A million or more died in Ireland’s devastating potato famine – and now scientists finally know what caused the tragedy.

It’s 2013, and scientists are working to untangle the precise identity of the pathogen that caused Ireland’s devastating Great Famine. Potato blight is known to have precipitated the tragedy, but only now can the researchers pinpoint the specific strain of the disease. And the answer has come as a complete surprise.

Humans first cultivated the potato between 7,000 and 10,000 years ago in what is now the border area between southern Peru and northern Bolivia. And it was the Spanish who brought the vegetable to Europe after they’d conquered the Incas in the late 16th century. The new crop was of great benefit, too, as some estimates state that the potato was responsible for around 25 percent of population growth in Europe, Asia and Africa between 1700 and 1900.

After the tuber first appeared in Ireland, meanwhile, it took root in the gardens of the upper classes. Initially, then, it seems, the potato was not particularly popular with the common people of the country. Nevertheless, the crop gradually caught on more widely – although the level of dependency that the Irish had on the potato in the early 19th century took many decades to establish itself.

What’s more, the potato had obvious advantages for the farmers of Ireland. You see, while many owned only tiny portions of land to cultivate – the majority of small tenant farmers possessed only 15 acres or less each to their names, in fact – they also typically had large families and many mouths to feed. It made sense, then, to grow the most productive crop available – which was the potato.

After all, potatoes not only grew well – even on poor land – but they were also easy to store for lengthy periods after harvesting. In terms of calories, too, the tubers are worth three times as much as the equivalent weight in grain. With all this taken into account, it stands to reason that potatoes were at the center of many Irish families’ meals by about 1800.

As a consequence, then, while farmers in Ireland once had diets based around grains and dairy products, the potato ultimately took over. But why had the country ended up so dependent on the vegetable? Well, the answer lies in both the structure of Irish society and the fact that the whole island was a British colony.

The British had gained the upper hand in Ireland after the defeat of Jacobite rebels at the Battle of Aughrim in 1691. And although the vast majority of Irish at the time – as many as 95 percent – were Catholics, discrimination against them was enshrined in law. Instead, Ireland was ruled by a minority of Anglo-Irish families who were Protestants.

Then came the Acts of Union in 1801, which actually made Ireland a part of the United Kingdom. The country was not, however, an equal partner within that union. Religious discrimination, for one, put the vast majority of Irish at an inescapable disadvantage to both their British cousins and their own ruling class.

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In turn, this system of oppression meant that huge numbers of the Catholic population lived as relatively destitute peasant farmers. The majority were also tenants with no property rights. And even though the worst of the laws discriminating against Irish Catholics were repealed in 1829, the position of Ireland’s poverty-stricken farmers did not improve as the 19th century rolled on.

In fact, the British government began to recognize that the ownership of Irish land and the hardship experienced by the peasant farmers were both problems that needed to be addressed. The poor could become a dangerous force if they were left in dire straits, and the country could not sustain the depredations of absentee landlords indefinitely.

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So, in 1843 the British established a Royal Commission to examine the problems of Ireland. And in its report, the commission wrote, “It would be impossible adequately to describe the privations which [the Irish laborer and his family] habitually and silently endure… In many districts their only food is the potato, their only beverage water.”

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At the conclusion of the report, moreover, the commissioners recognized a “strong sense of the patient endurance which the laboring classes have exhibited under sufferings greater, we believe, than the people of any other country in Europe have to sustain.” Yet as bad as things were for many of the Irish in the early 1840s, they were about to get much worse.

The potato blight is thought to have arrived in Ireland in 1844 after originating, it’s said, in Mexico’s Toluca Valley. And from Mexico, the disease had then spread to North America and then across the Atlantic to Europe. In the east of the United States, potato crops in 1843 and 1844 were devastated as a result.

And it’s possible that blight arrived in Europe aboard shipping from port cities like New York, Baltimore and Philadelphia. Potatoes would have been aboard transatlantic clipper ships bound for Europe, after all, as food for the passengers. But Ireland wasn’t the only European country to be affected; signs of the crop problem also appeared in northern France, Belgium, the Netherlands and the south of England.

In September 1845, though, reports emerged that suggested the blight had hit the Irish. On September 11 the Freeman’s Journal noted “the appearance of what is called ‘cholera’ in potatoes in Ireland, especially in the north.” Then, mere days later, The Gardeners’ Chronicle explained, “We stop the press with very great regret to announce that the potato murrain [blight] has unequivocally declared itself in Ireland.”

Yet at first the British government took a view that with hindsight looks delusional. In a letter written in mid-October 1845, Prime Minister Sir Robert Peel suggested that the Irish press was known for exaggeration, meaning this may also have been the case with the articles addressing the blight. But the truth was that at least one-third – and possibly as much as half – of Ireland’s potato crop had been lost to the phenomenon.

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Blight is caused by a microorganism known as Phytophthora infestans, which tends in turn to render potatoes inedible. Phytophthora infestans is actually a type of mold that thrives in wet, cool conditions such as those that were experienced in Ireland, with the first signs that blight has taken hold being dark patches on the leaves and stems of the potato plant.

And after the spores of the potato blight mold are spread by wind, they can infect and destroy crops. In particular, rain may move the spores from foliage to the soil below, where they can infiltrate a plant’s tubers. Patches of discoloration then appear on any potatoes, and they start to rot.

Image: Ronald Hutten, Laboratory of Plant breeding, Wageningen University

Even when seemingly healthy-looking potatoes are dug from the ground, in fact, they will later deteriorate if they have been infected with mold. And potato blight remains difficult to control today, although it can be contained with modern chemical treatments. Sadly, though, no such remedy was available to Irish peasants in the mid-19th century.

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After that first attack of blight in 1845, a second infestation came in the following year. In all, then, some three-quarters of Ireland’s potato crop was lost in the 1846 harvest. And although deaths due to starvation had already started after the 1845 failure, yet more Irish citizens would perish – not least because three million were reliant on the vegetable that the blight had affected.

Despite the apparent complacency of Sir Robert Peel towards the potato blight, however, the British prime minister had actually been concerned about the problem from as early as the fall of 1845. So, in secret, he ordered maize and cornmeal from America to the tune of £100,000. This was a vast sum at the time.

But this attempt at famine relief was not an unmitigated success. You see, the maize that the authorities shipped to Ireland – which arrived in February 1846 – had to be milled. Yet the mills in Ireland were not set up to deal with the grain. And even after milling, the maize was exceptionally coarse and difficult to digest; indeed, the Irish came to call the resulting substance “Peel’s brimstone.”

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Furthermore, even while the famine hit deeper in Ireland and people died of starvation, food was still being exported from the country. In 1847, when some 400,000 Irish citizens passed away as a result of not having enough to eat, around 4,000 merchant ships left Irish ports laden with comestibles bound for British ports such as Liverpool, Glasgow and Bristol.

And, incredibly, research by historian Christine Kinealy found that export of some foodstuffs actually increased in Ireland during the worst period of famine. Throughout that tragic time, well over 800,000 gallons of butter sailed across the Irish Sea to Britain, for example.

Yet people from around the world dug deep to donate money to relieve Irish suffering, with figureheads as diverse as the Russian Tsar, the Pope and Queen Victoria all giving cash. One potentially apocryphal story states that the Ottoman Emperor’s Sultan Abdülmecid I wanted to donate £10,000 to the famine effort. Apparently, though, he was asked to limit this to £1,000 or he would have outdone Victoria, the queen of Ireland, who had given only £2,000.

Americans were especially generous in the help they offered to the stricken Irish. Senator Henry Clay appealed to the philanthropy of the American people, and so 118 ships carrying nearly $550,000 worth of relief supplies crossed the Atlantic. Nor were American Catholics the only ones contributing to the efforts; members of other denominations – including Presbyterians, Jews, Quakers and Methodists – all gave donations, too.

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The reaction of the landlords, on the other hand, to these impoverished and starving Irish tenant farmers was enormously callous. As the tenants could no longer pay their rents, a series of mass evictions began in 1847. There was resistance to these actions, though, and in the latter part of 1847 six landlords were shot and killed.

Still, there was one escape route from the deadly famine: emigration. In the 30 years before the potato blight, around one and a half million people had left Ireland, so departing the country for new lands was already an established practice. During the “Great Hunger,” as it became known, however, the rate of emigration greatly accelerated.

Once the famine got under way, as many as 250,000 left Ireland each year, in fact. Favored destinations included North America and the other countries of the United Kingdom. But emigration was no easy way out. And there was a reason why the dilapidated and dangerous vessels that transported the Irish away from their homeland came to be known as “coffin ships.”

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Evidence of the terrible conditions faced by the emigrants can be seen by taking a look at the fates of the more than 100,000 people who left Ireland for Canada in 1847. One estimate puts the death toll among these citizens at one in five. And some 5,000 died on Grosse Isle on the Saint Lawrence River, where the emigrants had to wait in quarantine before being allowed to disembark at the city of Quebec.

The Irish also flocked to U.S. cities as the famine raged. In fact, by 1850 around 25 percent of the respective populations of Boston, Baltimore and New York were of Irish extraction. And the impact of the blight on the number of people in Ireland was extraordinary. In 1841, you see, a census showed that just over eight and a half million individuals lived in the European country. A decade later, however, the next such survey tallied just five million.

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And the Irish population continued to decrease, reaching a low of fewer than three million in 1961. Only after that year did the numbers of people living in Ireland begin to recover. But even now, the country is not populated to the extent that it was back in 1841; in 2016 less than five million dwelled in the nation.

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Yet despite our knowledge of the emigration records, pinning down the precise number of people who died of disease and starvation during the famine is difficult. And although the Catholic Church did keep track of the Irish population at the time, its documents on the matter are far from comprehensive. As a consequence, then, estimates of deaths attributable to the potato blight range from 775,000 to one and a half million. It’s thought, too, that between a further one and a half million and two million people emigrated from Ireland in that period.

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So what exactly was the pathogen that caused the potato blight? Well, for many years scientists believed that the culprit was a strain of Phytophthora infestans known as US-1. In fact, US-1 is behind the deterioration of crops right up to the present day and so ultimately costs nations billions.

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In order to reach solid conclusions, however, a team of scientists from Germany’s Max Planck Institutes and the U.K.’s Sainsbury Laboratory teamed up in a bid to pinpoint the exact strain of Phytophthora infestans that had caused the blight. Of course, an appropriate way to do this would generally be to analyze genetic material from infected plants.

But as we’re talking about events that happened over 170 years ago, how could samples of blighted potatoes from that period be found? The answer turned out to be in the collections of Kew Gardens in London and the Bavarian State Collection for Botany in Germany. Both locations, you see, included dried specimens of 19th-century potato plants.

Eventually, then, the researchers had elements from 11 separate blight-infested plants to work with. And while these samples had been collected by 19th-century scientists, only now could the information locked in the plants be uncovered. So the scientists set about mapping the genetics of the blight strains that had infected these preserved plants.

This work, moreover, involved analyzing both the 19th-century specimens and 15 contemporary ones. And in the process, the team were able to corroborate the theory that Phytophthora infestans had originally come from Mexico’s Toluca Valley. Yet they were not actually able to confirm that the strain of blight that had caused death and devastation in Ireland was US-1 – as had been expected.

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Instead, the mid-19th-century potatoes had been infected by a blight variant called HERB-1 – a strain that had not actually been known to scientists before this time. The researchers believe that HERB-1 probably arrived in Europe from the U.S. and that it’s probably extinct today.

But regardless of what caused the Irish potato blight, its impact and the resulting starvation continue to be felt in both the European country and the Irish diaspora. Every year, National Famine Commemoration Day marks the deaths of so many Irish men, women and children as well as the mass migration caused by the Great Hunger. It’s an occasion, then, on which to remember the terrible suffering of the nation’s people during the 1840s and beyond.