Milesina murariae

I have been looking around for a particular rust fungus which affects the diminutive fern, Wall Rue (Asplenium ruta-muraria), to be found quite commonly on old walls quite widely, as well as on limestone.
Having observed recently, that all the fern rusts (Milesina Spp.) seem to be having their season, I thought I would check any Wall Rue I came across, looking for the tiny whitish spore masses so characteristic of the genus Milesina (who says rusts have to look rusty?) and on the first plant I looked at, there they were. A couple of days later, I happened to notice a plant growing in an old school wall, as I walked past. I turned a frond over and there were the tiny white spots. I have since found it at a third site (I have only looked in three sites), so although M. murariae is described as rare, it is probably just uncommon and overlooked, just like a lot, if not most so called rare microfungi.

The white spore masses on the underside of a Wall Rue Pinna

If you want to give the fern rusts a go, then now is the time to get the hand lens out and start looking under those manky looking fronds.

Dryopteris Filix-mas, with the spores of, most probably M. kriegeriana, which is
the common rust found on Male Fern, but it is possible that it might be M.
carpatorum, which is regarded as rare. Only microscopic examination of the
spores will tell.

Uredispores of M. scolopendrii, on the underside a the frond of Hart's-tongue Fern.

Postscript: The Milesina on Dryopteris filix-mas, above, did indeed turn out to be M. kriegeriana.