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Report from the field: Wheat stem rust resistance screening at Njoro, Kenya

November 28, 2008

According to Kenyan researcher Joseph Macharia, a new, highlyvirulent form of the wheat disease known as stem rust is driving Kenyan wheat farmers off their land. “If farmers can’t grow wheat, they just abandon the field, or some may switch to maize,” says Macharia. “Wheat is a high-investment cereal, so if farmers lose their crop, they lose their investment and can’t continue.”

The culprit is Ug99, a strain of stem rust—a millenary disease of the crop worldwide—that first appeared in Uganda in 1998. It was subsequently detected in Kenya in 2002 and Ethiopia in 2005, in Sudan and Yemen in 2006, and in Iran in 2007. The pathogen is expected to continue its migration to South and Central Asia, through the Middle East and North Africa, riding on the winds or by other means. Most currently grown varieties in its path are susceptible, and the wheat areas at risk represent 20% of the global total and provide sustenance for 1 billion people.

In Kenya, Peter Njau, plant breeder and deputy director of the Kenya Agricultural Research Institute (KARI) research station at Njoro, says the loss of wheat harvests to the pathogen affect both farmers and the national economy. “Wheat is the second-most important crop in Kenya—we produce 350,000 tons every year,” he says, “but we need to import 450,000 tons more to meet national consumption demands.”

This year, Njau and his team worked with CIMMYT at the Njoro station to test 20,000 wheat lines from more than 15 countries for resistance to Ug99. Wheat scientists from most of these countries came to Kenya to evaluate their material and see first-hand the pathogen’s damage. “This is a hot-spot for the disease,” Njau says, referring to Nakuru District in the Central Rift Valley region of Kenya. “Disease incidence was so intense this year that 85% percent of the lines proved susceptible, and many supposedly resistant lines showed 20% greater infection than they normally would. New variants are appearing that overcome some of the most effective resistance genes in wheat.” But there is hope, too, according to Njau. “The experimental wheat variety Kingbird looked good under this year’s conditions, and has performed well in tests elsewhere.” Derived from CIMMYT germplasm, Kingbird is being used by the center to develop new varieties whose seed can be multiplied and quickly distributed to farmers in Ug99’s probable path of migration. Njau has also identified an experimental wheat variety from CIMMYT’s international stem rust resistance screening nursery that out-yielded the best reference variety by 27% and the average yield of varieties in the trial by 80%.

“Kenya and Ethiopia are doing the world a great service by conducting these trials,” says CIMMYT wheat breeder Davinder Singh, who is working in eastern Africa to combat Ug99. “The countries also benefit by having access to seed of resistant lines from international sources.” Ethiopian wheat researchers are also partners with CIMMYT in efforts to evaluate their research stations’ wheat germplasm from around the world for resistance to the pathogen. The work is part of the Borlaug Global Rust Initiative, led by Cornell University and supported by a growing number of donors, including the Bill & Melinda Gates Foundation, the USAID-seed project, USDA, ICAR-India, Australia, Canada, Arab Funds for Agricultural Development, FAO-training, and even a northwestern Mexican farmer association known as the Patronato