Plant disease resistance genes can protect crops against pathogens that have the corresponding avirulence (avr) genes. In many cases, the protection conferred by single, generally dominant, disease resistance (R) genes is short lived as the pathogen strains can evolve through mutation and selection, favouring strains with altered or missing avr genes that are therefore able to cause disease. Broad spectrum disease resistance that is quantitative, involving several genes, is generally expected to be more durable. However, this kind of resistance is more difficult to exploit in breeding programmes. The basis of this plant defence strategy is currently unknown. We have chosen quantitative resistance to Xanthomonas campestris pv campestris (Xcc) in Brassica rapa as a model for investigation. Xcc causes black rot of crucifers. This seed-borne disease is one of the most important diseases of Brassicas worldwide. In previous Brassica research at WHRI we have identified good sources of resistance and mapped the location of the major genetic components. In this project we will exploit the exciting new developments in Brassica rapa genomics and emerging information on the close relatedness of the Brassica and Arabidopsis genomes to characterise the resistance and identify the genes involved. The combination of the above resources in the proposed project will allow us to deliver novel and exciting information on the fundamental basis of quantitative disease resistance in a crop species. Key practical outcomes of this work will be resistant plant material with tightly linked molecular markers and information on the extent of natural variation for resistance in Brassica genomes that could be exploited in plant breeding to provide durable resistance to a very significant pathogen.