History Corner: An Irish Plant Pathogen

Donna Allen, 50% Irish
ISDH Field Epidemiologist, District 1

In honor of St. Patrick’s Day, March 17th, the following article describes a significant event which had an impact on the health of the Irish people. The Irish Potato Famine began in 1845 and robbed the Irish of their main sustenance for approximately five consecutive years.  The cause of the potato blight was a plant pathogen, Phytophthora infestans. This pathogen, which is similar to a fungus, contributed to approximately 750,000 deaths.  The actual number of deaths is unknown because many died unseen and were buried in shallow, unmarked mass graves. An additional two million Irish residents left their homes for Great Britain, Canada, and the United States. This famine was more destructive to human life than many modern day famines and is an example of how a single pathogen can greatly affect mortality, morbidity, and economic and social patterns of a country.

The Famine

During the 1800s, despite attempts at rebellion, Ireland was ruled by the British. Most of the land was owned by English landlords who sublet small parcels of the land to Irish tenants at high rates.  (Often these tenants were people whose families had historically owned the land.)  The tenants could afford the high rates because they sold or traded their crops.  Before the famine, Ireland’s population exceeded 8 million people. While harvests were bountiful, employment opportunities were few. In 1835, 75 percent of Irish workers lacked consistent employment.

During the 1800s, the Irish fed their growing population by planting potatoes. A farmer could grow three times as many potatoes as grain on the same plot of land. The potato provided 60 percent of the nation’s food needs, and many Irish consumed 8-14 pounds of potatoes each day. They planted a potato called the “lumper”.  Potatoes are propagated vegetatively, so diseased potatoes used as planters sprouted diseased shoots.  Moreover, the potatoes were all genetically identical to one another, so there was no genetic variation in immunity to pathogens.  In 1845, the “lumpers” became susceptible to Phytophthora infestans.  Not all of the potatoes were infected the first year, so farmers replanted the same potatoes during the next few years hoping for better results. The situation worsened as the spores spread.

The Irish relief effort soon came under the control of Charles Edward Trevelyan, Assistant Secretary of the British Treasury.  The English sent Indian corn from America to aid the Irish.  However, the corn had to be ground into digestible cornmeal, and there weren’t enough mills available. The cornmeal itself caused problems—it was difficult to cook, hard to digest, and caused diarrhea. It also lacked vitamin C, so scurvy, a disease previously unknown to the Irish, became a problem. Cornmeal stocks were depleted after the first year, and the Irish survived by selling their livestock and all their possessions to buy food. Trevelyan believed in the popular theory of the day, “Laissez-faire (let it be),” which advocated a belief that a situation would eventually solve itself through natural processes. The British economy was also suffering at this time, and food supplies were very tight throughout Europe.


The Irish Potato Famine gave birth to the science of plant pathology.  When the famine occurred in 1845, Louis Pasteur had not completed his work with bacteria, and the germ theory of disease had not been developed. The famine supported the idea that infectious microorganisms and plant pathogens can lead to disease.  Prior to this time, it was felt that diseases and crop failures were caused by bad omens, weather, and other superstitions. People had noticed localized outbreaks of the disease early in the 1800s, but it didn’t become a widespread problem until 1845.

Photo of a microscope slide showing Phytophthora infestansPhytophthora infestans, although similar to fungi, is actually part of the Protista kingdom, which also includes slime molds and brown algae. This pathogen is usually not a problem unless the weather is cool and wet. The water is necessary for the spores to float and infect the leaves of potato plants.  In wet weather, an enormous number of spores, called zoospores, are produced.  During the famine, the plant foliage turned to a putrid mass in a few days, and the tubers were affected to various degrees.  Under ideal weather conditions, Phytophthora can kill a field of potatoes in just a few days.

Diagram: disease triangle with three sides labeled "Favorable Environment", "Virulent Pathogen", and "Susceptable Host."The conditions leading to disease during the potato famine mirror the epidemiologic triad:  virulent pathogen, susceptible host, and favorable environment.  One of the most important lessons learned from the famine is the need for genetic variation in crops. Lack of genetic variation in the Irish potatoes contributed to the severity of the famine. Plant populations with low genetic variation are more susceptible to changing environmental conditions and diseases.  Because all of the potatoes were susceptible, the plant pathogen spread rapidly when it infected the Irish potatoes.  If potatoes of different genetic varieties had been planted, it is likely there would have been varying levels of resistance, and more resistant varieties could have been planted in the following years.
Sections of Ireland’s population were entirely dependent upon the potato. Ireland at this time had only 70 miles of railroad track, so efficient food distribution was not possible.  Fish remained out of reach in waters that were too deep, and starving fishermen had sold their nets and boats to buy food. In addition, the Irish had ancient cultural taboos against eating certain foods. For example, seals were thought to be reincarnated relatives. When food became available, the Irish had no money to purchase it.  Without money, residents were unable to pay rent, and many became homeless.  Phytophthora infections have occurred in the northeastern United States and in other countries.  However, because of a much more diversified diet, the effects have not been nearly as devastating as those resulting from the Irish Potato Famine.

The Irish Potato Famine demonstrates how a pathogen can devastate a nation. Most deaths were due to associated diseases, not from hunger.  The famine led to severe poverty and homelessness, which led to unsanitary and crowded conditions. Diseases such as cholera, dysentery, scurvy, typhus and lice infestations were common.  Typhus was called the “Black Fever,” because it blackened the skin and was carried by body lice from town to town by beggars and the homeless.  Many doctors, nuns, and others who attended to the sick in lice-infested dwellings also became ill.  Masses of bodies were buried without coffins just a few inches below the soil. 
Those who traveled to other countries did not fare much better.  They traveled in crowded conditions that were perfect for disease transmission.  Hundreds of men, women, and children were crowded together with no ventilation and no sanitary facilities. Another problem was the lack of potable drinking water. Water was stored in old, unwashed wooden casks that had previously stored wine, vinegar, or chemicals, which leached and contaminated the water. The trip to Quebec, Canada, took from 40 days to 3 months, depending on the weather. Shiploads of feverish Irish overwhelmed local medical facilities.

In Boston, during this same time, there was neither enforcement of sanitary regulations nor building or fire safety codes in place. A single-family three-story house was divided room by room into housing for 100 Irish. On average, adult Irish lived six years after stepping off the boat onto American soil.  Cholera was the predominant disease, and infant mortality rates were high.

The Irish Potato Famine teaches several lessons. It supported the fact that disease is a biological activity and not a mysterious event brought on by superstitions.  Many lessons in plant pathology were learned, from introducing a foreign plant into a new environment (corn) to the importance of producing genetically variable products (potatoes).  The famine also demonstrated the consequences of relying on one crop to feed the majority of people as well as the social, public health, and economic consequences that can occur as the result of starvation.


1.  Tom Volks’s Fungus of the Month for March 2001
2.  The Great Famine: Gone But Not Forgotten by Sean Henahan
3.  http://evolution.berkley.edu/ Monoculture and the Irish Potato Famine: cases of missing genetic variation
4.  http://www.apsnet.org/ The Irish Potato Famine and Birth of Plant Pathology by Gail Schuman