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It was a disaster that killed about 1 million people, devastating 19th century Ireland, but while the potato disease linked to the Irish famine is well known, a battle has raged over where it originated.
Scientists have long been divided over whether the fungus-like pathogen Phtytophthora infestans cropped up in the Andes or originated in Mexico.
Now one team of researchers say the question is settled after conducting what they describe as one of the largest whole genome studies of the microbe and its close relatives – with the results supporting a South American origin.
“This is how science works. There’s a hypothesis, people question it, test it, present data,” said Prof Jean Ristaino, the co-author of the research from North Carolina State University. “But over time, the evidence is really weighted in favour of the Andes, because the DNA doesn’t lie, the genetics show ancestry in that region.” She added that the conclusion tallied with historical documents.
“In 1845 when this blight hit Europe and the US immediately they were trying to figure out where it came from,” Ristaino said. “And there were reports that the disease had occurred and was known among the indigenous Andean Indians that grew potatoes.”
Ristaino did not rule out further debate but said the amount of data available would be difficult to counter.
Writing in the journal Plos One, Ristaino and colleagues report how they carried out an analysis of the microbe genomes and their close relatives from South America and Mexico, finding those from Mexico emerged more recently than the others.
The team found that modern populations of the potato blight microbe have a complex relationship, suggesting those from Mexico, the Andes and Europe have mixed in recent times.
But while modern populations of the microbe from the US, Mexico and South America all emerged from historic populations from the 19th century, the researchers say the Mexican populations are the most recent.
“If Mexico is the centre of origin, you would expect it to be the root of the tree, and that’s not the case,” said Ristaino.
Potato blight continues to pose a threat, with Ristaino noting there are fungicide resistant strains of the pathogen in Europe.
“The growers are having to look at different kinds of chemicals, and companies are trying to come up with different methods,” she said. “Maybe through breeding and gene-edited potatoes there may be a longer term solution, but right now it’s globally a problem in many parts of the world including in developing countries in Africa, South America, and in England.”
Ristaino said understanding where potato blight originated was important.
“When you know the centre of origin of a pathogen, that’s where you’re going to find resistance to it,” she said. “In the long run, the way to manage this disease is through host resistance, and [this work shows] the focus on breeding efforts needs to happen back in the Andes.”
Dr David Cooke of the James Hutton Institute, an expert on potato blight, who was not involved in the work, welcomed the research.
The origins of the pathogen remains up for debate, not least because the populations in South America reproduce almost entirely asexually, in other words by producing clones, whereas in Mexico populations mostly reproduce sexually – as would be expected for a centre of origin as this process maintains diversity and helps to prevent a species accumulating bad mutations over time.
“What we need as a smoking gun would be a survey in South America that reveals a sexual population,” he said.
