Hardy Potato Blight Strain Still Raising Ire of World’s Farmers
The scourge began in the late summer of 1845, with a spate of unseasonably cool weather and weeks of fog and rain leading into the fall. Lesions began to appear on the leaves of the potato plants, and a fluffy fungus traveled down the stems, carried by rainwater until it reached the tubers beneath the ground.
Within days, the plants were dead--shriveled to a few tiny tendrils of greasy black slime--but that was just the beginning. In the ensuing years, the Irish Potato Famine killed a million people and caused more than a million others to emigrate, most of them to the United States. Pre-famine Ireland had 8 million people. Post-famine, it has never had more than 4 million.
For 150 years, the famine has been a source of enduring rancor between the Irish and their then-masters from Britain. It has also been the source of one of the all-consuming mysteries of plant pathology, confounding scientists.
In many ways, the pathogen called Phytophthora infestans, or potato late blight fungus, is even more dangerous now than it was the year it made its apocalyptic debut. Thumbing its nose at modern science, it remains today the most pernicious and persistent plant disease on Earth. In the developing world alone, the blight causes $2.75 billion in crop losses a year.
“It has been pandemic for 150 years,” said Canadian government potato pathologist Harold Platt, chairman of the International Potato Center’s Global Initiative on Late Blight. “It is the most aggressive plant disease we have, without question.”
Over the years, the fungus has proved so hardy and so adaptable that scientists have never found an herbicide to which it cannot adjust, or a species of potato--including those that are genetically engineered--whose defenses it cannot breach. Any cure that scientists find lasts for five years at most before the blight mutates to overcome it. Sometimes the pathogen can adjust “in a single growing season,” Platt said.
It was partly for this reason that North Carolina State University epidemiologist Jean B. Ristaino decided to revisit the famine in an attempt to determine the origin of the late blight fungus: “You find out where it’s from, and you’re likely to find varieties of potatoes that are most resistant to it,” Ristaino said. “Understanding the source has always been a key question.”
The prevailing theory held that the most prevalent kind of modern blight, a variety called US-1 that reproduces asexually by cloning itself, was a direct descendant of the famine pathogen and probably came from Mexico’s high-altitude Toluca Valley, about an hour’s drive from Mexico City.
But in research reported last summer in the journal Nature, Ristaino described how she and a team of researchers had conducted genetic tests of pathogens taken from famine-era potato leaves stored in Britain’s Royal Botanic Gardens. Those pathogens did not match US-1.
Ristaino suspected the famine blight may have come from somewhere else, and in a report being prepared for publication later this year, the team will suggest that it matches a pathogen found both in Mexico and South America. So the search will continue.
Ristaino says she leans toward an “out of the Andes” theory, because the cradle of world potato cultivation lies in the high valleys and windblown moors of what are today the mountain cordillera countries of Bolivia, Peru and Ecuador.
By the time the Spaniards arrived in the 16th century, native Americans had been growing potatoes for nearly 8,000 years, and cultivation had traveled north and south along the ribbed highlands of the continental divide.
Evidence of pre-Columbian blight outbreaks is sparse, and when the Spaniards took the potato to Europe it was grown without serious disease for more than three centuries, becoming a staple ingredient in foods as diverse as shepherd’s pie, Russian vodka and the Hanukkah latkes of European Jews.
“As civilizations grew, certain types [of potatoes] were recognized as good food sources, and those were emphasized,” Platt said. Farmers went to clonal production of the most edible varieties--cutting up potatoes from one crop and using them as seed stock for the next. This practice, the cornerstone of potato production even today, “created a haven for late blight,” Platt said.
By the mid-19th century, Ireland was cultivating potatoes in two ways. Peasant farmers were nursing small plots for family and local use, and British landlords used large farms to produce for larger markets. The most popular potato at the time was a knobbed variety known as the “lumper,” Ristaino said, “the perfect shape for trapping the pathogen.”
Late blight arrived in Europe in July 1845, probably traveling on a shipment of Mexican potatoes imported after several years of bad European harvests, Ristaino said. Within weeks, the blight had hopped through potato fields in Flanders and other lowland producing areas. Then it jumped to Ireland, where it rampaged through a land where potatoes had been unknown only three centuries earlier.
The study of P. infestans began almost as soon as it was pinpointed by British clergyman Miles Berkeley as the cause of the blight, but early identification has not helped cure the disease.
Science has added sophistication. Herbicides, both prophylactic sprays and systemic substances that enter the plant itself, have had success. So, too, have selective breeding and genetic engineering for resistant potato strains.
But the pathogen always catches up: “With all the advances in modern agriculture, it has been able to adapt to all the chemical and host resistance we put out there,” Platt said. “When it has the opportunity, it will make changes around the world to ensure its survival.”
Famine-era blight can still be found everywhere, Ristaino said, but 150 years after its arrival, scientists still don’t know where it came from. If they did, they might learn how to stop it.
