Abstract

Since about two hundred years, downy mildew caused by Peronospora effusa is probably the most economically important disease of spinach (Spinacia oleracea). However, there is no information on the global phylogeographic structure of the pathogen and thus it is unclear whether a single genotype occurs worldwide
or whether some local genetic variation exists. To investigate the genetic variability of this pathogen, a sequence analysis of two partial mitochondrial DNA genes, cox2 and nad1, was carried out. Thirty-three specimens of Peronospora effusa from four continents were analyzed, including samples from Australia, China, Japan, Korea, Mexico, Russia, Sweden, and the USA. Despite the potential anthropogenic admixture of genotypes, a phylogeographic pattern was observed, which corresponds to two major groups, an Asian/Oceanian clade and another group, which includes American/European specimens. Notably, two of six Japanese specimens investigated did not belong to the Asian/Oceanian clade, but were identical to three of the specimens from the USA, suggestive of a recent introduction from the USA to Japan. As similar introduction events may be occurring as a result of the globalised trade with plant and seed material, a better knowledge of the phylogeographic distribution of pathogens is highly warranted for food security purposes.

• Keywords: obligate parasites; Oomycetes; phylogeographic distribution; plant pathogen; quarantine