This is a potato specimen from the Kew Garden herbarium, collected in 1847, during the height of the Irish famine. The legend reads "Botrytis infestans", because it was not known yet that Phytophthora does not belong to the mildew causing Botrytis fungi. Credit: Marco Thines/Senckenberg Gesellschaft für Naturforschung An international team of scientists reveals that a unique strain of potato blight they call HERB-1 triggered the Irish potato famine of the mid-19th century. 
 
 It is the first time scientists have decoded the genome of a plant pathogen and its plant host from dried herbarium samples. This opens up a new area of research to understand how pathogens evolve and how human activity impacts the spread of plant disease. 
 
 Phytophthora infestans changed the course of history. Even today, the Irish population has still not recovered to pre-famine levels. "We have finally discovered the identity of the exact strain that caused all this havoc", says Hernán Burbano from the Max Planck Institute for Developmental Biology. 
 
 For research to be published in eLife, a team of molecular biologists from Europe and the US reconstructed the spread of the potato blight pathogen from dried plants. Although these were 170 to 120 years old, they were found to have many intact pieces of DNA. 
 
 "Herbaria represent a rich and untapped source from which we can learn a tremendous amount about the historical distribution of plants and their pests - and also about the history of the people who grew these plants," according to Kentaro Yoshida from The Sainsbury Laboratory in Norwich. 
 
 The researchers examined the historical spread of the fungus-like oomycete Phytophthora infestans, known as the Irish potato famine pathogen. A strain called US-1 was long thought to have been the cause of the fatal outbreak. The current study concludes that a strain new to science was responsible. While more closely related to the US-1 strain than to other modern strains, it is unique. "Both strains seem to have separated from each other only years before the first major outbreak in Europe," says Burbano. 
 
 The researchers compared the historic samples with modern strains from Europe, Africa and the Americas as well as two closely related Phytophthora species. The scientists were able to estimate with confidence when the various Phytophthora strains diverged from each other during evolutionary time. The HERB-1 strain of Phytophthora infestans likely emerged in the early 1800s and continued its global conquest throughout the 19th century. Only in the twentieth century, after new potato varieties were introduced, was HERB-1 replaced by another Phytophthora infestans strain, US-1. 
 
 The scientists found several connections with historic events. The first contact between Europeans and Americans in Mexico in the sixteenth century coincides with a remarkable increase in the genetic diversity of Phytophthora. The social upheaval during that time may have led to a spread of the pathogen from its center of origin in Toluca Valley, Mexico. This in turn would have accelerated its evolution. 
 
 The international team came to these conclusions after deciphering the entire genomes of 11 historical samples of Phytophthora infestans from potato leaves collected over more than 50 years. These came from Ireland, the UK, Europe and North America and had been preserved in the herbaria of the Botanical State Collection Munich and the Kew Gardens in London. 
 
 "Both herbaria placed a great deal of confidence in our abilities and were very generous in providing the dried plants," said Marco Thines from the Senckenberg Museum and Goethe University in Frankfurt, one of the co-authors of this study. "The degree of DNA preservation in the herbarium samples really surprised us," adds Johannes Krause from the University of Tübingen, another co-author. Because of the remarkable DNA quality and quantity in the herbarium samples, the research team could evaluate the entire genome of Phytophthora infestans and its host, the potato, within just a few weeks. 
 
 Crop breeding methods may impact on the evolution of pathogens. This study directly documents the effect of plant breeding on the genetic makeup of a pathogen. 
 
 "Perhaps this strain became extinct when the first resistant potato varieties were bred at the beginning of the twentieth century," speculates Yoshida. "What is for certain is that these findings will greatly help us to understand the dynamics of emerging pathogens. This type of work paves the way for the discovery of many more treasures of knowledge hidden in herbaria." 
 
 Explore further: Bioscience researchers defeating potato blight 
 
 More information: Kentaro Yoshida et al. Herbarium metagenomics reveals the rise and fall of the Phytophthora lineage that triggered the Irish potato famine, eLife, in press, doi 10.7554/elife.00731 
 
 ||||| Image caption A potato specimen from Kew Gardens collected in 1847 
 
 Scientists have used plant samples collected in the mid-19th Century to identify the pathogen that caused the Irish potato famine. 
 
 A plant pest that causes potato blight spread to Ireland in 1845 triggering a famine that killed one million people. 
 
 DNA extracted from museum specimens shows the strain that changed history is different from modern day epidemics, and is probably now extinct. 
 
 Other strains continue to attack potato and tomato crops around the world. 
 
 The fungus-like infection causes annual losses of enough potatoes to feed hundreds of millions of people a year. 
 
 A team led by The Sainsbury Laboratory, Norwich, traced the global spread of potato blight from the early 1800s to the present day. 
 
 Until now, it has been unclear how early strains of Phytophthora infestans are related to those present in the world today. 
 
 The Irish Potato Famine Altogether, about a million people in Ireland are estimated to have died of starvation and epidemic disease between 1846 and 1851 
 
 Some two million emigrated in a period of a little more than a decade from 1845 
 
 Comparison with other famines suggests the Irish famine of the late 1840s, which killed nearly one-eighth of the entire population, was proportionally much more destructive of human life than the vast majority of famines in modern times 
 
 The famine began as a natural disaster, but historians say its effects were worsened by the actions and inactions of the Whig government of the time Source: BBC History 
 
 Researchers in the UK, Germany and the US analysed dried leaves kept in collections in museums at Kew Royal Botanical Gardens, UK, and Botanische Staatssammlung Munchen, Germany. 
 
 High-tech DNA sequencing techniques allowed them to decode ancient DNA from the pathogen in samples stored as early as 1845. 
 
 These were compared with modern-day genetic types from Europe, Africa and the Americas, giving an insight into the evolution of the pathogen. 
 
 "This strain was different from all the modern strains that we analysed - most likely it is new to science," Prof Sophien Kamoun of The Sainsbury Laboratory told BBC News. 
 
 "We can't be sure but most likely it's gone extinct." 
 
 Treasures of knowledge 
 
 The researchers believe the strain - HERB-1 - emerged in the early 1800s and continued to spread globally throughout the 19th Century. 
 
 Only in the 20th Century, after new potato varieties were introduced, was it replaced by another Phytophthora infestans strain, US-1, which is now dominant around the world. 
 
 Image caption Potato blight is a major problem today 
 
 The research, published in the new open-access scientific journal, eLife, suggests crop breeding methods may have an impact on the evolution of pathogens. 
 
 "Perhaps this strain became extinct when the first resistant potato varieties were bred at the beginning of the 20th Century," said Kentaro Yoshida from The Sainsbury Laboratory. 
 
 "What is certain is that these findings will greatly help us to understand the dynamics of emerging pathogens. This type of work paves the way for the discovery of many more treasures of knowledge hidden in herbaria." 
 
 Commenting on the study, Professor Sir David Baulcombe of the Department of Plant Sciences at the University of Cambridge said it shows how we can use herb specimens to track biodiversity. 
 
 "It might be a revival in the fortunes or relevance of dried plants," he said. "It illustrates very nicely the arms race over pathogens and their host." 
 
 Phytophthora infestans - which causes potato blight - emerged in the US in 1844, and spread to Europe the following year. 
 
 The summer of 1845 was mild but very wet, giving the perfect conditions for the blight to spread. 
 
 The failure of the crop in Ireland - which relied heavily on potatoes as a food source - led to the deaths of about a million people from starvation and disease between 1846 and 1851.
– Nearly two centuries later, researchers believe they have identified the pathogen that led to Ireland's deadly potato famine. To make their discovery, British, German, and American scientists sequenced DNA from samples of dried potato leaves collected between 120 and 170 years ago, reports PhysOrg. They identified the particular strain of Phytophthora infestans in samples dating to 1845, the year the pathogen arrived in Ireland, then compared the genetic information to that of today's plants, the BBC reports. As PhysOrg explains, researchers had previously believed US-1, today's major pathogen, was the culprit. Instead, they found that a new strain, dubbed HERB-1, triggered the outbreak. Scientists believe HERB-1 appeared at the beginning of the 19th century and wasn't replaced by US-1 until the 20th century, when new potato varieties were planted. ("We can't be sure but most likely [HERB-1 has] gone extinct," says one scientist.) Besides solving a 168-year-old mystery, the find has big implications: It marks the first time the genome of a plant pathogen has been decoded using dried, preserved plant specimens, and paves the way for similar research in the future.