IT IS sometimes called the “polio of agriculture”: a terrifying but almost forgotten disease. Wheat rust is not just back after a 50-year absence, but spreading in new and scary forms. In some ways it is worse than child-crippling polio, still lingering in parts of Nigeria. Wheat rust has spread silently and speedily by 5,000 miles in a decade. It is now camped at the gates of one of the world's breadbaskets, Punjab. In June scientists announced the discovery of two new strains in South Africa, the most important food producer yet infected.

Wheat rust is a fungal infection. Its most devastating form (Puccinia graminis) attacks the plant's stem, forming lethal, scaly red pustules. It has plagued crops for centuries. The Romans deified it, and believed that sacrificing dogs warded it off. It was the worst wheat disease of the first half of the 20th century, killing about a fifth of America's harvest in periodic epidemics.

Wheat rust once spurred the Green Revolution, the huge increase in crop yields that started in the 1940s. Now it could threaten those great gains. Norman Borlaug, the great American agronomist who died last year, conducted his original research into wheat rust. After ten years of painstaking crossbreeding, he isolated a gene, Sr31 (Sr for stem rust) that resisted P. graminis. By wonderful good fortune, Sr31 also boosted yields (and not only because plants were impervious to rust). Farmers everywhere adopted his seeds enthusiastically, saving millions of lives. So fast did his new varieties spread that by the 1970s, stem rust seemed to have been wiped out.

But in 1998 William Wagoire, a pupil of Borlaug's, was in south-western Uganda researching stripe rust, a less-deadly form of the disease that remains endemic there. While testing a new variety's resistance, he was alarmed to find stems scarred not by the yellow streaks of stripe rust but the angry pustules of stem rust.

At first neither he nor Borlaug's International Maize and Wheat Improvement Centre (CIMMYT) in Mexico could believe their eyes: stem rust could not have survived all those years. The awful truth dawned only after a second opinion, by Zak Pretorius of the University of the Free State in South Africa: stem rust had not only lived on in a remote corner of Africa's Great Lakes. It had evolved to overcome the previously unconquered Sr31. After decades without infection, most of the world's wheat crop was defenceless.

Wheat rusts spread as billions of spores in the wind. They usually move incrementally, from field to adjoining field, needing wet weather to thrive. But they can make larger leaps. They also mutate as they go. Rust, Borlaug once said, “never sleeps.”

The new variant is called Ug99: Ug for its country of origin; 99 for the year it was confirmed. It soon spread to Kenya and Ethiopia. In 2006 it made a leap over the Red Sea into Yemen, where it appeared in a more deadly form. In 2007 it showed up in Iran, apparently blown from Yemen. In June scientists announced they had found four new mutations of rust (making seven in all) and Mr Pretorius confirmed its presence in a harmful form in South Africa.

This could mark a final stage before disaster strikes. Rust's appearance in South Africa means the disease has pushed deep into the southern hemisphere for the first time. Mr Pretorius worries that westerly winds might blow spores as far as Australia, which is one of the world's top five wheat exporters. Iran borders Pakistan, which is among the top ten wheat producers and where roughly 100m people depend on the cereal to survive. David Hodson of the UN Food and Agriculture Organisation's Wheat Rust Disease Global Programme thinks it is only a matter of time before Ug99 appears in Pakistan.

To make matters worse, the threat from stripe rust (Puccinia striiformis) has also been worsening. Scientists at the International Centre for Agricultural Research in the Dry Areas (ICARDA) reckon it could affect 50m hectares and that some farmers could lose a third of their crops. Both figures are far worse than previously thought.

So far, wheat rust has not caused the disaster that scientists fear. But that, says Ronnie Coffman of Cornell University, has been mainly a matter of luck. So far, no giant producer (China, India, America, Russia) has been infected. The humid conditions that spread rust most readily have recently been lacking in most places. But where the weather was to its liking—Kenya in 2007, for instance—rust destroyed a quarter of the crop and affected four-fifths of all farms. Fungicides afford some protection but huge quantities of chemicals are an expensive and limited answer. Most small farmers in poor regions, including Punjab, cannot afford them anyway.

When the good luck runs out, stem rust can destroy the entire harvest in an infected area. A full-blown epidemic in a big wheat-growing area could therefore be catastrophic. Only a handful of wheat varieties have any resistance to Ug99, implying that harvests could fail even more completely than during earlier epidemics.

Such a failure would be felt on a vast scale. Wheat is the world's most widely planted crop and accounts for a fifth of humanity's calorie intake (rice has a similar share; all other foods combined account for the rest). Since its discovery, Ug99 and its descendants have shown up in eight countries ranging from South Africa to the Middle East; 26 others are on the FAO's danger list, accounting for about a third of wheat production. Kathy Kahn of the Gates Foundation says the disease is potentially “devastating”.

Scientists such as Mr Coffman say they know how to attack stem rust. The question is how fast and how completely they can do it, and at what cost. Borlaug used a mighty rust-resisting gene that lasted 40 years. The new approach uses four or five weaker defences, for example reducing the area of the wheat plant that Ug99 destroys, or slowing down the spread of the fungus. Each would be inadequate on its own. But put together, they achieve something close to full resistance. Multiple barriers also make it harder for Ug99 to overcome them all. A single gene will eventually be defeated—as Sr31 was. Knocking down a series of obstacles is much harder.

So far, researchers have developed about 60 experimental wheat varieties with multiple low-resistance genes. CIMMYT sent out a first batch of 15 for development and nine have made it into production and distribution on a wide scale. ICARDA has sent out two more seed lines and India and Pakistan have released rust-resistant seeds of their own. Scientists are also investigating how rust secretes the substances that enable it to break into the plant's cells. If the wheat can learn to recognise this as it is happening, it might also be engineered to generate its own Ug99- resistant proteins.

Separately, researchers have cracked the mystery of how stripe rust is able to overcome resistance in wheat so quickly. Until recently it was thought that stripe rust reproduces asexually. But new research found that, like stem rust, stripe rust increases its genetic variability by reproducing sexually on the leaves of another host plant (the barberry), making itself more adaptable—and more deadly.

Advances like these suggest the march of wheat rust can be halted, though at a cost. That is not just money, but a trade-off of lower yields for more resistance (ie, the opposite of what Borlaug achieved).

But if the signs in the laboratory look propitious, out in the fields the distribution problems are formidable. Over 47,000 hectares have been planted with the new seeds. But that is only 0.1% of the total area planted to wheat in the countries on the FAO's danger list. The astonishing spread of wheat rust makes quick containment impossible. In some places stem rust may become endemic before the outside world even spots it.

The high-yield seeds of the Green Revolution were not only developed but often marketed by state-financed agricultural institutions. In many poor African countries such institutions barely exist, whereas in wealthier ones spending on them has fallen over the years. Worst of all, farmers in earlier generations had a big incentive to get their hands on high-yielding seeds. Now, the vast majority have no experience of wheat rust. They may therefore see no reason for sowing rust-resistant seeds when they first appear—until the disease destroys their harvest. By then it will be too late.