NAIROBI (AFP) - A new rapidly-evolving pathogen spreading in east Africa could annihilate wheat plantations worldwide, posing a "catastrophic" threat to crops unless steps are taken quickly.

The genetically-variable stem rust strain, dubbed Ug99 after its discovery in Uganda in 1999, has moved into neighboring Kenya and Ethiopia, where it has nearly affected all plantations and could spread further, scientists warned.

"The risk and losses will be tremendous if this gets on the loose," said U.S. botanist and 1970 Nobel peace laureate Norman Borlaug.

Borlaug, now 91, who helped develop resistant wheat varieties 50 years ago, urged a restoration of the cooperation that helped suppress a stem rust outbreak in North America that destroyed nearly 70 percent of wheat plantations in 1950.

"The stem rust strains are changing through mutation ... coming up with new types," he told a news conference in Nairobi to release a report on the problem compiled by experts organized by the International Centre for Maize and Wheat Improvement (CIMMYT). "Maybe we got too complacent," he said.

"We have to restore the cooperation (as the current threat) is potentially more serious than it was 40 to 50 years ago."