Genetic and phenotypic diversity analysis reveals dominance of a clonal lineage in the Phytophthora infestans from Northern Shaanxi, China.

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 characterised phenotypically and genotypically. A mating type assay showed that 94 (97) isolates were A1. 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 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 do not occur. Potential implications for disease management are discussed.