Pucciniastrum epilobii
Pucciniastrum epilobii is a fungal rust that affects Willowherb species (including Rosebay Willowherb) and Fuchsia species - it alternates on Pine species. On Willowherb it causes yellowing of the upper surface of the leaf whilst the underside bears the orange uredinia.

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. The species of host plant should also be named.
Wherever the host plants occur.
When the host plants are in leaf.
Widespread and fairly frequent in Britain.
Status in Leicestershire and Rutland not known.
Leicestershire & Rutland Map
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MAP KEY:
Yellow squares = NBN records (all known data)
Coloured circles = NatureSpot records: 2020+ | 2015-2019 | pre-2015