A powdery mildew not well recorded in Scotland but probably overlooked. It occurs on many species of woody Rosaceae.

References: 
Braun, U. & Cook, T.A. (2012). Taxanomic Manual of the Erysiphales (Powdery Mildews).
Ellis, M.B. & Ellis, J.P (1997 new enlarged edition). Microfungi on Land Plants: An Identification Handbook.