The overall aim of the research project was to evaluate the potential of the fungal pathogen Ascochyta caulina as a biological control agent against Chenopodium album, a major weed in arable crops. A number of isolates of Ascochyta caulina were evaluated but isolate W 90-1 from Holland proved to be the most promising candidate because of its high virulence. It was therefore selected for more detailed greenhouse and field studies into the environmental parameters required for infection and disease development. Results of these studies showed that in order to achieve the maximum infection, a temperature of between 20-30°C, a relative humidity of >95% for 24 h and a spore density of approximately of 1-2 x 10⁶ spores per ml spore suspension were required. Mortality and plant necrosis levels after application of A. caulina decreased with plant age and treatment of C. album shortly after emergence or to juvenile plants (before 4-leaf growth stage) was most effective. The requirement for long periods of high relative humidity and the inability of A. caulina to cause satisfactory disease after the 4 leaf growth stage are the most important limiting factors for the development of A. caulina as a bioherbicide for C. album. A range of spore formulations was studied with the aim of reducing the requirement for long periods of high relative humidity for disease development. Studies indicated that disease development could be increased by incorporation of surfactants (Tween 80 or Sylgard) and nutrients (Czapek-Dox Broth and Yeast Extract) into inoculum suspension. Results of field trials indicated that if application were properly timed and optimum environmental conditions can be achieved the pathogen can give satisfactory control of the weed.