Poplar rust caused by Melampsora larici-populina has become the main disease affecting poplars in northern Europe after several cultivars selected for complete resistance became susceptible following pathogen adaptation. Although understanding pathogen adaptation requires working at the genetic and molecular levels both in the cultivated host and in the pathogen, understanding the whole process also requires looking at all other partners of the interaction at landscape level. We will present the outputs from a 3-year project aiming at elucidating the role of wild poplars in the adaptive potential of the pathogen to an exotic and non-commercially-deployed resistance gene. Melampsora larici-populina isolates were sampled in different environments with contrasted selective pressures. The finding that up to 13% of the isolates collected on wild P. nigra trees in environments where the resistance gene is absent were however able to defeat the gene demonstrates that wild host populations act as a reservoir for pathogen adaptation. We will also present the first results from an ongoing project where ornamental poplar and larch (aecial host) trees have been recorded, mapped and characterized in urban and rural areas outside the natural range of sympatry between alternate hosts. This project aims at elucidating how scattered trees outside the forest impact the earliness and rate of rust epidemics.