Looks like a fungal infection to me. Rust?

Someone else will be better informed than me.

it is red spider mite! spray the plant with insecticide on the underside of the leaves as that is where they like it most,if there is no improvment check the roots and see if there is any other parisite there (if there is a bug in the roots remove or soak potting mixture in malathion). but if there really is no improvemet then throw the plant away.i suggest you start now as red spider mite must be dealt with as soon as possible!!

Hi, it isn't red spider mite, it is probably 1 of 2 types of fungi, that rhubarb plants are host to, which are; ramularia rhei, or ascochyta rhei, have a look here,
http://www.extension.umn.edu/garden/yard-garden/vegetables/rhubarb-leaf-spot/
Derek.

Thank you Alexth I think it is a fungal. I would also like to thank Wildifebal I looked up Google on red spider mite they said to check under the leaf and see if you can spot them, they said they should be coloured orange at this time of year, and try spraying with soapy water.

Thank you Derek I have looked up the website you said, and I think you have hit it on the head.

A very big Thank you to all for taking the time to answer me

Johnjoe.

Wildlife, Malathion has been withdrawn from sale. If you still have any use it with extreme caution and don't get any on your skin. I got some on my hands and finished up in the accident unit overnight.

It's oxalic acid burning the leaves. Rhubarb produces a lot of it and gradually burns its own leaves.

Thank you Alanharmony...,