Adaptation to the most abundant host genotype in an agricultural plant-pathogen system--potato late blight.

This study investigated local adaptation of Phytophthora infestans populations, the causal agent of potato late blight, to two susceptible potato cultivars, each grown for a number of years and over large areas in separate French regions. We measured aggressiveness (quantitative pathogenicity) of each pathogen population to sympatric and allopatric hosts in a reciprocal cross-inoculation experiment. There was no evidence for specific host adaptation in this pathosystem. At both local and regional scales, the distribution of aggressiveness fits a pattern of adaptation to the most common host genotype. Our observations support the theoretical predictions that large pathogen dispersal rates and genetic drift, revealed by the comparisons of the genotypic structures of the populations tested, can lead to a local adaptation pattern detectable only at a large spatial scale. The unravelling of adaptive patterns at different spatial scales can be used for a more efficient management of the disease.