In poplars, varietal improvement is based on the valorisation of interspecific hybrids in the form of clones resulting from the combination of the Eurasian species Populus nigra L. with P. deltoides, a poplar native to North America. The Eurasian parent has a good behaviour with regard to leaf rust (Melampsora larici-populina, Mlp), this native species coevolving with the populations of the pathogen. However, we found a strong Genotype x Strain interaction for partial resistance in P. nigra, and we identified specific effect QTLs according to the Mlp strain used. In a context of development of cultivars with durable resistance, it is essential to consider these interactions. One of the strategies is to consider the behaviour of each individual to several strains and thus define a reaction norm that can be the target of selection. In the end, such a characterization would make it possible to propose a cocktail of poplar genotypes covering all the resistances. Reaction norms are a mean of describing the plasticity of organisms to their environment (Sanchez et al., 2013). They are often modelled (in the form of a function) according to gradients of a continuous environmental variable (temperature, light, water availability ... Marchal et al., 2019). Very few studies have focused on characterizing pest gradients. The proposed research work is part of the European B4EST project (http://b4est.eu/). The objectives are: - To develop reaction norms to describe the interaction Black Poplar / Mlp. For this, pathological tests under controlled conditions will be performed on a set of families (300 individuals) against 7 Mlp strains. These strains are already characterized and highlight GxS interactions on a collection of 26 P. nigra genotypes from natural populations. - To elucidate the genetic control of these standards through GWAS (Genome Wide Association Study) using parameters describing the reaction norms.