A white mould that affects Winter Heliotrope, causing dark purplish leaf spots. Greyish tufts of conidia emerge from the stomata mainly on the leaf undersides.

Butterbur is affected by a different species of Ramularia.  Other organisms may cause similar purplish spots

host

Photograph the leaf spots and white tufts of conidia on the host.  Ensure you have noted the host-plant species in your comments

Roadsides, parks, etc., on Winter Heliotrope leaves