A powdery mildew associated with trees and shrubs in the Rosaceae family, including Hawthorns (Crataegus). Blackthorn and Cherry (Prunus). 

The fruiting bodies are called chasmothecia and produced on the mycelium on the underside of the leaf, visible as minute spheres later in the season.  These are yellow or brown when developing and black when mature.  The chasmothecia appendages of Phyllactinia are straight and needle-like and swollen at the base; they are just visible under low magnification.  This is the sexual stage, with ascospores developing inside the asci with the central spherical part of the chasmothecium.   To see them clearly, x100 magn. is recommended.

In the asexual stage, conidia are produced.

Several other powdery mildews affect various tree species in the Rosaceae; refer to Woods et al for details.  Other Phyllactinia on different host species

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 may be needed

Various species of trees and shrubs, including Hawthorn, in the Rosaceae family