Global historic pandemics caused by the FAM-1 genotype of the Irish potato pathogenPhytophthora infestans

The FAM-1 genotype of Phytophthora infestans caused late blight in the 1840s in the US and Europe and was responsible for the Irish famine. We examined 140 herbarium specimens collected between 1845 and 1991 from six continents and used 12-plex microsatellite genotyping (SSR) to identify FAM-1 and the mtDNA lineage (Herb-1/ Ia) present in historic samples. FAM-1 was detected in approximately 73% of the historic specimens and was found on 6 continents. The US-1 genotype was found in only 27% of the samples and was found later on all continents except Australia/Oceania. FAM-1 was the first genotype detected in almost all the former British colonies from which samples were available. The data from historic samples suggest the FAM-1 genotype was widespread, diverse, and spread more widely than US-1. The famine lineage spread to six continents over 140 years, and likely spread during global colonization from Europe.