarded and start a new plant in another location in disease free soil. Rhubarb does best in a soil with lots of organic matter (composted manure) but needs to be well drained. Try planting your rhubarb in mounded soil. If the roots get too wet and stay that way too long, you will get rot -- a fungal disease.
a rust fungue
Rhubarb is ripe when the stalk is a dark red. If its still mostly green, it's not ready to be picked yet, unless you want it really sour! Actually, there are two types of rhubarb and several varieties of each type. Red rhubarb emerges with a red stalk that is completely red. It's ready to pick whenever you're ready to pick it, and a rhubarb that is constantly harvested will continue to send out new stalks until it gets too cool at night. Green rhubarb emerges green and will never turn completely red, no matter how long you wait. Some varieties will have a reddish base when the stalks get very thick. Both varieties do not get 'sweeter' as the stalks get bigger.
Rhubarb actually comes in several color varieties; from green to speckled pink to ruby red. It is safe to eat the stalks of green rhubarb, but never eat them if they have been frostbitten, and never ever eat the leaves of the rhubarb plant, as they are poisonous.
Since the leaves turn red in the fall it is not evergreen... the leaves are dying and that's why they turn red... Soon after the leaves turn red they will fall to the ground.
Red. Anthocyanin turns leaves red. Yes, it is spelled correctly!
That may be the colour of the variety that you are growing, other than that it is not yet ripe
swiss chard and rhubarb
fall
The stem has xylem in it. the xylem carries water to leaves, so the leaves turn pinkish.
Because of lack of Phosphorus
not enough sunlight
If you soak a flower in red dye it will turns the vains inside the stem and leaves red.
It ranges between pink and red.