PCR amplification of the Irish potato famine pathogen from historic specimens

Late blight, caused by the oomycete plant pathogen Phytophthora infestans, is a devastating disease of potato and was responsible for epidemics that l ed to the Irish potato famine in 1845 (refs 1- 5). Before the 1980s, worldw ide populations of P. infestans were dominated by a single clonal lineage, the US-1 genotype or Ib mitochondrial DNA (mtDNA) haplotype, and sexual rep roduction was not documented outside Mexico, the centre of diversity of the pathogen(6,7). Here we describe the amplification and sequencing of 100-ba se-pair fragments of DNA from the internal transcribed spacer region 2 from 28 historic herbarium samples including Irish and British samples collecte d between 1845 and 1847, confirming the identity of the pathogen. We amplif ied a variable region of mtDNA that is present in modern Ib haplotypes of P . infestans, but absent in the other known modern haplotypes (Ia, IIa and I Ib)(8). Lesions in samples tested were not caused by the Ib haplotype of P. infestans, and so theories that assume that the Ib haplotype is the ancest ral strain need to be re-evaluated(4,7). Our data emphasize the importance of using historic specimens when making inferences about historic populatio ns.