Irish Potato Famine pathogen *Phytophthora infestans* was not a fungus

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 a similar way, and was classified by scientists as one for decades.

It took 180 years to determine what it really is, where it originated, and how many related organisms are destroying crops around the world.

The Irish potato disaster begins

In 1845, potato plants throughout Ireland started to blacken in the fields. Within only a few growing seasons, the Irish Potato Famine had caused the deaths of about 1.5 million people and forced approximately the same number to emigrate.

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

Phytophthora infestans: “Plant destroyer”

Two scientists began to alter that understanding. 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 responsible organism.

De Bary named the species Phytophthora infestans. Drawn from Greek, its 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 make plants ill, helping to establish an entire scientific discipline.

Cataloguing a dangerous genus

Almost 180 years later, two researchers have devoted much of their working lives to cataloguing what followed that original 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 nearly five decades of work, have published a feature article documenting the complete history.

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

Tracking pathogens through genetics

For over a century, researchers identified Phytophthora species solely by their appearance: spore shape, cell-wall characteristics and growth behaviour under a microscope. The approach was effective, although slow.

DNA sequencing then changed the process. From around 2000, scientists could compare complete genetic signatures rather than relying on physical traits. The number of accepted species rose rapidly.

A 2023 paper recorded 212 species in the genus. The Abads’ latest article raises that total to 261. Certain additions are truly newly identified pathogens.

Others are species that could never have been separated from one another through outward appearance alone.

The origins of the Irish Potato Famine pathogen

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

Some supported central Mexico, where wild relatives of Phytophthora infestans still occur in the highlands. Others argued for Ecuador and Peru, the region in which potatoes themselves originally grew.

A 2025 genomic study examining dozens of isolates found that the species originated 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 worldwide, supporting South America as its centre of origin.

Why old specimens still matter

Species names can shift over time. As methods advance, older descriptions may prove to refer to two organisms, or sometimes to none at all.

To maintain accuracy, taxonomists use ex-type cultures: the original preserved laboratory samples that establish the identity of every species.

The Abads devote a substantial part of their article to explaining the importance of these cultures. Without them, matching a new sample to a description made 50 years ago becomes an exercise in guesswork.

How Phytophthora infestans spread

Phytophthora has proved far larger than anyone could have foreseen in 1876. There are now 261 recognised species, most of which attack something valued by a grower or forester.

They cause 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.

The genus is responsible for billions of dollars in agricultural losses annually and puts food security at risk across continents.

Some species also damage natural ecosystems with no commercial value, making them more difficult to detect before they spread.

Lessons from Phytophthora infestans

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

It also resolves a long-standing argument: the Peruvian Andes, rather than Mexico, are the most likely origin of the Irish Potato Famine pathogen, Phytophthora infestans.

What has changed is the set of tools available for action. Identifying a new outbreak no longer requires months of painstaking microscope work.

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

That speed may reduce the interval between an outbreak and action, a gap which in 1845 extended into 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.”