The mtDNA haplotypes of the plant pathogen Phytophthora infestans present in dried potato and tomato leaves from herbarium specimens collected during the Irish potato famine and later in the 19th and early 20th century were identified. A 100 bp fragment of ribosomal DNA (rDNA) specific for P. infestans was amplified from 90% of the specimens (n=186), confirming infection by P. infestans. Primers were designed that distinguish the extant mtDNA haplotypes. 86% percent of the herbarium specimens from historic epidemics were infected with the Ia mtDNA haplotype. Two mid-20th century potato leaves from Ecuador (1967) and Bolivia (1944) were infected with the Ib mtDNA haplotype of the pathogen. Both the Ia and IIb haplotypes were found in specimens collected in Nicaragua in the 1950s. The data suggest that the Ia haplotype of P. infestans was responsible for the historic epidemics during the 19th century in the UK, Europe, and the USA. The Ib mtDNA haplotype of the pathogen was dispersed later in the early 20th century from Bolivia and Ecuador. Multiple haplotypes were present outside Mexico in the 1940s–60s, indicating that pathogen diversity was greater than previously believed.