Identity of the mtDNA haplotype(s) of Phytophthora infestans in historical specimens from the Irish Potato Famine K& AQ1 J. MAY and Jean B. RISTAINO* Department of Plant Pathology, North Carolina State University, Box 7616, Raleigh, NC 27695-7616, USA. E-mail : Jean_Ristaino@ncsu.edu Received 24 June 2003; accepted 2 March 2004. 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. mtDNA primers were designed that distinguish the extant 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 1950s, indicating that pathogen diversity was greater than previously believed. INTRODUCTION Phytophthora infestans is a fungus-like, straminipilous pathogen and the causal agent of potato and tomato late blight, a devastating disease of potato and tomato worldwide (Fry & Goodwin 1997). The pathogen causes a destructive foliar blight and also infects potato tubers, tomato fruit, and a number of solanaceous hosts. The disease was first observed on potatoes in the USA in 1843 and was reported in areas around the ports of Philadelphia and New York (Stevens 1933, Peterson, Campbell & Griffith 1992). Potato blight subsequently spread to Canada, the northeastern United States and the Midwestern states by 1845 (Bourke 1993). Epidemics were observed in Belgium in June 1845 and spread to other regions in Europe that season. By mid-October 1845 the disease was observed in Britain and Ireland (Bourke 1993). Epi- demics caused by P. infestans in the mid-19th century in Ireland led to what is known as the Irish potato famine. Approximately three million people died from starvation or were forced to migrate to other regions of the world. Migration has played an important role in the spread of P. infestans. Three theories have been proposed to describe the migration of P. infestans to the USA and Europe and then to the rest of the world. It was pro- posed that the pathogen might have migrated in the mid-19th century from Mexico to the United States and then to Europe (Fry et al. 1993). A second theory suggests that the pathogen may have migrated to the United States and Europe from the Andean region near Peru where the disease was reported in the 19th century and earlier (Tooley, Therrien & Ritch 1989, Bourke 1993). A final theory suggests that the pathogen migrated from Mexico to Peru then to the USA and Europe (Andrivon 1996). Once introduced to Europe, P. infestans could have been distributed to rest the of world via international trade in seed potatoes (Fry et al. 1993). Another migration of P. infestans out of Mexico to Western Europe in the late 1970s has been well documented (Hohl & Iselin 1984, Cooke, Swan & Currie 1985, Shattock et al. 1990, Fry et al. 1992, 1993, Drenth, Turkensteen & Govers 1993, Drenth, Tas & Govers 1994). In Europe, it appears that the new popu- lations displaced the old populations in a few years, as the new genotypes were probably more aggressive than old ones. A single genotype dominated extra-Mexican populations sampled from locations not affected by * Corresponding author. Mycol. Res. 108 (0): 1–9 (& 2004). f The British Mycological Society 1 DOI: 10.1017/S0953756204009876 Printed in the United Kingdom.