ARKPORT, N.Y. -- A perverse sort of historical symmetry is at work in Murray Mahany's potato fields.
Almost 150 years after the potato blight drove his great-great-grandfather out of Ireland, a virulent new cousin of // the same disease is threatening to drive him out of the only business he has ever known.
Mr. Mahany found the potent, fast-moving fungus known as late blight in one corner of his 600 acres of potatoes this summer and immediately plowed it under.
But he has no way of knowing whether he has controlled it, and no way of knowing whether he will earn a dime from his $5 million investment, until he harvests his crop in September.
In what scientists are calling the worst threat to America's potato crop in decades, an aggressive new strain of the blight that devastated Ireland in the 19th century is sweeping across the nation, showing remarkable resistance to all chemical fungicides.
First reported in the United States in the late 1980s, it has spread up the East Coast from Florida to Maine.
Last year, it reached New York state, where the crop was down 3 percent. This year, it has reached the potato-belt states of Idaho and Oregon, where heavy rains have fueled its growth.
Federal agricultural officials said they would not know until year's end how many acres have been destroyed by the disease, which cost Maine farmers more than $25 million last year and put several major growers out of the potato business.
If it wreaks similar damage on the Western states' crops, it could drive up the price of potato products by the fall, federal officials said.
"This is the worst crisis to have hit the United States potato industry in history," said Dr. Kenneth Deahl, a research microbiologist with the U.S. Department of Agriculture in Maryland, "and the worst internationally since the Irish blight of the 1840s."
The new fungus has also been reported in major potato-growing areas across the world, and some scientists predict that it could cause famine in South America if it is not brought under control soon.
Its march across the world's potato fields has inspired the creation of a special mailbox on the Internet, where scientists and growers share tips on how to fight the blight.
In New York, potatoes are the state's third-largest cash crop, after corn and hay, generating about $50 million a year. The fungus has been found this year in the Finger Lakes region and the southern tier counties, where Mr. Mahany lives, near Pennsylvania. Sparse rain has helped curb the spread of the fungus, but high humidity is encouraging its growth.
On a recent afternoon, Mr. Mahany watched stoically as his son, Gary, snapped open his Swiss army knife and cut a potato plant. Its green leaves and delicate white flowers seemed perfect to the untrained eye, but nestled along the stem was the telltale fuzzy gray of the late blight fungus, whose scientific name is Phytophthora infestans.
"This could take out this whole field in five days," said Mr. Mahany, the fifth generation in his family to farm potatoes in America. "My return, the boys' livelihood, everything we've made in this life could be wiped out."
Scientists believe that the new strain of blight originated in Mexico, the birthplace of the blight that ravaged Ireland, starving 1 million people and forcing 2 million others to emigrate to America.
The new strain is especially virulent because it can reproduce sexually, with cells from different plants combining to form new organisms. By allowing greater genetic variation, sexual reproduction has produced strains that spread faster, survive harsher environments and resist more chemicals.
"When it gets going, it's like a freight train," said Paul Tooley, a research plant pathologist with the U.S. Department of Agriculture. "It cannot be stopped."
Scientists say the only sure way to defeat the new strains is by breeding potatoes that can resist the disease, but such breeds may not be available for several years.
That may not be soon enough to keep Mr. Mahany and others like him in the potato business.
"We've had freezes; we've had too much rain; we've had drought," he said. "Nothing has scared us like this."