Studies were carried out to assess the potential of the plant pathogenic fungus Ascochyta caulina as a mycoherbicide against Chenopodium album L., a worldwide weed in arable crops. A. caulina is associated with necrosis of leaves and stems of plants belonging to genera of Chenopodium L. and Atriplex L. The research of this thesis was focused on recognition of factors that may enhance pathogenicity and determine the impact of the pathogen not only on its target host (growing both alone and in competition), but also on a range of temperate crop species. These studies showed that formulation of A. caulina spores with nutrients, especially V-8, and surfactants resulted in faster and greater in vitro germination. During an investigation of in vitro pathogen growth, it was observed that A. caulina grows most prolifically in the nutrient rich liquid medium, V-8. Simple nitrogen and carbon sources also induce a positive effect on the pathogen's growth. Nutrients also promoted the pathogen's enzyme activity of a-amylase and cellulase. The in vitro and in planta enzyme activity varied significantly between A. caulina isolates and plant growth stage affected the a-amylase activity but not cellulase. Observations of A. caulina on detached leaves and whole plants of C. album and several crop species, showed that the surface deposits on C. album leaves are part of a defensive mechanism, and that the pathogen can develop pycnidia on the dead leaves of the non-host plants. A. caulina caused no infection symptoms on any of the crop species. However, under some circumstances it reduced the dry biomass accumulation of some of the non-target plants. Results of the three-way interaction between Beta vulgaris, C. album and the potential mycoherbicide A. caulina showed that the pathogen can reduce the competitive effects of the weed over B. vulgaris by suppressing the growth of the former.