A white mould occuring on the leaves of some umbellifers, including Hogweed.  (See Chater et al (2021), as R heraclei, for a full host-list)  Discoloured leaves and dark leaf-spots and mottles are present.  Grey-white caespituli can be seen emerging from the spots, mostly on leaf undersides.  Conidia are oval or ellipsoid and formed in chains.

Other Ramularia species are found on umbellifers; check host carefully

host

Photograph both sides of leaf; note host plant species in your comments.  

Umbellifers, including Hogweed.  Also reported from Wild Parsnip, Fool's Watercress and other species - see Chater et al (2021) for a list of host plants on which it has been found

This may be a complex of species - see Chater et al, 2021, under R heraclei.