Determining the gene responsible for a QTL controlling broad-spectrum resistance to pathogens Phytophthora agents are the most devastating pathogens of Dicotyledons. Chemical controls against these pathogens are insufficiently effective, and have serious consequences on the environ ment and human health. An alternative more environmentally friendly is the use of pathogen-resistant varieties. However, monogenic resistances are rare and often specifie to a strain of the pathogen (narrow spectrum); they are thus frequently overcome because of the emergence of new virulent strains. Polygenic resistances have often the advantage of being broad-spectrum. ln addition, they have been demonstrated for some of them more durable. But their use in selection is limited because the controlling loci (named QTLs for Quantitative Trait Loci) are difficult to identify due to their number and their partial individual effect on resistance. ln addition, it is not known in what resistance QTLs differ from monogenic resistance genes. Consequently, we aim to determine the molecular basis of a QTL involved in polygenic resistance to Phytophthora capsici, the agent of late blight in pepper (Capsicum annuum). From the fine genetic map to the identification of a candidate gene: chromosome walking, seguencing and gene annotation The QTL Phy-PS confers a high level of broad-spectrurn resistance towards P. capsici. Identified in several resistant genitors of C annuum, it was mapped to chromosome P5. To facilitate its fine mapping, we derived a population that segregates for the QTL alone. Then a physical map of the locus was constructed by chromosome walking based on two BAC libraries (BACfor Bacterial Artificial Chromosome), and anchored to the fine genetic map. BAC clones covering the locus have been sequenced by classical and new generation sequencing approaches. An automatic annotation pipeline has been adapted to the pepper genome to characterize sequence repeats that are very common in this species, and to predict the coding sequences corresponding to hypothetical functional genes. An analysis of nucleotide and protein homology and a search for protein domains have been achieved to identify among them candidate genes. Finally, severa 1 methods for functional validation have been considered and developed to study the role of candidate genes in the resistance to P. capsici.