Potato Famine Pathogen Traced to South American Andes

Researchers at North Carolina State University (NC State) have confirmed that the Irish potato famine pathogen, Phytophthora infestans, originated in the South American Andes Mountains. This discovery, the result of a comprehensive genetic study, provides vital evidence that P. infestans spread from South America to North America, ultimately devastating Ireland’s potato crops in the 1840s and causing a catastrophic famine.

The study is the result of extensive research that delves into the genetic makeup of P. infestans as well as closely related pathogens in the Phytophthora genus, particularly Phytophthora andina and Phytophthora betacei, both of which are native to South America. The findings of the research underscore the genetic similarity of these three species, pointing strongly to South America, specifically the Andes, as the cradle of the disease that would later reshape global agricultural landscapes.

Breaking New Ground: Whole-Genome Analysis of P. Infestans

One of the key components of this study is the comparison of the whole genomes of P. infestans with those of its South American counterparts. The research is significant because it is one of the largest genome analyses conducted on both P. infestans and its closest relatives. The team, led by Jean Ristaino, a prominent professor in Plant Pathology at NC State, utilized advanced sequencing technology to compare the genetic blueprints of these pathogens. This allowed the researchers to trace evolutionary patterns, migrations, and their long-standing co-evolution with specific host plants.

Ristaino emphasizes that the Andes region has proven to be a “hot spot” for the speciation of Phytophthora pathogens. Speciation refers to the evolutionary process through which new and distinct species arise from a common ancestral lineage. The findings show that the Andean region, with its unique environmental conditions, likely played a crucial role in the origin of P. infestans. The path of migration then followed a trajectory from the Andes into North America and then, notably, into Ireland in the 19th century. Ristaino’s conclusion aligns with earlier evidence but provides clearer, more direct genetic proof of the pathogen’s origin.

This study also adds critical context to the ongoing debate about the origins of P. infestans. While some scientists previously suggested a Mexican origin for the pathogen, the comparison of genomes indicates significant differences between P. infestans and two related Mexican species: P. mirabilis and P. ipomoea. These findings strongly argue for a South American origin rather than the Mexican one proposed by some earlier researchers.

Significance of Host-Pathogen Coevolution

An important aspect of the research emphasizes the need to consider the origin of host plants when searching for sustainable resistance to P. infestans. Much of the resistance research in recent years has focused on the wild potato species, Solanum demissum, which is native to Mexico and has long been used to breed resistant potato strains in agricultural practices. However, Ristaino argues that it’s crucial to examine where both the potato and its pathogen have evolved together over time — not just where the pathogen has spread recently.

This aspect is particularly timely given the challenges posed by climate change. The increase in drought conditions in the high-altitude regions of the Andes is threatening potato biodiversity, including wild species that could offer valuable insights into natural resistance mechanisms to late blight disease caused by P. infestans. Ristaino warns that the current threat to these wild potato populations due to climate shifts may result in the loss of potentially useful resistance traits before scientists can adequately study them.

The Pathogen’s Migration: From South America to Mexico and Beyond

The study also explores the historical migration patterns of the pathogen. According to Allison Coomber, the former graduate student and lead author of the paper, there have been more substantial migrations of P. infestans into and out of South America compared to movements into and out of Mexico. Notably, P. infestans likely first migrated from the Andes into Mexico, with significant movement occurring much later, when Mexican potatoes were introduced to the Andean region.

Coomber and her colleagues discovered that while there is some evidence of gene flow in both directions between the Andes and Mexico in modern times — including the transfer of Mexican potato breeding programs into the Andes — historically the pathogen’s movement was largely southward to Mexico, rather than the reverse. This supports the theory that P. infestans first emerged in the Andes before later impacting agricultural practices in regions such as Mexico.

One of the most interesting insights from the study pertains to the historic populations of P. infestans that were collected between 1845 and 1889. These earlier samples showed the most genetic divergence from any other P. infestans populations found today. Ristaino and her colleagues observed that these historical populations represented the first stages of differentiation, providing valuable insights into the initial spreading patterns and evolutionary processes that led to the global pandemic of late blight disease.

Modern Implications: Climate Change and the Role of Global Trade

The modern implications of this study are vast, especially in the context of global agricultural systems. P. infestans continues to pose a threat to the potato and tomato industries worldwide. Its devastating effects are still felt across continents as the pathogen continues to evolve, in part driven by global trade practices, including the movement of crops and plants around the world.

The study suggests that contemporary global trade patterns are responsible for mixing populations of P. infestans in South America and Mexico, adding to the genetic pool of the pathogen. This mixing complicates the development of long-term solutions, as the pathogen exhibits considerable genetic diversity. Amanda C. Saville, a research specialist in the Ristaino lab, explained that modern-day trade has led to a kind of “gene flow” between the South American and Mexican populations of P. infestans, furthering the pathogen’s capacity to adapt and mutate in response to growing resistance efforts.

The Future of Potato and Tomato Protection: Looking to the Andes

Looking ahead, the research highlights the importance of continuing to investigate the wild potato species found in the Andes, a region that has likely been integral to the ongoing co-evolution between host and pathogen. With climate change threatening many of these plants’ survival in their native habitats, there’s an urgent need for further research into the genetic potential these wild species might hold in the fight against late blight disease. Only by understanding how both the pathogen and its hosts have evolved in the same ecosystems over millennia can we hope to develop robust strategies for managing and mitigating the spread of this destructive disease.

As Coomber concludes, “The genetic evidence we have amassed clearly shows that the pathogen’s origins lie in South America, and the challenges we face now demand that we rethink traditional methods for managing disease resistance in potatoes. We cannot ignore the Andes — we must protect these native species before it’s too late.”

Conclusion

This comprehensive study provides a significant step forward in understanding the origins and migrations of Phytophthora infestans. The research by NC State’s team, led by Jean Ristaino, offers compelling evidence that the Andes Mountains in South America are the primary origin of the potato famine pathogen, reshaping scientific approaches to plant disease management, especially in light of climate change and the continuing evolution of the pathogen.

As researchers continue to explore ways to combat late blight, this study serves as a reminder of the complex connections between environment, agriculture, and evolution. The road to finding a lasting solution to late blight is still long, but this research lays an important foundation for the future of global agriculture and food security.

Reference: A pangenome analysis reveals the center of origin and evolutionary history of Phytophthora infestans and 1c clade species, PLOS One (2025). DOI: 10.1371/journal.pone.0314509

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