Probably Puccinia caricina var, ribesii-pendulae which seems to be very common on Carex pendula at the moment.
If you examine the underside (abaxial) surfaces of enough leaves, you may find a few spots where sori have burst through the leaf epidermis, and dark-chocolate-brown teliospores can be seen.
See this post: – http://fungi.org.uk/viewtopic.php?f=9&t=3
Regards,
Mike.