Killer fungus spells disaster for wheat

A WHEAT disease that could destroy most of the world’s main wheat crops could strike south Asia’s vast wheat fields two years earlier than research had suggested, leaving millions to starve. The fungus, called Ug99, has spread from Africa to Iran, and may already be in Pakistan. If so, this is extremely bad news, as Pakistan is not only critically reliant on its wheat crop, it is also the gateway to the Asian breadbasket, including the vital Punjab region.

Scientists met this week in Syria to decide on emergency measures to track Ug99’s progress. They hope to slow its spread by spraying fungicide or even stopping farmers from planting wheat in the spores’ path. The only real remedy will be new wheat varieties that resist Ug99, and they may not be ready for five years. The fungus has just pulled ahead in the race.

Ug99, a virulent strain of black stem rust (Puccinia graminis) was identified in Uganda in 1999. Since then it has invaded Kenya and Ethiopia and, last year, Yemen. From previous fungal invasions, scientists expected the prevailing winds to carry Ug99 spores to Egypt, Turkey and Syria, and then east to Iran, a major wheat-grower, buying them some time. But on 8 June 2007, Cyclone Gonu hit the Arabian peninsula, the worst storm there for 30 years.

“We know it changed the winds,” says Wafa Khoury of the UN Food and Agriculture Organization in Rome, because desert locusts the FAO had been tracking in Yemen blew north towards Iran instead of northwest as expected. “We think it may have done that to the rust spores.” This means, she says, that Ug99 has reached Iran a year or two earlier than predicted. The fear is that the same winds could have blown the spores into Pakistan, which is also north of Yemen, and where surveillance of the fungus is limited.

There could be more unpleasant surprises in store. On mature wheat, the fungus reproduces asexually to release billions of identical spores. If the spores drift onto a barberry bush (Berberis vulgaris), however, they switch to sexual reproduction, and so could swap genes with other stem rusts to produce completely new variants. Iran is a hotspot for barberry.

Scientists have now found out how Ug99 took hold, says Rick Ward of CIMMYT, the wheat breeding institute in Mexico that started the Green Revolution. “It turns out most of Kenya was planted with a wheat variety that contained only one gene for rust resistance, SR24,” he told New Scientist.