Erysiphe pisi is a powdery mildew which grows on members of the clover and vetch family, Fabaceae.  There are several varieties, depending on host. 

Erysiphe trifoliorum also occurs on Clovers, Bird'-foot Trefoils, Melilots and various vetches and vetchlings.  Erysiphe baeumleri occurs on Tufted Vetch and Hairy Tare.  See Woods et al (2024) for full host lists.  These species are difficult to tell apart, even with microscopic examination

host

Photograph on host plant, and add host species to the comments for your record.  Check host in Woods et al (2024); depending on host, microscopic examination or confirmation by an expert before sending to NatureSpot is needed to separate from similar species. 

On various species in the legume family, Fabaceae, including Hairy Tare, Smooth Tare, Vetchlings, Medicks, Red Clover, Common Vetch, Bush Vetch and Broad Bean - see Woods et al (2024) for full host list.