Dominance of a single clonal lineage in the Phytophthora infestans population from northern Shaanxi, China revealed by genetic and phenotypic diversity analysis

Late blight caused by Phytophthora infestans is the most serious disease of potato worldwide. To understand the P. infestans population structure in northern Shaanxi, an emerging potato production region in China, 125 single-lesion isolates were randomly collected from farmers' fields in 2009 and characterized phenotypically and genotypically. A mating type assay showed that 94 isolates were A1 mating type. Virulence determination of selected isolates on a set of differential potato lines containing R1 to R11, respectively, showed the presence of two pathotypes, of which the pathotype lacking avirulence genes Avr3, Avr4 and Avr10 was dominant. Isolates lacking all avirulence factors Avr1 to Avr11 were detected but at lower frequency (13.6%). Analysis for mtDNA haplotype showed all 61 examined isolates were IIa. A total of seven multilocus genotypes were distinguished among 125 isolates, as determined with seven polymorphic microsatellite markers. The genotype SG-1 was dominant in the population with a frequency of 75.2% and was present throughout the region. Analysis of the phenotypic and genotypic structures of P. infestans populations indicated strict clonal reproduction of the pathogen and suggested that sexual reproduction probably does not occur. Potential implications for disease management are discussed.