By Sudeep Chand
Science reporter, BBC News, Guildford
About one million people died in the famine in the 1840s and 50s
The killer mould behind potato blight has a giant genome, say scientists.
Potato blight was a major cause of the mid-19th Century Irish famine, when starvation killed a million people and drove a million more to emigrate.
The water mould Phytophthora infestans still destroys crops worldwide worth $6bn each year. In 2003 it wiped out the entire crop of Papua New Guinea.
Scientists hope the genome, published in the journal Nature, will help in developing new defences to the blight.
The mould thrives in cool, wet weather and can infect potatoes and tomatoes, causing a "late blight" that can destroy entire fields in just a few days.
The problem appears to be getting worse as the organism continues to evolve.
Potato farmers have to spray chemicals each week to keep the mould at bay, but even then it can come back with a vengeance.
In recent years, growers in the UK have been forced to increase chemical spraying by up to 30%.
In Ireland, farmers have described this season as the worst in living memory.
How the mould adapts so rapidly and becomes resistant to chemical attack has long puzzled scientists.
"This pathogen has an exquisite ability to adapt and change, and that's what makes it so dangerous", said Dr Chad Nusbaum, a lead scientist on the project from the Broad Institute in Cambridge, US.
The international scientific team mapped the genome of the mould, and found it contained at least twice as much DNA as its relatives.
The genome is a mixture of gene-dense regions and much bigger gene-light regions.
The mould can insert itself into the leaves of potato plants
The scientists found 700 key genes in the gene-light regions. Some of them are known to help the mould attack the immune systems of potatoes.
Brian Haas, co-lead author of the study, said: "The 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.
"As a result, these critical genes may be gained and lost so rapidly that the hosts simply can't keep up."
Paul Birch from the Scottish Crop Research Institute, another of the scientists involved, said that modern farming techniques were part of the problem.
He said: "There is a real epidemic in Europe.
"In the EU we grow certain types of potato for mass production, forcing natural selection.
"But the Maris Piper that you find in Sainsbury's or Tesco has certain traits that people want."
The "invader mould" remains a critical threat to global food supplies.
It is hoped that the information will help researchers devise new ways of protecting crops.
However, one of the researchers doubted that the potato could win its arms race against the old enemy.
"We now know our enemy and may find an Achilles' heel," said Sophien Kamoun from the Sainsbury Laboratory in Norwich, UK.
"We will aim to breed a potato that is more durable; but don't bet against the pathogen," he told BBC News.
Douglas Kell, chief executive of the Biotechnology and Biological Sciences Research Council (BBSRC), said: "I am delighted that several UK and BBSRC-funded institutes and laboratories have been able to participate in the experiments leading to this important milestone."
Professors Kamoun and Birch were speaking at a presentation of the genome at the British Science Festival.