1. Department of Plant Pathology, North Carolina State University, Raleigh, North Carolina 27695, USA
2. Present address: USDA-ARS, New England Plant, Soil, and Water Laboratory, Orono, Maine 04469, USA.

Correspondence to: Jean B. Ristaino¹ Correspondence and requests for materials should be addressed to J.B.R. (e-mail: Email: Jean_Ristaino@ncsu.edu). DNA sequence data can be found in GenBank under accession nos AF004277–AF004280 (ITS and rDNA of four modern isolates); AY027652–AY027655, AF349576–AF349587 (PINF/Herb amplification of spacer region 2 from four modern isolates and herbarium specimens); AY003908–AY003919 (mtDNA P2 region for three modern isolates of each haplotype); AF348598–AF348609 (mtDNA P4 region for three modern isolates of each haplotype); and AF349588–349601 (P2F4/R4 amplification of mtDNA from four modern haplotypes and herbarium specimens).

Abstract

Late blight, caused by the oomycete plant pathogen Phytophthora infestans, is a devastating disease of potato and was responsible for epidemics that led to the Irish potato famine in 1845 (refs 1,2,3,4,5). Before the 1980s, worldwide populations of P. infestans were dominated by a single clonal lineage, the US-1 genotype or Ib mitochondrial DNA (mtDNA) haplotype, and sexual reproduction was not documented outside Mexico, the centre of diversity of the pathogen^{6, 7}. Here we describe the amplification and sequencing of 100-base-pair fragments of DNA from the internal transcribed spacer region 2 from 28 historic herbarium samples including Irish and British samples collected between 1845 and 1847, confirming the identity of the pathogen. We amplified 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 IIb)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 ancestral strain need to be re-evaluated^{4, 7}. Our data emphasize the importance of using historic specimens when making inferences about historic populations.