The Irish potato famine

A lack of genetic diversity was a major factor in Ireland’s biggest catastrophe, a tragedy that became known as the Irish Potato Famine.

The potato is believed to have originated in the Andes Mountains of South America (and even today the Andes are a source of a great diversity of potatoes whose relatives are not found elsewhere). Spanish explorers took the plant home to Europe around 1570, and it is said that the British explorer Sir Walter Raleigh introduced it in England a few years later.

A potato plant. Note that the part most people eat, the enlarged root (tuber), is underground. For larger views of this and the sweet potato, click on the image. (International Potato Center)

The potato was not popular at first (somebody started a rumor that it was poisonous), but it caught on after an unusual promotion campaign: Big-name landowners and members of royalty urged their tenants to plant and eat the crop. Potatoes were especially popular in Ireland, where the cool, damp climate favored their production. They were so popular there, in fact, that the common potato is still known today as an “Irish potato.” One source indicates that Irish farm workers in the 19th century ate more than 3 kg (6.6 pounds) of potatoes each day!

Potatoes do not easily grow from seed, as do many other foods. They reproduce from enlarged underground root parts called tubers that are growing on existing plants. Potatoes are, in other words, clones of their mother plants.

There is a great deal of genetic diversity in potatoes. Some of the varieties are very strong at resisting diseases. But the potatoes that were growing in Ireland and Europe in the mid-1840s represented a very limited number of varieties. They lacked resistance to a fungus named Phytophthora infestans. The result was a terrible famine in Ireland, where so much agriculture depended on a single crop.

When Phytophthora infestans infected some potatoes in Ireland, it rapidly spread to others in what has been called the Irish potato blight, or famine. Edward R. French, a bacteriologist at the International Potato Center in La Molina, Peru, points out that the fungus got its name from the Greek for “plant destroyer.” The blight, French says, is “the most dramatic of any plant disease.” He likens it to “a dragon that spews flames and just burns up the crop. It leaves nothing; it just burns everything to the ground.”

Smelly black rot.
Hectare after hectare of Irish farmland turned into a smelly black rot. The fungus, which is distributed by spores on the wind, quickly moved to Europe. Its effects were less serious there, because European farmers had other crops on which they could rely. Irish Catholic farmers, however, who paid their rent to British and Protestant landlords in crops, had no crops to hand over, and many were evicted from the land.

The starving farm families were also forced to eat the rotten produce, which sickened them. Cholera and typhus, both deadly diseases, joined with starvation to kill an estimated 1 million people. The British landlords and government did little to help, and even today this neglect is a cause of anti-British and anti-Protestant feeling in Ireland.

Hundreds of thousands of starving Irish tried to leave their disease-ridden country. They crowded on to unstable vessels, which came to be called “coffin ships,” and set out for America and other countries. Those who made it were part of a huge wave of Irish immigration to the United States, and the movement seriously affected the makeup of Ireland itself. Before the famine, Ireland had a population estimated at 8 million people; after the famine ended around 1850, the population had been reduced by death and migration to 5 million. In the meantime, Irish-Americans became prominent and important parts of the U.S. population.

Would more diversity have prevented the Irish Potato Famine? Because the potatoes in Ireland are essentially biological copies of each other, it would have been difficult (but not impossible) to use genetic diversity to quickly replace them with varieties that resisted the blight. The economics of trade between Irish farmers and British landlords had a lot to do with the seriousness of the tragedy.

But farmers learned one important lesson about genetic diversity: They stopped relying on just one crop. They began to appreciate the importance of planting a number of food crops, each serving as insurance in case another one failed. This is a lesson that Andean farmers had learned long ago. They encouraged diversity (and still do) by growing many different potato varieties in the same field.

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Poster-size drawings of potato and sweet potato plants

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