The plant pathogen Phytophthora infestans causes late blight disease in potatoes and was responsible for the Irish potato famine of 1845. In the June 7 Nature, Ristaino et al. report a molecular characterization of historic potato samples from the nineteenth century epidemics (Nature 2001, 411:695-697). They used PCR amplification and sequencing of short mitochondrial DNA fragments, to investigate the history of P. infestans haplotypes. Their results confirm that P. infestus infections accompanied the Irish potato famine. Furthermore, the historic lesions did not contain the mitochondrial DNA haplotype 1b, challenging current theories about the ancestry of modern P. infestus strains. The authors make an impassioned plea for the importance of herbarium archive collections in historical analysis.