Basal rot of Narcissus caused by Fusarium oxysporum f.sp. narcissi is the major limiting factor for the increased export of ornamental bulbs from the United Kingdom. There is no known resistance to the disease amongst commercial cultivars. The purpose of this study was to develop techniques for the rapid detection and identification of the pathogen in bulbs, culture and soil, and to develop serological and biochemical tests to investigate differences amongst isolates of this forma specialis from a range of countries. Significant differences in virulence were observed between the isolates tested, two being identified as avirulent. Results suggest that there is wider morphological variation within this forma specialis than previously recorded. Isolates stored at -80°C have remained viable over a three year period. Thirty three species and varieties of Narcissus and seventeen cultivars were screened for resistance to the pathogen. Accessions were either wholly resistant, partially resistant (a variable portion of the bulbs infected) or susceptible. Accessions identified as resistant are now being used as parents in a resistance breeding programme. F. oxysporum Esp. narcissi isolates of low virulence and avirulence showed greater agglutination of microconidia with Narcissus pseudonarcissus lectin. In addition increased lectin levels were found in cultivars showing greater disease resistance; this suggests a possible host pathogen interaction. Polyclonal antisera were elicited against extracts of isolates and a specific and sensitive enzyme-linked immunosorbent assay was developed for the detection of the pathogen in bulbs. Little cross-reactivity was shown by other bulb-rotting fungi. A direct correlation was observed between positive results in the ELISA and recovery of the pathogen on selective media. The ELISA which was developed will be field evaluated with growers during the coming season. In indirect ELISA and Western blotting of fungal extracts, all fungi outside the genus Fusariwn showed consistently low binding. Isolates of F. oxysporum f.sp. narcissi could be discriminated from other Fusarium species and further split into a series of sub-groups. Avirulent isolates or isolates of low virulence showed differential binding to the PAbs. Other sub-groups were not related to virulence. Western blotting allowed greater differentiation of isolates and bands specific to F. oxysporwn f.sp. narcissi were identified. Serological detection of the pathogen in soil proved possible through the inclusion of a soil enrichment stage with a selective medium.