Phytophthora infestans and the Irish Potato Famine: The Organism Behind the Disaster

The organism behind the Irish Potato Famine is not a fungus. Yet most people familiar with it presume that it is: it resembles a fungus, spreads in much the same way, and was classified as one by scientists for decades.

It took 180 years to determine what it truly was, where it originated and how many related organisms are destroying crops worldwide.

The Irish potato disaster begins

In 1845, potato plants throughout Ireland started blackening in the fields. Over just a few growing seasons, the Irish Potato Famine killed around 1.5 million people and forced approximately the same number to emigrate.

Nobody knew what was devastating the crop. Weather was blamed by some, while others pointed to the soil. Plant pathology, the field that might have provided an answer, had not yet been established.

Phytophthora infestans: “Plant destroyer”

Two scientists began to shift understanding of the crisis. In 1846, English naturalist Miles Joseph Berkeley suggested that a fungus was to blame.

Thirty years afterwards, German botanist Heinrich Anton de Bary confirmed that suspicion and named the culprit.

De Bary gave the species the name Phytophthora infestans. Derived from Greek, the genus name translates almost literally as “plant destroyer.” The pathogen now had an identity.

Their research achieved more than the identification of one pathogen. It demonstrated that microscopic organisms could cause plant diseases, opening the way for an entire scientific discipline.

Cataloguing a dangerous genus

Almost 180 years later, two researchers have devoted much of their working lives to documenting what followed that first discovery.

Z. Gloria Abad and Jorge A. Abad, who recently retired from the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) after almost five decades of work, have published a feature article charting the complete history.

Their timeline follows the gradual, painstaking work of describing species individually. It also examines the technological advances that ultimately enabled scientists to separate the genus into 261 distinct species.

Genetic tracking of pathogens

For over a century, researchers classified Phytophthora species solely by their visible characteristics: spore shape, cell-wall traits and growth patterns viewed through a microscope. Although effective, this approach was slow.

The arrival of DNA sequencing changed that. From around 2000, scientists could compare complete genetic signatures instead of physical traits. The number of recognised species then rose rapidly.

A paper published in 2023 recorded 212 species in the genus. The Abads’ latest article raises the total to 261. Some of these are truly newly identified pathogens.

Others are species that could never have been separated by outward appearance alone.

The origin of the famine pathogen

For many decades, scientists disputed where the famine pathogen first emerged.

Some favoured central Mexico, where wild relatives of Phytophthora infestans still inhabit the highlands. Others argued for Ecuador and Peru, the region in which potatoes themselves first developed.

A 2025 genomic study examining dozens of isolates concluded that the species arose in the Andes, where its nearest relatives still survive in the same mountain range.

The Abads’ review brings together the wider evidence, now based on more than 1,700 pathogen samples collected globally, supporting a South American origin.

Why old specimens still count

Species names can shift with time. When methods improve, older descriptions may prove to refer to two organisms, or perhaps to no organism at all.

To maintain accuracy, taxonomists depend on ex-type cultures: the original preserved laboratory specimens that fix the identity of every species.

A substantial part of the Abads’ article argues for the importance of these cultures. Without them, matching a new sample to a description written 50 years ago becomes little more than speculation.

How Phytophthora infestans spread

Phytophthora has proved far bigger than anyone in 1876 could have foreseen. The genus currently includes 261 recognised species, most of which attack something valued by a farmer or forester.

Its diseases include late blight in potatoes and tomatoes, crown rot in strawberries, black pod rot in cacao, sudden oak death in forests across California and Europe, and cypress dieback in Patagonia.

Every year, the genus creates billions of dollars in agricultural losses and puts food security at risk across multiple continents.

Certain species also damage natural ecosystems that have no commercial worth, making them more difficult to detect before they spread.

Lessons from Phytophthora infestans

The Abads’ review formally documents something that did not previously exist: a complete 180-year account of how scientists came to identify 261 species in a genus that began with one.

It also settles a longstanding argument: the Peruvian Andes, rather than Mexico, are the most probable origin of the Irish Potato Famine pathogen, Phytophthora infestans.

The key difference today is the set of tools available to respond. Identifying a new outbreak no longer requires months of meticulous microscopic examination.

Several days of sequencing can establish where a sample belongs within the 261-species genus, allowing regulators to determine the appropriate action.

That speed may reduce the interval between an outbreak and a response - an interval that, in 1845, lasted for years.

“This timeline of 180 years of the taxonomy and identification of Phytophthora is truly fascinating,” the Abads wrote.

“There were many challenges, but also many important technological contributions that have cemented Phytophthora as a solid and unique genus of plant pathology.”