One of the biggest challenges in the production of biomass in agro-forestry is the growth loss and mortality caused by diseases. Septoria poplar canker, caused by the Dothideomycete fungal pathogen Mycosphaerella populorum =( Sphaerulina musiva), is the most important disease of poplar, an important bioenergy agro-forestry crop. This pathogen reduces growth by causing leaf necrosis (called leaf spots) that affect growth through reduction of photosynthesis. But the pathogen can also cause mortality by attacking the woody stems and branches of susceptible poplars. These cankers can severely weaken the trees and result in stem breakage and tree mortality. The presence of the disease in a poplar plantation often results in plantation failure. A promising avenue in tackling the challenge of Septoria poplar canker is the deployment of resistant poplar genotypes. We have generated genomic sequencing and re-sequencing resources of over 1000 P. trichocarpa provenances and conducted genome-wide association studies (GWAS) to identify genomic determinants associated with resistance to the pathogen. These resources promise to help breeders with early screening for selection of more resistant progenies or to perform genomic selection for future deployment of resistant material. However, pathogens can be highly variable and adaptable and resistance in crop species has proved to be easily overcome by pathogens. A close monitoring of the pathogens and a deep understanding of their evolutionary potential are essential to favor the deployment of sustainable resistance. In order to better understand the adaptive potential of the Septoria poplar canker pathogen, we have re-sequenced samples of the pathogen and have discovered a very diverse and structured pathogen population in North America. This includes the presence of sub-specific variants with different genomic and pathogenic profiles. We speculate that this variation in genomic diversity can drive adaptive processes that help these populations to succeed in new areas and on new host species. Phenotyping studies of different populations of the pathogen for pathogenicity on different genotypes of P. trichocarpa have revealed a significant adaptation of certain fungal isolates to host genotypes. This indicates that we cannot necessarily predict the level of resistance of a poplar genotype by infection with a single fungal strain. It also suggests that M. populorum may have the capacity to evolve new virulence to overcome host resistance. The presence of phenotypic diversity in virulence could translate into host-specialization and result in rapid selection for novel adapted races. This would defeat the deployed resistant material. Our objectives in this project are to build upon the genomic and biological resources developed to advance the characterization and understanding of the interaction between poplar and the poplar canker pathogens. In particular, we are interested in understanding what makes some pathogen more successful at attacking their host. We propose to use our phenotypically and genomically-characterized poplars and Mycosphaerella collections and generate transcriptomics, metabolomics and proteomics resources of various pathogen collections. This will allow us to elucidate the question of what makes a highly aggressive Septoria poplar canker pathogen and how poplar trees can be selected to have the broadest resistance to the pathogen attacks.