Cytology of the host-pathogen interactions between Lolium perenne and Drechslera dictyoides

The interactions between Drechslera dictyoides and Lolium perenne resulting in the development of the net blotch lesion were examined using light and electron microscopy. Following appressorium formation a reorganization of the network of cytoplasmic strands in underlying epidermal cells resulted in the accumulation of cytoplasm and the migration of the nucleus towards the appressorium. Successful penetration was followed by extensive colonization of the invaded epidermal cell. Abortive penetration attempts often resulted in the formation of hyphae within the epidermal cell wall. During lesion development, advancing hyphae came into contact with unmodified host cells which responded with the production of wall appositions. Hyphae were sparsely distributed within the developing lesions and their sparingly branched, predominantly linear pattern of growth resulted in the formation of discrete net lesions. Prior to the commencement of sporulation, the density of hyphae increased and the invasion of moribund cells of the senescent host tissue was observed.