Powdery mildew genera are grouped into five tribes:
Phyllactineae, Erysipheae, Blumeriae, Golovinomyceteae, and Cystotheceae.
The tribes can be identified by several criteria, including the type of host plant (Ex. Phyllactineae infects trees and shrubs), and by microscopic examination of their chasmothecium and appendage morphology, as well as shape and structure of spores.
Cindy and I collected took Pee Em from a highly infected Lilac bush and brought it back to the lab. The dirty little %$#@ fungus wouldn't talk tho, and that's when things got a bit rough. We made the parasite spill its guts, all of them.
Host plant (Lilac) and appendage morphology ("spines") positively identifies genera as Phyllactinia. Organism is most probably P. fraxini, which is known to infect Lilac leaves.