Leptosphaeria maculans is one of the major fungal pathogens on oilseed rape (Brassica napus), causing stem canker disease. The closely related Brassica species Brassica nigra, Brassica juncea, and Brassica carinata display extreme resistance toward stem canker. In this study, we demonstrate the nonhost status of B. carinata toward L. maculans in France through field experiments and inoculations performed in controlled conditions. A few isolates moderately adapted to B. carinata, in controlled conditions, were recovered in the field on B. nigra leaves, allowing us to investigate the unusual B. carinata / L. maculans interactions using molecular, macroscopic, and microscopic analyses. A cross between a L. maculans isolate adapted to B. napus and an isolate moderately adapted to B. carinata allowed the generation, in the lab, of recombinant L. maculans strains better adapted to B. carinata than the natural parental isolate obtained from B. nigra, and highlighted the polygenic determinism of the adaptation of L. maculans to B. carinata and B. napus. This biological material will allow further investigation of the molecular determinants of the adaptation of L. maculans to nonhost species and elucidate the genetic resistance basis of B. carinata.