Box Rust - Puccinia buxi

This dark brown rust (Only telia formed) seems to be confined to older Box plants, not affecting younger ones. The rust galls the underside of leaves, and a corresponding bulge may occur on the upper surface. 

Unless identified by a recognised expert, a photo is required. If the photo doesn't show the key ID features then in the comments box describe the size and identifying characters you have observed.

Where mature Box (Buxaceae) plants occur.

Most months of the year.

It is monophagous on Box (Buxaceae), there is no hostplant alternation.

Widespread and fairly frequent in Britain.

Status in Leicestershire and Rutland not known.