Confidence: It's likely to be this, but I can't be certain.
Notes: I'm afraid I have to disagree with the above ID. Really this is simply a Rumex obtusifolius leaf with red lesions. There is no visible fungus, and without the fungus we cannot be sure what we do have. Puccinia phragmitis lesions are larger and will normally have visible fungus, so I am sure it is not that. While there are other possibilities, such as Venturia rumicis, it is most likely in this case that the causative fungus is a species of Ramularia, which form tiny, white tufts of fungus emerging from the stomata on the leaf underside, just visible with a lens. Much the commonest Ramularia on R. obtusifolius is R. rubella, but at least two other species occur on this host, and unless the conidia have been measured and their septation (if any) checked (i.e. microscope work), a definite record is not possible. In this case the lesions are probably immature. If recording, I would have ignored this leaf and looked at an older one.