Sunflower downy mildew caused by Plasmopara halstedii is one of the most potentially important diseases. So far, a complete, major gene resistance (Pl) has been used successfully, but with the appearance of eight races in France since 2000, research on more durable resistance was undertaken. In this study, we present new results concerning the evolution of pathogenicity of P. halstedii under conditions of re-enforced infection and different Pl gene selection pressures. Moreover, we imagine the evolution of virulence and aggressiveness of P. halstedii under a mixture model of sunflower inbred lines carrying the two types of resistance which may enhance durable resistance against it. Examples of host-parasite interactions including the influence of plant mixture models against pathogens are equally presented to understand how the pathogen develops its pathogenicicity.