A powdery mildew affecting Hogweed and other species in the umbellifer (Apiaceae) family. The sexual stage produces fruiting bodies called chasmothecia on the mycelium on the underside of the leaf, visible as minute spheres later in the season ( (in older sources, these are sometimes called cleistothecia)   These are yellow or brown when developing and black when mature.  The appendages are numerous and branched.  In the asexual stage, conidia are produced on the mycelium.

host

Photograph the underside of the leaf, showing the chasmothecia, and you must note the host plant species

Plants in the Apiaceae (umbellifer) family, including Ground-elder (Aegopodium podagraria), Fool's Parsley (Aethusa cynapium), Cow Parsley (Anthriscus sylvestris), Angelica, Water-parsnip (Berula erecta), Chervils (Chaerophyllum), Hemlock (Conium maculatum) Wild Carrot (Daucus carota), Hogweeds (Heracleum), Sweet Cicely (Myrrhis odorata), Wild Parsnip (Pastinaca sativa), Burnet Saxifrages (Pimpinella), Upright Hedge-parsley (Torilis japonica).