That is the beginnings of a flower, so it has bolted. Stress of some kind can lead to Rhubarb going to flower, in an attempt to set seed before dying. Too little water is as likely this year.

Hi, welcome to GoY, it's run to seed, or 'bolted' this can happen with older plants, or if we have a mild spring, Rhubarb prefers cooler conditions, cut out the stalk as close to the base as possible, using a clean sharp knife, and keep it well watered, cut it as soon as possible, the longer you leave it, the more it saps the strength of the plant, and the longer it will be before it produces edible stems, Derek.

Sorry Owdboggy, nothing here when I started,? , Derek.

Would also suggest that the 'crown' or 'stool' needs to be move away from the fence where it is in a rain shadow...