Although Avr sequences are often polymorphic, the contribution of these polymorphisms to virulence diversity and host range in natural pathogen populations remains largely unexplored. We determined how natural sequence polymorphisms of the avirulence factor Avr3D1 in the wheat pathogen Zymoseptoria tritici contributed to adaptive changes in virulence and showed that there is a continuous distribution in the magnitude of resistance triggered by different Avr3D1 isoforms. These results demonstrate that natural variation in an Avr gene can lead to a quantitative resistance phenotype.

We further showed that homologues of Avr3D1 in two non-pathogenic sister species of Z. tritici are recognized by some wheat cultivars, suggesting that Avr-R gene-for-gene interactions can contribute to nonhost resistance.

We suggest that the mechanisms underlying host range, qualitative resistance and quantitative resistance are not exclusive.