Three species of Drechslera were isolated from ryegrass; D. dictyoides f. sp. perenne; D. siccans and D. nobleae. The species were distinguishable by taxonomic characters; conidiophore length, arrangement of conidia, conidial shape, the manner of germination and cultural characters; morphology and growth rate. D. dictyoides was characterised by tapered conidia produced at intervals on the conidiophore. Germination was predominantly from the basal cell of the conidium and growth in culture was slower than that of the other two species. The life history of D. dictyoides on L. perenne was investigated using optical and electron microscopy. Changes in the ultrastructure of the host and pathogen were followed during infection. The vegetative fungal wall consisted of two layers but modifications, usually involving reductions in one or more of these layers, during infection were important in the successful establishment of a parasitic relationship with the host. Appressoria formed predominantly over the anticlinal wall of the host epidermis but germ tube length was variable. Penetration was usually into an epidermal cell but subcuticular infection was common. In the period of lesion development, hyphae spread intercellularly with limited host reaction until host senescence began, at which time intracellular hyphae were again found. D. dictyoides was interpreted as a hemibiotrophic pathogen and formed a close nutritional relationship with its host. Intrahyphal hyphae were occasionally found within lesions. Conidium development was interpreted as holoblastic and discussed in relation to theories concerning 'blastic' ontogeny. A field trial indicated the presence of a wide variation in resistance of perennial ryegrass lines to net blotch. Drought conditions were experienced during much of the field trial, but increases in disease intensity usually followed periods of rain, with a major increase occurring in March 1982. The ryegrass epidermis was examined in relation to the effect on net blotch development. Epicuticular wax projections were present on the adaxial surface but absent on the abaxial surface and this resulted in greater spore accumulation on the latter. In contrast, the outer wall of the adaxial epidermis was approximately half as thick as that of the abaxial epidermis. Given sufficient moisture, penetration of the adaxial surface was more rapid than that of the abaxial surface. Host nuclei migrated towards the site of attempted penetration of the leaf and cytoplasm accumulated in this region. Glasshouse and laboratory assessments of the effect of leaf age showed that infection was greater on older leaves. The interaction of exposure time and leaf senescence was responsible for the observed differences. Infection was possible over a wide temperature range but the optimum was close to 20°C. The Lolium endophyte was present in most perennial ryegrass plants but appeared to have no affect on net blotch disease even when hyphae of both fungi were closely associated within a leaf lesion. Spore accumulation on the leaves of susceptible trap plants was greater than on those of more resistant lines, resulting in a higher density of infections on leaves of susceptible lines which was compounded over several disease cycles. This appeared to be largely responsible for the difference in disease intensity between lines in the field.