A powdery mildew found on blackthorn and plum/damson (Prunus spinosa and P domestica).  Conidia have fibrosin bodies, and the chasmothecia have a few long appendages branched at the apex.  The appendages all arise above the 'equator' of the chasmothecium and under the microscope they tend to appear in a cluster all on one side of the spore.

Several other species of powdery mildew occur on blackthorn and plum/damson; microscopic examination is needed.  See Woods et al (2024) for full host species list and notes regarding identification.

microscopy

Photograph the chasmothecia or mycelium on the host and identify the host plant in your notes; it is important to identify the host to species level.  Detailed photographs of chasmothecia will be needed for verification.

Prunus spinosa and Prunus domestica