Hieracium species are a severe weed problem in the high country native tussock grasslands of New Zealand. This thesis reports on the potential for fungal biological control of Hieracium, in particular with a rust pathogen, Puccinia hieracii var. piloselloidarum. Isolates of Hieracium rust were collected from throughout Northern, Central and Southern Europe, and the British Isles. One thousand four hundred and twenty four isolates were screened on New Zealand Hieracium pilosella to identify the most infective strains for potential use as biological control agents. The rust isolates most pathogenic to New Zealand H. pilosella, were from the south of Ireland. They had a shorter latent period and higher infectivity compared to other isolates. One isolate infected representatives of all New Zealand H. pilosella sites as well as H. praealtum and H. x stoloniflorum. Hieracium rust was common throughout Europe with large seasonal fluctuations. Most dissemination, infection and effect was seen in a main peak in spring and a secondary peak in autumn. The rust could survive through winter conditions within host tissue allowing rapid re-establishment of symptoms during occasional periods of suitable milder weather and, eventually, with the onset of spring. In an intensive field study of one Edinburgh area, the level of rust infection on patches of H. pilosella was found to be affected by several site factors, particularly the density of patches. The infection process of Hieracium rust was studied. Spore germination was fastest in the dark and occurred over a wide range of temperatures. Inoculations of hosts was either on to detached leaves kept on water agar or on to whole rosettes. Infection of detached leaves was generally higher than on whole rosettes and may allow a wider host range of subgenus Pilosella taxa. Infection rarely occurred on all inoculated plants. This was attributed in part to the effect of host condition. A genetic resistance component of the non-susceptibility remains possible although one rust isolate was able to infect representatives of all identified genotypes of New Zealand H. pilosella. The variation present in New Zealand Hieracium species was investigated by chromosome analysis and isozyme electrophoresis. H. pilosella from 34 collections throughout New Zealand were predominantly pentaploid with a hexaploid found in just one population; the pentaploids included variants, according to electrophoresis results and morphological characters. Hieracium rust showed potential as a biological control agent. The rust significantly affected the growth of H. pilosella and displayed strict host-specificity, with no hosts outside the subgenus Pilosella. Powdery mildew, Erysiphe cichoracearum, is common and very pathogenic on Hieracium spp. throughout Europe. However, Hieracium powdery mildew grew on two New Zealand endemic species, Embergeria grandi/olia and Kirkianella novaezelandiae. Several other Hieracium pathogens were noted but their potential for biological control requires further investigation.