Potato blight: fingerprinting casts new light on old famine.© SPL.

Scientists probing 150-year-old plant samples have cast doubt on the prevailing theory of what strain of potato blight caused the Irish potato famine of the 1840s, and from where this devastating pest originated¹.

Jean Ristaino of North Carolina State University, Raleigh, and her team performed DNA fingerprinting on dried British and Irish potato leaves infected with Phytophthora infestans, the pest that causes late blight in potatoes. Considered the most serious potato disease, late blight caused the Irish potato famine, in which one million people died. Today the pathogen costs the developing world alone up to US$3 billion a year.

The researchers discovered that DNA isolated from the minute amounts of the pest still lurking on the leaves did not match DNA from the strain of P. infestans, known as US-1, thought to have caused the famine. Work is now under way to find out exactly which strain did kill Ireland's potato plants.

The theory blaming US-1 for the famine is based on the strain's current distribution and DNA analysis of twentieth-century potato leaves. Ristaino's team says the latest experiment is the first time that DNA from samples in dried plant libraries have been used to learn about historic plant epidemics such as the Irish Potato Famine.

"We decided to go back to the old samples because that's where the answers lie," says Ristaino. This wasn't possible before because the technology wasn't advanced enough to isolate and analyse the minuscule quantities of P. infestans DNA found on old and fragile leaves.

Received wisdom has it that US-1 is the ancestral strain of potato blight, and that it originated in Mexico. But Ristaino and colleagues also question this, because US-1 is found all over the world except Mexico. They hint that the pathogen may have originated in South America, the ancestral home of the potato.

But Stephen Goodwin, a plant pathologist at Purdue University in West Lafayette, Indiana, says that this argument and the results from Ristaino's study aren't convincing enough to topple the Mexican-origin theory, which is supported by 70 years of research. Goodwin developed the hypothesis that US-1 migrated out of Mexico to Europe and triggered the Irish potato famine.

"The body of evidence is very strong that P. infestans originated in Mexico," he says. Because the genetic diversity of P. infestans is greatest in Mexico, says Goodwin, this suggests that the pathogen has lived in Mexico the longest - allowing it to evolve into many different genetic forms.

But the potato pest's origin is still an open debate. Even though Mexico is the pest's centre of diversity, that doesn't necessarily mean Mexico is its centre of origin, says Juan Landeo, a potato geneticist and breeder at the International Potato Center in Lima, Peru.

Ristaino agrees and is currently examining hundreds more historic potato samples, including some from South America, in the hope of finding definitive evidence of P. infestans' ancestral home.

Where the disease originated is irrelevant to researchers trying to breed potatoes resistant to late blight and to farmers fighting this devastating disease with costly fungicides, says Landeo.

It's more important to know P. infestans' centre of diversity, which is Mexico, says Landeo. Pathogens become more diverse when the organisms they infect also evolve and diversify. Where there is P. infestans diversity, there is also potato diversity, something breeders look for when hunting for naturally resistant potato varieties. 

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