Bluebell Rust (Uromyces muscari) affects various Bluebell species and causes yellowed areas on the leaves filled with dark brown/black telia.   There is no host-plant alternation, and only telia are known.

Note that Bluebell Rust is given the scientific name Uromyces hyacinthi in other parts of Europe, but this is currently treated as a synonym of U. muscari in the UK.

host

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

Bluebell leaves, including hybrid varieties naturalisedin gardens, roadsides and woodlands, etc.

March to May