An international team of scientists has sequenced the genome of the organism that causes potato blight, the disease that caused the 19th-century Irish potato famine and continues to threaten crops.

Decoding the entire genetic fingerprint of the organism, called P. infestans, could allow researchers to come up with new ways to control potato blight, which is notoriously difficult to fend off. The disease, also called late blight, still costs farmers worldwide $7 billion every year.

The study, led by researchers at the Sainsbury Laboratory in the U.K. and the Broad Institute of MIT and Harvard, found that the organism's genome is unusually large, more than twice as long as related organisms, which enables a rapid evolution of genes. The blight can quickly adapt to new plants, even potatoes that have been bred specifically to resist infection.

The scientists working at dozens of institutions around the world sequenced the entire genome of the organism and compared it with its close relatives, one that infects soybeans and another that causes "sudden oak death."

The genome of P. infestans was found to have 240 million base pairs of DNA, or 2½ to four times more than the genomes of the other pathogens. And most of that additional gene sequence is repetitive DNA, once considered "junk" DNA.

Lengthy DNA aids adaptation

The researchers concluded that this unusual genetic structure allows the organism to adapt quickly to changes in its environment and to a plant's natural defence mechanisms, making it particularly efficient at infecting its hosts.

"We now have a comprehensive view of its genome, revealing the unusual properties that drive its remarkable adaptability. Hopefully, this knowledge can foster novel approaches to diagnose and respond to outbreaks," said the study's senior author Chad Nusbaum, co-director of the Genome Sequencing and Analysis Program at the Broad Institute, in a release.

The Great Hunger, or Irish potato famine, of the 1840s was caused by late blight, and led to the deaths of more than one million people and a wave of immigration from Ireland to North America.

P. infestans, the organism that causes the blight, was once considered a fungus, but is now known to be a single-celled protozoan similar to the malaria parasite.

Late blight starts on the leaf of the potato and works its way down into the tuber, making it inedible. It can wipe out a field of potatoes in just a few days. The disease thrives in cool, wet weather, and can also infect tomato plants.

Some varieties of potato have been bred for resistance to blight, but the vast majority of potatoes eaten around the world are russet potatoes, which are cheap to produce, have long tubers ideal for making french fries, and are extremely vulnerable to P. infestans.

Nik Grunwald, a USDA plant pathologist and member of the research team, said farmers have to use up to 15 chemical sprays a season to keep the blight from wiping out russet potato crops.