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Irish potato blight pathogen mystery solved: study


A potato leaf collected in 1847 during Ireland's Great Famine, now housed in the Kew Gardens herbarium. Researchers decoded the genome of the blight that caused the famine by collecting genetic material stored in herbariums. (Marco Thines/Senckenberg Gesellschaft für Naturforschung)


Using crumbling leaf specimens from 19th century herbariums, an international team of researchers has unravelled the mystery of the potato disease that caused Ireland's Great Famine by decoding the pathogen's genome.

Researchers have long known that the great potato blight -- which actually affected all of Europe in the 19th century, but which caused such devastation to the Irish that population levels have never recovered -- was caused by a pathogen called Phytophthora infestans. Modern incarnations of the disease still wreak havoc on the important potato crop. 

But the exact strain of the disease that led to the deaths of a million Irish from 1845 to 1849 has been a mystery.

Molecular biologists gathered 11 historical samples of dried potato leaves from Europe, the British Isles, and North America and managed to extract enough genetic material to sequence the genomes of the pathogens they found on those plants. The specimens had been housed at Kew Gardens, London, and Munich's Botantical State Collection.

"The degree of DNA preservation in the herbarium samples really surprised us," said one co-author, the University of Tübingen's Johannes Krause, in a statement.

The team discovered something surprising: that the potato blight was not caused by a Phytophthora infestans strain known as US-1, as has long been thought, but one entirely new to science that they dubbed HERB-1. 

They also compared HERB-1 to other modern strains from around the world and related Phytophthora strains to map how the disease evolved over time. 

Researchers already knew that the Irish blight originated in Toluca Valley, Mexico. And the team discovered that in fact, the first contact between European explorers and Americans in Mexico was also a period of tremendous genetic diversity for Phytophthora infestans. 

HERB-1 probably emerged in the early 19th century and made its way over to Europe, where it devastated crops for most of that century. Only the introduction of new potato varieties halted the spread of that strain.

The team hopes to answer more historical riddles by digging through data preserved in old herbariums.

Kate Allen is the Star's science and technology reporter. Find her on Twitter at @katecallen


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