Even without looking I can tell you that this is Rust.
What you can see are the fruiting bodies full of spores. Remove all the
leaves carefully and destroy.
If you only have a pot or two I would either tip the pot upside down and
remove the top inch of compost, or better lift the bulbuls and rinse them
in a fungicide and replant in fresh compost, then spray every couple of
weeks with a fungicide to prevent the rust taking hold again.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk