A powdery mildew affecting roses, including Rosa canina and hybrids/cultivated varieties of rose grown in gardens.  Leaves, stems, flower stalks, calyces and petals masy be affected.  It may also affect Forsythia and some species of Prunus.  

Several other powdery mildews can affect Prunus species - see host-lists in Woods et al (2024) more more details. 

Rose/Forsythia Prunus (microscopy)

Photograph the mildew on the host plant, and add host species to the comments for your record.  If recording on Prunus, microscopic examination will be needed to separate from similar species.

primarily on Roses

It is not considered to be a gall-causer, despite occasionally causing some leaf or shoot distortion (ref.: Redfern, M & Shirley, P.  2023.  British Plant Galls. FSC)