#096: The Great Famine of Ireland and Phytophthora infestans

by Thomas Roehl · Published July 3, 2015 · Updated September 19, 2016

In celebration of Independence Day, I have decided to discuss a fungus-related event which significantly impacted the history of the United States (Don’t worry; I will get to the remaining mushroom toxins in the next few weeks). America is a country of immigrants, so on the day that we celebrate the founding of our country, it only seems fitting that we take a moment to remember how immigration has shaped our history.

The Great Famine of Ireland (known to most people in the United States as the Irish Potato Famine) was caused by a unique convergence of biological, economic, political, and societal factors. The results of the famine were dramatic: the Irish population dropped from over 8 million to under 6 million and has not yet recovered to pre-famine levels. An estimated 1 million people died as a result of the famine (either from starvation or disease) and roughly 2 million people emigrated, with most heading to United States. The consequences of the Great Famine of Ireland helped shape and are continuing to shape British/Irish history as well as American history.

As you probably know, the Great Famine of Ireland was caused by the loss of nearly the entire Irish potato crop. Potatoes were originally cultivated in South America’s Andes by the Incas. The potato was first introduced to Europe by the Spanish sometime in the late 1500’s. Since it belongs to the nightshade family (which also includes tomatoes), Europeans assumed the potato was poisonous. They were mostly right, as all parts of the potato except the tubers are toxic to humans. In the 1700’s, it was finally realized that the potato was edible. Potatoes contain carbohydrates, protein, and vitamin C. When combined with the calcium and vitamin A from milk, they can provide almost all of a person’s nutritional requirements. Potatoes grow easily in the Irish climate, which, together with their nutritional value, made them the perfect staple crop for poor Irish farmers. By 1800, about 90% of the Irish population relied on potatoes for the bulk of their diet. For an adult of the time, this meant eating at least 6.5 lbs of potatoes a day.

The Irish potato crop was composed entirely of a variety known as the “lumper.” Clippings of potato plants can easily be used to start new plants, and this method was used to spread the potato across Ireland. Unfortunately, this “vegetative propagation” results in plants that are genetically identical to one another. Plants in such a “monoculture” are all susceptible to the same diseases, which makes it extremely easy for a pathogen to infect every single plant.

The disease responsible for wiping out the Irish potato crop in the mid 1840’s is now known as Late Blight. Late Blight is caused by the oomycete Phytophthora infestans. The class Oomycota is currently placed in the kingdom Straminopila, which also includes a variety of algae, such as giant kelp. Prior to this, the Oomycota were included in the Fungi. Their modes of growth and reproduction are very similar to those of fungi and, as a consequence, they are still studied by mycologists. Like the other oomycetes, P. infestans has flagellated zoospores, which can swim through water. Because these spores get to new hosts by swimming, the pathogen does better in years with lots of rain. An abundance of water in the soil means the spores can easily find new hosts.

At the time of the Great Famine of Ireland, the island was under the control of the British Empire. The land of Ireland had been given over to British landowners, who leased small lots of land to the Irish. Ireland also experienced a population boom in the early 1800’s, which meant that the lots of land were subdivided further. In order to pay for the use of the land, the Irish had to sell most of their crops to the British. With all their grain being exported, the Irish were left to eat nothing but potatoes.

No one knows exactly when Late Blight first showed up in Europe. It was probably brought over with a batch of potatoes from the Americas, where the disease evolved alongside its host. In the early 1800’s, there were a number of localized outbreaks of Late Blight in Ireland and elsewhere. Late Blight caused regional food shortages, but did not seem to be a widespread problem.

In 1845, however, the entire island experienced unusually wet weather. When September rolled around, the rapidly-spreading disease could be found in roughly 40% of the Irish potato crops. First, the stems turned black, seemingly all at once. By the next day, the leaves of infected plants had also died and started rotting. The edible tubers did not rot as quickly, but they did eventually turn to black goo. The tubers that survived were picked early, so many of them rotted before the end of winter. With a loss of 40% of their potatoes, many Irish were forced to eat their seed potatoes.

Another cool, wet period happened in July of 1846. With all the spores lying around from the previous year’s outbreak, nearly 100% of the potato crop failed. In 1847, there were very few seed potatoes available and farmers were afraid to plant any, lest their crops all died again. Livestock were also fed potatoes, so many of the pigs and cows died. This was bad news for the Irish, who needed milk from cows to fulfill their dietary needs. The blight did not return for the next four years, but the damage to the Irish food supply had already been done and famine conditions persisted.

The Irish put much of the blame for the famine on the British, and rightly so. Although there was a real food shortage, the British could certainly have done more to mitigate the famine. The British did set up soup kitchens, set up a public works project, and repeal the Corn Laws. Unfortunately, all of these actions were too little, too late. The soup kitchens fed 3 million Irish people daily in 1847, but were only in operation for six months. The public works project gave the Irish jobs during the winter of 1847-8, but they were not paid enough to afford the high prices of imported food. The Corn Laws imposed tariffs on imported grains (the American word “grains” corresponds to the British word “corn,” while the American “corn” corresponds to the British “maize”), keeping the price of British grain artificially high. This made it very costly to send corn to Ireland, preventing relief efforts. An attempt to repeal the Corn Laws early on in the famine was shot down. The laws were eventually repealed, but it was far too late to prevent the famine. Additionally, any grain that did make it to Ireland did not provide enough nutrients to fulfill dietary requirements. Without key nutrients, the Irish could not fight off diseases and the grain ultimately did no good. Throughout the famine, the British continued to support landowner evictions of Irish people who were unable to pay for the land, leaving millions homeless and unable to support themselves. Various organizations did set up poor houses to provide relief, but these were poorly managed, overcrowded, and ineffectual.

While these actions (or, more accurately, inactions) may seem callous by today’s standards, they were supported by the prevailing ideologies at the time. Laissez-faire capitalism was a big factor in limiting relief. Any relief scheme would involve money, and the British government thought that capitalism would make the landowners support their tenants (dead tenants, of course, cannot make the landowners any money). However, large farms were on the rise, so a decrease in tenants was actually beneficial to the landowners. Another complicating factor was religion. The mostly protestant British were in the process of trying to convert the mostly Catholic Irish. Some British saw the famine as punishment for the sinful Catholics, which made them reluctant to offer help. Many British also saw themselves as morally superior to the Irish, which offered another reason against sending aid to the Irish.

With the dwindling Irish food supply, many Irish decided to leave their island in search of better opportunities. Many Irish made the journey to the United States of America. Some of these people went unwillingly, when landlords promised them food, clothing, and money with a trip to America. None of these promises were fulfilled, and the Irish ended up being packed into sailing ships bound for Canada. These ships were often barely seaworthy and were crammed full of more people than they could support. About 20% of those making the voyage died, giving these ships the nickname “coffin ships.”

Once they arrived in Canada, most Irish crossed the border into the United States. Irish immigrants tended to be met with hostility by the Americans. They had to take low-paying, unskilled jobs (for example, working on docks and railroads) and extremely low-rent housing. Most of them settled in the same areas as previous Irish immigrants in an attempt to find some sort of support network and community. Unfortunately, Americans reacted to the immigrants with skepticism and hostility. New York and Boston took in most of the Irish immigrants, but many others settled in various cities across the Northern and Western United States. As a result of the sheer number of Irish immigrants, anti-Irish and anti-Catholic sentiments became prevalent in American culture. This resulted in various stereotypes of the Irish and fewer economic opportunities for the immigrants. As the Irish became more established in cities, Americans often chose to fight against the perceived invaders. There were a number of anti-Irish mobs that provoked violence in a few large, Eastern cities. It was not until after the American Civil War that things began to turn around for the Irish. They still took menial jobs and ended playing a large role in building the infrastructure for the Industrial Revolution. It was not until the election of the first Irish Catholic president (John F. Kennedy) in 1960 that it was widely viewed that the Irish had been accepted into American society.

If you are interested in the Great Famine of Ireland or in the history of the Irish in America, I invite you to read more about the subject. What I have provided here is extremely short and leaves out many elements to the story.

Late Blight continued to be a problem after the Great Famine of Ireland, and is still a problem today. P. infestans destroys $5 billion worth of potatoes every year, making it the number one biotic threat to global food security. The potato is still a major staple crop, so understanding Late Blight and the Great Famine of Ireland is of global significance. After the Great Famine, resistant varieties of potatoes were discovered and cultivated. Combined with effective fungicides, this kept Late Blight at bay for a long time. However, this all changed about 30 years ago when a second mating type made it into the United States. Before that, most of the P. infestans around the world had the same mating type (roughly the fungal equivalent of gender), which meant that the oomycete could not sexually reproduce. This was beneficial to farmers, since a lack of genetic recombination greatly slowed down the development of resistance to fungicides. Once the second mating type arrived, sexual reproduction became an option. This has led to a rise in resistance to fungicides and has given the oomycete the ability to produce more resilient spores (sexual oospores). To make matters worse, the other mating type belongs to a more virulent strain of the pathogen, making the disease even more concerning.

In an interesting twist, scientists recently analyzed the genetics of P. infestans from a preserved potato leaf taken during the Great Famine of Ireland. They found it to be a completely different strain of the fungus than any found in the world today. Apparently, the Great Famine strain spread around the world soon after the potato. It persisted as the dominant strain until the 1970’s, when it completely died out and was replaced by modern strains.

We certainly have a lot to learn from the story of the Great Famine of Ireland, which contains valuable lessons for biology, history, disaster relief, and even modern culture. What is history trying to teach you?