Divergent evolution of the Jackie pathotype of Puccinia striiformis f. sp. tritici between Australia and New Zealand

Based on historical data, Australia and New Zealand (NZ) form a single epidemiological unit for cereal rusts. The dominant westerly wind pattern produces a one-way pathway of pathogen movement from Australia to NZ. Until 2002, pathotype analysis of cereal rust pathogens for NZ was conducted at the University of Sydney, Plant Breeding Institute. Over that time, windborne dispersal of members of the Pst 104 pathotype lineage to New Zealand was confirmed. Historically, pathotypes of Pst introduced to New Zealand have taken different evolutionary pathways to their Australian relatives, including a higher diversity of step-wise mutant isolates, often with different virulence profiles. A preliminary screen of Pst in NZ was conducted in January 2013 and a broader survey was conducted in 2014. Initial results confirmed that the Australian pathotype (pt.) 134 E16 A+ YrJ+ had crossed to NZ. The designation “YrJ+” was allocated to indicate virulence for an unidentified, probably rye-derived, resistance gene in the Australian triticale cultivar ‘Jackie’. The divergent evolution of this pathotype in NZ relative to Australia is of interest. In NZ, this pathotype subsequently acquired virulence for Yr10 to produce pt. 150 E16 A+ YrJ+. In Australia, Yr10 virulence had previously evolved in pt. 134 E16 A+, the progenitor of pt. 134 E16 A+ YrJ+. Only two mutational derivative pathotypes have evolved from pt. 134 E16 A+ YrJ+ in Australia. The first acquired virulence for an adult plant resistance gene in another triticale variety, ‘Tobruk’, and the second acquired virulence for Yr27. Despite being present in both Australia and NZ, pt. 134 E16 A+ Yr17+ has dominated the Australian Pst population whereas in NZ the predominant pathotype appears to be 134 E16 A+ YrJ+. Since the rust resistance genotypes of NZ varieties are poorly characterised, no conclusions can yet be reached as to whether this difference in dominant pathotype is due to selection or chance.