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.

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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.
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