The yearly losses incurred by bacterial blackspot disease are high. Often trees are asymptomatic, with the pathogen either in the resident phase or latent stage of infection. Detection of the pathogen in these asymptomatic trees is one of the most important means of controlling the disease. Isolates which consistently differed in virulence were isolated from symptomatic mango plants. These isolates could be categorised into four groups based upon differences in virulence. Monoclonal antibodies (mAbs) were successfully raised using separate and pooled isolates for immunisation. MAbsraised were of the lgG class and reacted with a proteinaceous epitope. These monoclonal antibodies could distinguish between different virulence groups of Xanthomonas campestris pv. mangiferaeindicae by means of Western Blot analysis. These antibodies were used along with a selective medium, BVGA for detection of epiphytic populations as well as latent infections in mango. An enrichment step prior to the enzyme- linked immunosorbent assay (ELISA) is important, since bacterial counts on trees with latent infections are too low to result in a positive signal. These techniques in combination are thus useful for detection and monitoring of the pathogen, which may play an important role in controlling the spread of the disease. / Dissertation (MSc Agric)--University of Pretoria, 1993. / gm2014 / Microbiology and Plant Pathology / Unrestricted