In the mid-19th century, a microscopic invader swept across Ireland, leaving a trail of devastation in its wake. The potato blight, caused by the fungus-like pathogen Phytophthora infestans, triggered a famine that killed over a million people and forced millions more to flee. For over a century, scientists have debated where this deadly organism first emerged. Was it the rugged Andes, where potatoes were first domesticated? Or was it the highlands of Mexico, a region teeming with similar pathogens?

Now, a team of researchers claims to have settled the question. In one of the largest genetic studies of its kind, they have traced the origins of the potato blight to the Andes. The findings not only settle one of the darkest long-standing debates but also reveal a complex web of evolution, migration, and hybridization that shaped the history of one of the world’s most infamous plant diseases.

A Pathogen’s Journey

Even today, Phytophthora infestans continues to wreak havoc on potato and tomato crops worldwide, causing billions of dollars in losses each year. Learning where it originates could help scientists predict and combat future outbreaks.

The debate over its birthplace has been fierce. Some researchers argued for a Mexican origin, pointing to the pathogen’s sexual reproduction in the region. Others, citing genetic evidence, proposed an Andean origin. The new study, led by Allison Coomber and Jean Ristaino of North Carolina State University, brings a wealth of genomic data to the table.

The team analyzed whole-genome sequences from P. infestans and six closely related species, including P. andina and P. betacei, which are found in South America, and P. mirabilis and P. ipomoeae, native to Mexico. They also included historic samples of P. infestans collected during the Irish Potato Famine.

The results were clear. The Mexican species, P. mirabilis and P. ipomoeae, formed distinct genetic groups, separate from P. infestans. In contrast, P. infestans was closely intertwined with the Andean species P. andina and P. betacei. These latter three species form a complex with indistinct boundaries. They’re more like siblings than distant cousins.

“This is how science works,” said Jean Ristaino, a co-author of the study and a professor at North Carolina State University. “There’s a hypothesis, people question it, test it, present data. But over time, the evidence is really weighted in favor of the Andes, because the DNA doesn’t lie. The genetics show ancestry in that region.”

Historical records also point to the Andes. “In 1845, when this blight hit Europe and the U.S., people were immediately trying to figure out where it came from,” Ristaino added during an interview with The Guardian. “There were reports that the disease had occurred and was known among the indigenous Andean Indians who grew potatoes.”

The Andean Crucible

According to the genetic analysis, the common ancestor of P. infestans and its Andean relatives diverged from the Mexican species around 5,000 years ago. Over time, P. infestans spread from the Andes to other parts of the world, including Mexico and Europe, thanks to increased overseas trade and globalization.

The study also revealed surprising levels of gene flow between P. infestans and its Andean relatives. Migration rates between these species were much higher than those involving the Mexican species. This suggests that the Andean region is not just the birthplace of P. infestans but also a hotspot for ongoing evolution.

One of the most intriguing findings was the blurred line between P. infestans, P. andina, and P. betacei. These Andean species are so closely related that they often hybridize, creating new genetic combinations. It’s like a melting pot with all these microbial species swapping genes, which could lead to new strains with different virulence traits — some of which could overcome plant resistance.

Understanding where the devastating potato blight originated has major practical implications for managing this disease, which remains a global threat.

Potato blight continues to wreak havoc worldwide. In Europe, fungicide-resistant strains have emerged, forcing farmers to seek new chemicals and methods. New breeding and gene-editing methods could offer a long-term solution.

“When you know the center of origin of a pathogen, that’s where you’re going to find resistance to it,” Ristaino said. “In the long run, the way to manage this disease is through host resistance. This work shows the focus on breeding efforts needs to happen back in the Andes.”

As the world grapples with the challenges of food security and climate change, studies like this one are more important than ever.

The findings appeared in the journal PLOS ONE.