Podosphaera clandestina is a powdery mildew on the leaves of various trees and shrubs in the Rosaceae, including Hawthorn (Crataegus).  The chasmothecial appendages are branched at the tip, and do not have swollen bases.

Phyllactinia mali can also affect Hawthorn and other host species; the chasmothecial appendages are straight with a swollen base.  Podosphaera leucotricha can affect Pear and other hosts.  Other powdery mildew species affect other shrubs in Rosceae.   Refer to Woods et al (2024) for more details of hosts.

host/microscopy

Photograph the mildew and chasmothecia on host plant, and add host species to the comments for your record.  Microscopic examination or verification by an expert before submission to NatureSpot will be needed

Certain trees and shrubs in Rosaceae, including Hawthorn