Puccinia millefolii

A rust affecting leaves and stems of Yarrow Achillea millefolium.   There is no host plant alternation, and only dark brown telia on yellowed spots are formed.  Teliospores are two-celled, on a short pedicel.

other species in the Asteraceae are affected by different species of rust.

host

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments. 

Yarrow, grassland

Apart from host, the rust is very similar to Puccinia cnici-oleracei which affects some species of thistle, and some sources regard them as conspecific.  See Plant Parasites of Europe account