For the first time, scientists have used dried herbarium samples to decode the genome of a plant pathogen and its plant host. The pathogen in question is Phytophthora infestans, and its host, the simple potato—a duo responsible for the devastating famine that swept through Ireland in the mid-1800s, causing a mass exodus out of the Irish countryside as well as a ghastly death toll (due to both starvation and the rapid acceleration of disease) from which the country’s population is still recovering today. Through their research, a team of European molecular biologists have been allowed to confidently pin the agricultural disaster on a special strain of potato blight called HERB-1.

In order to examine the pathogen, molecular biologists had to reconstruct the spread of the potato blight pathogen using dried plants, a difficult feat that was helped immensely by samples so well preserved that, despite being 120 to 170 years old, they still contained several intact pieces of DNA. A fungus-like oomycete (microscopic, absorptive microorganisms that are often referred to as water molds), Phytophthora infestans has evolved over time. A different strand of the pathogen called US-1 was long thought to be the cause of the famine, but in their comparison of the historic samples with samples from today, the scientists have been able to conclude that US-1 is actually an evolved version of a separate, previous strain: HERB-1.

Thanks to the surprisingly well-preserved pathogen samples, the researchers have been able to estimate that the HERB-1 strain probably appeared in the early 1800s and continued to spread throughout the 19th century. Only in the 20th century, with the dawn of new potato varieties, did US-1 appear to take the place of HERB-1. While unique from US-1, the newly discovered HERB-1 shows definite signs of relation to its descendant strain. “Both strains seem to have separated from each other only years before the first major outbreak in Europe,” says Hernàn Burbano of the Max Planck Institute for Developmental Biology.

This conclusion is a huge breakthrough in a new area of research that seeks to understand the way pathogens evolve, as well as the effects of human activity on the growth of plant disease. "Perhaps this strain [HERB-1] became extinct when the first resistant potato varieties were bred at the beginning of the twentieth century," speculates Kentaro Yoshida from The Sainsbury Laboratory in Norwich. "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."