The spore sizes seem a bit big? (Me = 31.2 x 24.1 µm ; Qe = 1.3)
Ellis and Ellis have 18 – 25 x 15 – 21
Wilson & Henderson 18 – 26 x 15 – 22
Bioimages M. Stor 26 – 28 x 20.5 for one example
And an 1888 book 19 – 26 x 17 – 20
Given that, I can’t think of anything else it could be.
Thanks for this. Unfortunately, unless these are conidial spores(?) they do not match P. obscura or P. luzulae (the two species in E&E on wood rush). Nor do they match P. poarum. Info on P. obtusa is proving elusive (two further species which have been recorded by the BMS on wr). Of these four rusts I have found info on three of them and they all have septate spores.