Ultrastructure of the host-pathogen relationship in Entomosporium leaf spot disease of Photinia

Entomosporium mespili appears to be a hemibiotroph on infected Photinia lea ves. This fungal pathogen produced distinctive haustoria in living host cel ls in young lesions. Each haustorium possessed a long slender neck with a s ingle septum and an enlarged distal body that contained a single nucleus. A collar of host cell wall material was associated with the haustorial neck. Intact haustoria also were found in necrotic cells of older lesions. Howev er, by this stage of disease development, the pathogen also possessed an ex tensive system of branched, septate hyphae that grew indiscriminately betwe en and through dead and dying host cells. These hyphae eventually gave rise to a subcutaneous layer of sporogenous cells that formed conidia.