The deployment of plant varieties carrying resistance genes ( R ) exerts strong selection pressure on pathogen populations. Rapidly evolving avirulence genes ( Avr ) allow pathogens to escape R ‐mediated plant immunity through a variety of mechanisms, leading to virulence. The poplar rust fungus Melampsora larici ‐ populina is a damaging pathogen of poplars in Europe. It underwent a major adaptive event in 1994, with the break...