Rhubarb disease/stress/other?

Why does the rhubarb on the left have these red, rust marks? I have 3 plants and this is the only one affected. They were planted in the ground last year, having been in very large pots for a year beforehand. All grew flowers this year, so I assumed they were happy, but this one clearly isn't. It was growing well until a month ago. Any ideas?

 

In my experience it’s best not to allow rhubarb to flower. You should break and pull the flower stem away at the base as soon as it appears. It uses too much of the strength of the plant and stresses it … I think the stress has made yours susceptible to rust.

In fact if a rhubarb plant is really happy it may well never flower. Mine is about 12 years old and has never flowered.

I would give yours a good watering if it’s at all dry at the roots, (and even in wet spells the leaves act as an umbrella and the roots can be quite dry so check thoroughly) and a mulch … and then leave it alone and not pick any more this year so the crown can build up strength … Fish. Blood & Bone and a good mulching again in the autumn and hopefully you’ll have some good strong stalks for your crumbles next spring.

 

Thanks @Dovefromabove. Yours looks fabulous! I should have said, at any sign of a flower, I did cut them off straight away. None got larger than a 50p coin size and before any had actually opened and flowered. I was worried it might be a disease, but just on one. I shall keep an eye on water on nutrients. thanks.

 

My rhubarb plant has never flowered either. Presumably they only do it when they suspect that the end is nigh.

 

Update- Since I took the original pic of the rusty rhubarb earlier this month, it has since grown regular, green, unaffected leaves! It looks the same as the neighbouring rhubarb now. I'm still unsure what caused the 'rust' but its happy and healthy again. Hooray!