Potato Blight: The 19th Century Irish Famine and 21st Century Threat

Potato Blight Herbarium Specimen from 1845 – Copyright image courtesy of Jean Ristiano from N.C. State University, used with permission.

Due to the political climate of the era, Irish citizens in the 1800s depended on potatoes for their sustenance.

According to Christopher Cumo’s “Potato,” in 1845, the Irish planted millions of acres to potatoes, expecting a robust crop. Disaster struck when an attack of a new organism, subsequently named potato blight, devastated this crop.

The effect of this blight was widespread famine and death, with one million Irish dying between 1845 and 1849. One and a half million of the survivors emigrated to the United States, Canada, and Australia.

The 2011 study, Understanding and exploiting late blight resistance in the age of effectors, states that potato blight remains a major threat to potato crops throughout the world to this day, causing global losses of 16%. Researchers actively study the biology of this pathogen in hopes of ameliorating its effects.

Fortunately, scientists preserved herbarium specimens of infested potato leaves and tubers from the time of the original Irish famine and from other areas of Europe in the late 1800s. This enabled researchers to do molecular detective work and compare the strains from the 1800s with those of today, thus providing new hypotheses on how the strains originally spread throughout Europe as seen in the 2013 study, Reconstructing genome evolution in historic samples of the Irish potato famine pathogen.

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The Stage Was Set for Famine to Strike

Planting whole crops of the same variety is always risky, since having a uniform genetic makeup makes the crop vulnerable if a new pathogen evolves that can cause disease on that variety. Irish farmers in the 1840s had little choice but to rely on potatoes for their sustenance, however.

These farmers grew large quantities of grain, but it went to their landlords. Not having title to their land, these Irish farmers had to pay high rents for the land they cultivated. The grain they produced went to pay their landlords, but potatoes grew well in their climate and provided sustenance for their families and even livestock.

Plant breeders in the early 1800s created a new variety called the Lumper, and farmers in Ireland planted this new high-yielding variety to the exclusion of all others. When the weather turned wet and rainy in September of 1845, this set the stage for the pathogen to strike, destroying the entire potato crop in weeks.

With nothing to eat and no help from their landlords or the British monarchy, the effects were fatal to the Irish. Famine struck on a large scale, resulting in death for one million Irish peasants over the next several years. The survivors emigrated en masse, forever altering the cultural makeup of the Western hemisphere.

The New Potato Blight Pathogen and the Start of Plant Pathology

Potato growers were first faced with this new disease in 1843 when it struck in Pennsylvania. By 1845, potato blight had spread to much of Europe. Scientists avidly investigated this previously unknown organism. While some argued that the potatoes had died as a result of the wet soil, others pursued a new angle, claiming that a fungus was responsible.

They were proven correct in 1861 when the German scientist Anton de Bary was able to cause the blight by putting tissue from blighted potatoes on healthy plants. De Bary named the pathogen Phytophthora infestans and started the science of plant pathology. The organism is still known by this name, although it is no longer considered a fungus.

While plant pathologists have been intensely studying the biology of this pathogen, much remains unknown about its genetics and biology. New research published in 2013 has shed light on how the strains spread throughout Europe. Scientists had assumed that a single strain spread from the United States to Europe, wreaking havoc as it went.

As shown in their 2013 study, Dr. Michael Martin and his team used a technique called shotgun sequencing to analyze all of the DNA from strains of the 1840s on preserved herbarium specimens of potato tubers and compare them to those of today. Their results comparing the DNA of the historical strains allowed them to put the strains in a family tree, based on the similarity of their genetic material.

Several of the strains from the 1800s appeared to significantly differ from each other, suggesting that there were multiple introductions of the pathogens into Europe. This molecular detective work also identified major genetic differences in the historical strains compared to those that cause disease today. Dr. Martin told Decoded Past that “It appears that the original potato blight-causing strains have died out over time and have been replaced with even more virulent ones.”

Evolution of Potato Blight

From its first attack in the U.S. in 1843, the organism that causes potato blight went on decimate much of Ireland’s population from 1845-1849. From this auspicious introduction to the present day, potato blight has proven that it is a durable and hard to control pathogen that still causes significant losses of potatoes and tomatoes.

Phytophthora infestans continues to be an aggressive pathogen that rapidly evolves resistance to new varieties of potatoes. Recent research on the DNA of the potato blight organism shows that the isolates from the 1800s differed greatly from the strains of today. This provides a window into the evolution of pathogenicity in this versatile organism that rapidly overcomes man’s best effort to control it.