Genetic epidemiology of late blight in Australia using ancient DNA

Late blight caused by Phytophthora infestans is the most destructive disease of potatoes worldwide and is most notable as the cause of the Irish potato famine of the mid 1840’s. Whole mitochondrial genome sequences were generated from 44 Australian, two UK and one US isolates of P. infestans stored as either herbarium specimens or as recent samples on FTA cards, spanning the period 1873–2019. Mitochondrial (mt) genome sequence comparison confirmed that over the past c. 110 years, an old A1 strain of P. infestans has been present in Australia. There was evidence, however, that P. infestans had been introduced on multiple separate occasions in the early 1900s prior to the introduction of quarantine measures. The 44 Australian samples included six infected specimens of the Australian native kangaroo apple (Solanum aviculare) collected in 1911. The P. infestans mt genome sequences from these isolates clustered with P. infestans from infected potatoes collected in Victoria (1910, 1911, 1986, 1989), South Australia (1909, 2003) and Tasmania (1910, 2019), supporting the hypothesis that Australia has one old strain that has survived since the first arrival of the pathogen into the country. The study highlights the success of Australia’s potato biosecurity setting and supports the need for ongoing surveillance and biosecurity measures to prevent the introduction of the newer more aggressive strains of the pathogen.