In order to understand the evolutionary potential of complex traits in wild populations, it is necessary to determine their genetic architecture. In an agronomical context, the evolutionary potential of plant pathogens drives their ability to adapt to cultivated host plants. However, the evolution of pathogen traits is rarely taken into account in breeding strategies. In particular, aggressiveness, the quantitative component of pathogenicity, combines several complex traits related to the pathogen fitness and life cycle (including morphological traits and developmental characteristics). In this work, we consider the hypothesis of a change in aggressiveness following the deployment of new plant cultivars with quantitative resistance. To this aim we use three complementary approaches: (i) a study of the divergence of aggressiveness traits and neutral molecular markers to assess the evolutionary potential of complex traits; (ii) an analysis of G matrices to highlight covariance among traits and putative trade-offs; and (iii) a genome-wide association study on quantitative (aggressiveness) and qualitative (virulence) traits to identify their genetic underpinnings. To explore these questions, we focus on the poplar rust fungus, Melampsora larici-populina, in the context of a major poplar resistance breakdown. It occurred in 1994 in Belgium and the North-East of France and led to a complete replacement of the pathogen population (Persoons et al., 2017). Therefore, our study is based on a historical collection of living samples (“frozen genotypes”) collected before, during and after this resistance breakdown event. This temporal sampling allowed us to retrace the evolutionary trajectory of these populations and test for the consistency of trade-offs through time Analyses of phenotypes measured in a common garden lab experiment enable to decipher the joint evolution of complex traits and a genomic region potentially involved in this resistance breakdown.