Hard to tell, but if the red dots have a white centre then it is one of the Spot fungus. Eventually the centre of the red dots will drop away leaving a hole. No treatment as it is not harmful to the plant.
Or it could just be the weather!

I'm sure I've seen this on mine before now and never thought anything about it. I wouldn't worry.

Thanks for the replies :-))

As it is only the one plant that is affected I don't think it can be the weather which also occurred to me

It's not spots either so I think we can rule out the spot fungi as being the culprits.

I think I will wait & see what happens to the plant. I didn't notice any other Rhubarb plant between Gerry's plot & the others I passed on my way to his plot. Not that I was looking particularly! But most plots have a plant or two. I shall have to pay more attention the next time I go down to see if there are any more like this.