A taxonomic study was conducted of Marssonina species pathogenic to poplars. From descriptions of conidial morphology in the literature and examination of type material 3 species were recognised viz: M. populi, M. castagnei and M. brunnea. Study of worldwide collections of 280 herbarium specimens confirmed the existence of the three species and further established the validity of type specimens as species representatives. The stability of selected differential taxonomic criteria was evaluated both in the laboratory and field under varying environmental conditions. Subsequent comparative morphological studies under defined laboratory conditions established that features of conidium morphology (shape, mean dimensions and septum location) were valid differential taxonomic criteria. Further, the existence of 'large-conidium variants' was demonstrated. Microconidia were of no value in species delimitation. Host range and pathogenicity tests revealed two host specific forms of M. brunnea: (i) M. brunnea f. sp. trepidae pathogenic to P. tremula and P. tremuloides, (ii) M. brunnea f. sp. brunnea pathogenic to P. deltoides and P. X euramericana. Conidium and microconidium ontogeny of the three Marssonina species was annellidic and not phialidic as previously reported. A taxonomic study of Drepanopeziza species pathogenic to poplars established the synonymy of D. tremulae and D. punctiformis and the close morphological similarity of D. tremulae, D. populorum and D. populi-albae. Production of apothecia of D. tremulae was induced in the laboratory and the rate of maturation shown to be temperature dependent. Incubation temperature had no significant effect on dimensions of asci and ascospores. Seed transmission studies established that M. brunnea was transmitted on imported poplar seedlines as a contaminant. Seed-borne contamination was effectively controlled by dusting seed with a number of fungicides, the benzimidazole derived compounds being particularly effective. Studies on the pathogenesis of Marssonina species to poplars showed that germtubes penetrated poplar leaves directed by enzymatic activity, the infection peg being naked. Within host tissue hyphae ramified indiscriminately and completely disrupted cellular contents. The resistance of leaves to infection increased significantly with maturity, this being attributed to the ultrastructure of the mature cell wall. In depth host range studies of New Zealand and overseas isolates of M. brunnea established the wide host range and strong pathogenicity of this species. In many instances gross differences in pathogenicity were revealed between isolates. The solution to this disease as seen in the breeding of resistant material is discussed.