Potato late blight (PLB) has become a resurgent disease following the introduction and spread of the A2 mating type and new strains of the A1 mating type of P. infestans into North America and Europe. New aggressive populations of the A1 and A2 mating types have now spread around the globe with the trade in seed potatoes. In Australia, PLB occurs infrequently in NSW, South Australia, Victoria, but more commonly in northern Tasmania, during atypical summer weather patterns characterised by periods of moist air combined with stagnant or slow moving depressions.
In an early study, 20 isolates of P. infestans collected in Tasmania during PLB epidemics in the summers of 1998/99 and 99/2000 were characterised by RG57 probe analysis as single strain and of the A1 mating type. Diseased leaflets collected during PLB epidemics in 2010/11 and 2011/12 from NSW, Victoria and Tasmania (202 samples) were preserved as leaf squash preparations on FTA™ Classic Cards for microsatellite genotyping. Simple sequence repeat (SSR) genotypes were determined using the standard 12-plex Euroblight set of P. infestans SSRs. Initial analysis of 125 samples identified a single novel genotype of P. infestans not found anywhere else in the world and identical to the genotype of freeze-dried Australian isolates collected a decade previously. Consistent evidence of a single linage confirms that Australia does not have the A2 mating type, or the new aggressive strains of both the A1 and A2 mating types of P. infestans found elsewhere in the world.