High Genetic Diversity of the Cypress Canker Pathogen Seiridium cardinale in the Southern Hemisphere

Cypress canker, caused by several species of Seiridium , affects Cupressaceae worldwide. Of the seven described pathogens that cause this disease, S. cardinale is one of the most aggressive and widespread. This study examined the genetic diversity of S. cardinale isolates from the Southern Hemisphere, with a focus on South Africa and New Zealand. Three S. cardinale genomes were sequenced to evaluate the utility of existing microsatellite markers and to identify and develop new microsatellite markers. Sixty isolates from the Southern Hemisphere, together with 22 reference strains from Europe and California, were genotyped at 10 loci. Genetic diversity was high for the South African ( H exp = 0.80) and New Zealand ( H exp = 0.67) populations, though rarefaction indicated even greater diversity in California, the presumed native range of the pathogen. Southern Hemisphere populations showed no evidence of recombination and isolates were closely related to one another and to Californian strains, whereas European strains formed a distinct lineage. Three genetic lineages were detected among the New Zealand isolates, all closely related to South African strains. Collectively, these results suggest multiple introductions of S. cardinale into Southern Hemisphere countries from California, likely facilitated by trade in Cupressaceae trees, as well as subsequent accidental movement of the pathogen between South Africa and New Zealand.