Résumé
South American Leaf Blight (SALB), due to the fungus Microcyclus ulei, is potentially the major phytosanitary threat for rubber tree (Hevea brasiliensis) cultivation all around the world. After fifty years of breeding efforts, attempts to create resistant cultivars failed, the selected resistances being each time rapidly overcome by the fungus. QTL analyses of genetic resistance were performed on either field conditions (Le Guen et al., 2003, TAG 108: 160-167) or under controlled inoculations using non sporulating and weakly pathogenic fungus isolates (Lespinasse et al., 2000, TAG 100: 975-984) and revealed unexpected complexity of genetic determinism for either partial or complete resistance. More recently, we analysed resistance to weakly, highly and very highly pathogenic isolates in order to study QTL bypassing. The new isolates were inoculated on the same segregating population used in the previous study, allowing precise comparison of genetic determinism of complete and partial resistance. Our results confirm that both complete and partial resistance to M. ulei can be mono- or multigenic traits and reveal that minor and major QTLs can be simultaneously overcome by a single fungus isolate. (Texte intégral)