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Scientists used modern genetic sequencing and rotten old museum samples to peer back in time at the cause of the potato blight that led to more than 1 million deaths in Ireland in the 1840s.

The fungus-like water mold that ravaged the country’s potato crop sent hungry Irish survivors fleeing for far-flung new countries — which is why so many people now justify getting wasted every St. Patrick’s Day, saying they’re sure they have an Irish ancestor somewhere in their family tree.

What the scientists found was a strain of Phytophthora infestans that is different from similar water molds that are still ravaging the world’s crops. From the BBC:

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.”

Thing is, the scientists can’t figure out what made the water mold so devastating. From an article in Nature:

[Plant Geneticist Detlef] Weigel’s team also found nothing in the nuclear genomes of the famine strains to explain their ferocity. In fact, the strains lack a gene found in modern strains of P. infestans that overcomes the plant’s resistance genes. And, surprisingly, the famine strain seems less lethal than the P. infestans strains that now cause US$6 billion in crop damage per year. “It seems rather that the potatoes were unusually susceptible,” he says.

OK, all very interesting. But given that the mold strain responsible for the Irish famine appears to have gone extinct, we have some advice for the scientists who are done analyzing the infected old potato leaves: Burn them.