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How blight becomes a killer

Amanda Gevens / UW-Madison
These two potatoes are infected with late blight disease.

Scientists have unraveled the genome of the parasite that sparked the Irish potato famine of the 1840s, revealing why it was such a killer back then and why it's still a scourge today.

The critter is known as "late blight," and although it's often called a fungus, it's more correctly classified as a type of water mold. More than 150 years ago, Irish farmers didn't know what it was; they only knew that something was killing off their potatoes just when they were getting close to harvest.

The repeated failure of potato crops led to the great Irish migration of the late 1840s and early 1850s. And one of those emigrants, my great-grandfather, ended up in Iowa. He was much luckier than the estimated 1 million Irish who died as a result of "the Great Hunger."

The blight is not merely a historical tragedy, however. Experts estimate that the late blight organism, which carries the scientific name Phytophtora infestans, costs farmers around the world $6.7 billion every year.

In countries such as Peru, where potatoes are a staple crop, late blight always looms as a potential peril and may be exacerbated by global warming. The parasite can hit tomatoes as well as potatoes. This summer, reports of late blight's spread stirred concerns here in the United States, in regions ranging from New York and Ohio to Minnesota and North Dakota.

Fungicides can fight the blight, and plant breeders have been working to develop blight-resistant strains of potatoes and tomatoes. The pest seems to bounce back every time it's hit, however. Now that the pest's genome has been decoded, scientists are finding out why.

"This pathogen has an exquisite ability to adapt and change, and that's what makes it so dangerous," Chad Nusbaum, co-director of the Genome Sequencing and Analysis Program at the Broad Institute of MIT and Harvard, said in a news release issued by the University of Wisconsin at Madison.

Nusbaum headed up the international sequencing project for the blight genome. UW-Madison's David Schwartz, the principal inventor of an optical mapping system for deciphering genetic code, participated in the project as well. The research team's findings were published online today by the journal Nature.

The researchers found that the late blight's DNA coding comprised about 240 million bases - making the organism's genome more than twice as large as those of its relatives. About 75 percent of that inflated genome consists of repetitive sequences of DNA. Those sequences are specialized for attacking plants, and they appear to be the key to the blight's virulence, Nusbaum said.

"The repeat-rich regions change rapidly over time, acting as a kind of incubator to enable the rapid birth and death of genes that are key to plant infection," said the Broad Institute's Brian Haas, another leading member of the team behind the research.

It looks as if Phytophtora infestans mixes and matches the DNA from its huge genetic toolkit to overcome the genetic defenses thrown up by blight-resistant plant strains - all part of an evolutionary arms race between the parasite and its host. "These critical genes may be gained and lost so rapidly that the hosts simply can't keep up," Haas said.

That's the bad news. The good news is that the geneticists are on to the parasite's game. "We now have a comprehensive view of its genome, revealing the unusual properties that drive its remarkable adaptability," Nusbaum said. "Hopefully, this knowledge can foster novel approaches to diagnose and respond to outbreaks."

Geneticists have already found a promising target for their fight against the blight, known as RLXR, and more targets are likely to emerge as they take a closer look at the now-published genome.

Update for 9 p.m. ET Sept. 10: Here are a couple bonus links about the blight:

Late blight can be a concern for home gardeners as well as for geneticists and commercial food processors. Take a look at this expert guide and this blog posting to find out how to fend off the parasites.

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