The Irish Potato Famine killed about 1 million people in the mid-19th century, but the exact strain of the potato blight that caused massive crop failures has never been identified, until now.

According to research to be published in the journal eLife, the Great Famine was caused by a previously unidentified strain of the fungus-like pathogen Phytophthora infestans. Scientists have long known that this pathogen caused the famine, but the events in Ireland had previously been linked to a strain of the pathogen called US-1.

A research team led by Sainsbury Laboratory in the U.K. wanted to know if that was true. They extracted DNA from several museum samples collected in the 1840s — potato plant leaves that contained traces of the blight — and compared them to modern-day strains of the pathogen. They found that it was not US-1 and was, in fact, something new. "This strain was different from all the modern strains that we analyzed — most likely it is new to science," Sainsbury Laboratory's Sophien Kamoun told BBC News. They have dubbed the strain HERB-1.

Kamoun said the research revealed something else: the HERB-1 blight may no longer exist. "We can't be sure, but most likely it's gone extinct," he said.

The researchers say HERB-1 likely originated in Mexico, which supports long-held suppositions. Their genetic tests found that it was similar to US-1, which is still found around the world. As they wrote in the abstract to their paper, "HERB-1 is distinct from all examined modern strains, but it is a close relative of US-1, which replaced it outside of Mexico in the 20th century."

HERB-1 may have only existed on Earth for a few decades, and possibly only a few years before it began its deadly impact. The US-1 and HERB-1 strains "seem to have separated from each other only years before the first major outbreak in Europe," co-author Hernán Burbano from the Max Planck Institute for Developmental Biology said in a news release about the discovery.

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