By David N. Leff

Science Editor

During a single week in the summer of 1846, a fungus called Phytophthora infestans wiped out the remains of Ireland’s entire potato harvest. That colossal crop failure caused more than 1 million of Ireland’s 8-million population to die of starvation between 1842 and 1847. Twice as many survivors fled the country – most of them emigrating to the U.S. Within a decade, its population was down to 4 million.

Potatoes, the common root vegetable Solanum tuberosum, were literally Ireland’s staff of life. Their sudden loss 150 years ago led its victims to blame the famine on divine or diabolical intervention. The germ theory hadn’t been developed, and people didn’t understand that microorganisms can cause disease.

However, an English clergyman and amateur mycologist named Miles Berkeley assiduously collected infected leaves from the blighted potato plants, and confirmed that Phytophthora – Greek for “destroyer of plants” – caused the disease.

“Berkeley was the mycologist who did some of the early microscopic diagnosis,” observed plant pathologist Jean Ristaino at North Carolina State University in Raleigh. “One of the early pioneers, he collected a lot of different fungi. His leaf samples from the 1840s are really important in this work today.”

To Ristaino, those ancient, carefully preserved, fungus-tainted dried leaves are the botanical equivalent of flies trapped in amber a la Jurassic Park’s DNA-restored dinosaurs, or woolly mammoths awaiting reincarnation. She is senior author of a paper in today’s Nature, dated June 7, 2001, titled: “PCR amplification of the Irish potato famine pathogen from historic specimens.”

“The 28 samples we report on,” Ristaino told BioWorld Today, “were collected in the mid-19th century by scientists in various countries of Europe, including Ireland itself. I collected them from three different herbaria – the Royal Botanic Gardens in Kew, England; the U.S. National Fungus Collections in Beltsville, Md.; and Harvard University’s Harlow Herbarium.”

Russian Roulette With Four Fungal Strains

No one doubts that the fungal perpetrator of the Irish potato famine was Phytophthora infestans. The open question, which Risteino tackled, was: Which of four genetic variants – or haplotypes – in the genome of P. infestans was its guilty strain?

She explained: “There are four different haplotypes – 1a and 1b, 2a and 2b – which are all present in modern populations. Eight or nine years ago,” she recalled, “scientists did some RFLP [restriction fragment length polymorphism] analysis of several P. infestans populations. They found one they called the US1 genotype, which has the 1b mitochondrial DNA haplotype.

“There were various polymorphisms in the mitochondrial genome,” Ristaino recounted, “that allowed us to distinguish different types. So we used the mitochondrial polymerase chain reaction method – like forensic fingerprinting – to amplify DNA from these leaf samples. The theory is that this 1b strain came out of Mexico and entered North America. Potato-blight epidemics occurred in the U.S. two years prior to the outbreak in Ireland. Presumably, the strain infected potatoes in North America, then went over to Ireland, France and other European countries.”

Ristaino continued: “I developed a diagnostic assay that would enable me to amplify and sequence DNA from the pathogen out of these dried potato leaves. I used those PCR primers to amplify R. infestans ribosomal DNA, and confirmed that we had the right pathogenic DNA. Then we used primers specific for various genes in the mitochondrial genome.

“The question we were asking was whether a particular haplotype, 1b, was the ancestral strain that caused the Irish famine. We found that it was not; we’ve eliminated the 1b as a possibility. Now we’re in the process of amplifying DNA from more samples from various regions of the world, including South America, to determine what actual strains did cause the disease.”

South America is the potato’s point of origin. Spanish conquistadors found native Indian populations cultivating it in the high Andes of Peru and Bolivia. They introduced the vegetable to Spain in 1538, and by 1586 it had reached a hungry Ireland. Today, Solanum tuberosum is the world’s top vegetable crop, and the fourth leading foodstuff product on Earth – after wheat, rice and corn.

“Potatoes were brought into Europe via Spain,” Ristaino observed, “but there’s no evidence this pathogen was brought with it. In fact, potatoes were grown for several hundred years without any problems, so it’s more than likely that the pathogen came later. If it had been introduced earlier, it would have wiped things out.”

Planet Earth is still scourged by recurrent epidemics of the fungal blight. Right now, the principal sufferer is Russia, where P. infestans is currently ravaging the potato crop.

“Russians are highly dependent on the potato for food – as the Irish were,” Ristaino pointed out. “There’s a lot of poverty, with people consuming potatoes for their main source of food. They’re having problems over there, although they have fungicides now. The Irish didn’t have those kinds of chemicals available 150 years ago.

“Potato blight is common,” she continued. “Small-scale epidemics occur every year in Mexico, Ireland, Ecuador and the U.S. It’s a re-emerging disease, present in a lot of developing countries, which can’t afford fungicides. And it’s still endemic in the Andean region of South America.”

Aiming To Curb P. infestans’ Spread, Resistance

She and her team are now “doing further analysis with leaf samples from Central and South America to try to figure out whether that was actually the location and migration path of the infestation. The diagnostic assay can be useful to minimize the spread of the pathogen in potato tubers,” she pointed out. “You can use it to determine whether you have infected potatoes, and that’s the main means this pathogen moves around. The present-day population has been infected with seed potatoes that are shipped and planted from one region of the world to another.”

Ristaino made the added point, “Knowing where the pathogen came from is important because we can develop potato varieties that have resistance to R. infestans. If we know the center of origin we are more likely to find resistant host plants there.

“This study,” she concluded, “is the first ever to extract and analyze mitochondrial DNA from historic specimens. Previous studies by others, which implicated the 1b haplotype, used only DNA collected from modern-day late-blight outbreaks.”