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« Five Questions About Oomycetes | Main | Genomic Secrets of P. infestans, the Master of Potato Blight »

November 18, 2009

A Mold That Changed the Course of History

In this, the second of our three posts focusing on the oomycetes, we are pleased to offer a post from the blog by Mercè Piqueras, La lectora corrent (The Common Reader), translated here from Catalan.

by Mercè Piqueras


Micrograph of an oospore of Phytophthora
. Source.

Recently, Nature published an article about the genome sequence and analysis of Phytophthora infestans, the oomycete that caused the potato blight in the nineteenth century that changed the course of history. Phytophthora infestans is the reason that today about 11% of the population of the United States is of Irish origin and that the population of Ireland was lower at the end of the 20th century than it had been in the first half of the 19th century.


Symptoms of potato late blight on potato
leaves. Source.

People who migrated to the United States in the 19th century did so for the same reason that pushes many people to migrate today to Europe from Latin American or African countries: to improve their living conditions, sometimes simply to survive. Hunger to the point of starvation was the main cause that pushed many Irish to go to the United States. That hunger even has a name of its own: The Great Famine in English and Am Gorta Mór in gaèlic The cause was potato blight, a pest that is still difficult to control today, although there are several ways to try to prevent it. Potatoes, originally from the Andean regions of South America, were introduced in England in the mid-16th century by Sir Walter Raleigh, who planted them in the fields of his estate in Ireland. In the 17th century the crop spread throughout the island, supplementing the Irish diet that had consisted mainly of cereals and dairy products. By the early 18th century, potatoes had become the staple food of the poor during the winter, and its culture became more and more widespread.

Infected potato

Infected potatoes. Source.

The Great Famine covers a period between 1845 and 1852. Potato blight also spread to other countries in the Europen continent, but nowhere in Europe did it have such disastrous consequences as in Ireland. In fact, the diet of a portion of the Irish people depended on potatoes. In addition, those who raised potatoes for their living were left without income. During those years, Ireland lost 20-25% of its population either by diseases resulting from malnutrition or by emigration.


The Emigrants' Farewell. Source.

Great Britain had been the traditional destination of Irish emigrants, but during the Great Famine they had to set off for other havens. Those were bad years also for Great Britain, where many workers had lost their jobs due to economic recession. To prevent the influx of immigrants from the neighboring island, the English and Scots enacted a draconian "Poor Law." According to that law, Irish immigrants could be returned to Ireland if they could not prove they had lived in Great Britain for at least five years. This might seem to be an immigration law, but we must bear in mind that, at that time, Ireland was part of the United Kingdom—the United Kingdom of Britain and Ireland. The United States was a young and developing country where Irish immigrants were not met by such barriers. Sometimes, the landowners in Ireland encouraged the farmers that had been working in their fields to leave for America. As the case today for many immigrants, the new country did not always fulfill their expectations.

That was how the great wave of Irish immigration to the United States began. Without the plague caused by Phytophtora infestans, the history of the United States would have been different. And the history of Ireland might also have been different. The Great Famine, and the way the Irish people felt they were treated by the British, may have been one of the causes that sparked Irish nationalism and made it possible for most of the island to become an independent country: the Irish Free State, currently the Irish Republic.

To learn more about the effects of potato blight on Irish society, click here.


Mercè hosts a delightful eclectic blog in Catalan where she shares her passion for words, science, and writing.


It is surprising to know that even very small things like molds have impact in history. This grows creature have somehow advantages.

As I read the history of the Irish famine, the mold was necessary but not sufficient to explain the disaster. As usual, politics contributed. The British could not by themselves feed all the Irish, but they has passed a protectionist law called the corn laws that prohibited importation of cheaper cereals into the UK (Ireland was part of the UK). That law had to be repealed and by doing so the Tories lost control of parliament. There was also famine in other parts of Europe, limiting the options of the Irish and the English. Finally, there was a cholera epidemic, and perhaps bacillary dysentery that killed lots of Irish made vulnerable by malnutrition.

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