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What caused the Irish Potato Famine of 1845?



June 25, 2013
Kew Herbarium Phytophthora infestans specimen 1847

Kew Herbarium Phytophthora infestans specimen 1847

Scientists have extracted DNA from 166 year old herbarium specimens of the dried leaves from infected potatoes, held since a potato late blight devastated crops in the mid 19th century. Potato losses caused severe famines in both Ireland and the Scottish Highlands in which more than a million people starved to death, as well as significant social upheaval throughout Europe.

Phytophthora infestans effects on potato. USA Dept Agriculture

Phytophthora infestans effects on potato. USA Dept Agriculture

A team led by the Sainsbury Laboratory in Norwich used DNA ‘shotgun’ sequencing techniques to show that the blight in museum samples from the UK, Germany, USA and Canada, and collected between 1845 and 1896, all belong to a single lineage called HERB-1 that is related to but different from the US-1 group blights that infect potatoes today. It’s likely the 19th century HERB-1 strain would have become extinct once resistant potato varieties were bred in the early 1900s.

The fungal-like water-bornĀ Phytophthora infestans emerged in the early 1800s and spread from the USA throughout the world over the following decades. Its arrival in Ireland during the wet and mild summer of 1845 allowed the blight to spread rapidly, destroying the population’s main food crop, causing over one million Irish to starve to death in the following 5 years and a wave of Irish migration to the USA, Canada and Australia.

The spread of Phytophthora infestans HERB-1 and US-1 Yoshida et al in eLifeBoth HERB-1 and US-1 are thought to have arisen from meta populations of potatoes growing in North America, rather than from the potato’s place of origin in Mexico. Usually pathogens develop alongside their host in a sort of ‘arms race’ with the pathogen at first gaining the ascendancy and then the plant developing naturally resistant mutations through its genetic diversity. When pathogens arise in clonal meta populations, they can have much more devastating effects like the potato blight of the 19th century.

Read the full paper at eLife – The Early Days of Late Blight

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