Abstract
South American Leaf Blight (SALB) caused by the fungus Microcyclus ulei is a serious threat to worldwide rubber tree cultivation and also responsible for the poor development of this culture in South and Central America. The breeding of resistant cultivars is considered as the best way to anticipate an accidental introduction of SALB in disease free countries and to develop rubber tree cultivation in diseased areas. The main resistance source described until now proved to be ineffective against the most aggressive fungus isolates. Another resistance source identified in a Peruvian cultivar holds for more than thirty years in high infestation conditions. Genetic mapping carried out on the progeny of a cross between this cultivar and a susceptible one revealed two major resistance genes, one located on linkage group g15 and effective against M. ulei isolates from French Guiana, and the other one on linkage group g13 effective against isolates from Bahia state (Brazil). Diversity analysis in South-West Amazonian natural populations shows populations structured in three main clusters corresponding to the Brazilian states of Acre, Rondônia and Mato Grosso. The Madre de Dios population in Peru is part of the Acre genetic cluster. Differentiation among populations is mainly explained by isolation by distance and secondarily by the existence of hydrographical basins. The linkage disequilibrium between linked neutral genetic markers is wider in Acre cluster than in Rondônia cluster, but remains low in both cases. A genetic association with the SALB resistant trait is detected with a microsatellite marker located close to the major resistance gene in linkage group g15. Another association is also detected in a genomic area where no resistant locus was expected until now. Importance of these results for breeding of new SALB resistant cultivars is discussed.