Stem Rust

About this project

DRRW - Phase II

The purpose of the Durable Rust Resistance in Wheat project is to reduce systematically the world's vulnerability to stem rust diseases of wheat through an international collaboration unprecedented in scale and scope.

The ability of the world's farmers to meet current and future demand for wheat is threatened by the highly virulent stem rust population emerging from East Africa. This project will mitigate that threat through coordinated pathogen surveillance activities, and breeding initiatives. Together, these efforts will replace susceptible varieties in farmer's fields with seed of durably resistant varieties, created by accelerated multilateral plant breeding, and delivered through optimized developing country seed sectors.

Background and Rationale

Stem Rust: Historical Perspective

Stem rust is the most feared disease of wheat. Stem rust spores arriving as late as one month before harvest can turn a previously healthy crop into a tangled mass of stems, which produces little to no grain. Moderately infected fields can produce as many as 1011 spores/hectare, which are picked up by wind currents, resulting in the movement of astronomical numbers of rust spores hundreds or thousands of kilometers to infect other regions. In the 1953 pandemic in North America, rust spores produced in fields in Kansas were deposited across 100,000 km2 of wheat 1,000 km north in North Dakota and Minnesota at a rate of over eight million spores per hectare. The result was the loss of 40% of North America's spring wheat crop. Although individual fields can be annihilated, average yield losses on a regional basis under epidemic conditions are commonly 10%. A loss of this magnitude is enough to have disastrous humanitarian effects on wheat producing countries in the developing world, as well as substantial secondary impacts on the entire global economy. Pandemics have been noted throughout history, with significant events occurring in South Asia, China, Central Asia, East and Central Europe, North America and elsewhere in the past 130 years. The last major continental pandemic was that described above in North America. The episode triggered international collaboration that helped lay the groundwork for the development of the wheat varieties of the Green Revolution. Although stem rust epidemics have occurred since 1955, they were localized (e.g. 1973 and 1974 in Australia and the Southeast United States, respectively; and 1992 in Ethiopia). This 40-year period when stem rust was quiescent was a result of genetic resistance.

Ug99

In 1998 a Ugandan crop scientist, Dr. William Wagoire, collected samples of a stem rust variant that appeared to be virulent on the previously undefeated and widely used major gene Sr31. Subsequent bioassays demonstrated that the Uganda isolate represented a unique pathotype (or race), which had a unique and potentially dangerous virulence pattern. In scientific terms, this race is designated TTKSK, but it is widely known by the colloquial name given the isolate: Ug99 ("Ug" for Uganda; "99" for the year its discovery was published). By 2004 Ug99 had colonized wheat fields of both Kenya and Ethiopia. An expert panel report, Sounding the Alarm on Global Stem Rust, issued May 29, 2005, declared Ug99 to be a threat to world wheat production (see www.globalrust.org). The report predicted that Ug99 would migrate across the Red Sea to Yemen, then to the Middle East, and subsequently to Central and South Asia. The predicted immediate path of Ug99 to South Asia covers a region that produces 19 percent of the world’s wheat (ca. 117 million tons) and has a population of one billion people. It is likely that either wind currents or inadvertent human transport will eventually carry Ug99 to North Africa, Europe, West Asia, China, Australia, and the Americas.

Collaborative research between early 2005 and 2007 established that Ug99 defeats virtually every race-specific resistance gene used in commercial varieties grown throughout the world. Ug99 defeats more of the 50+ known major resistance genes than any previously known stem rust lineage. In 2006, researchers discovered a variant of Ug99 in Kenya (TTKST) that defeats the widely used stem rust gene Sr24. This even more dangerous race will follow the same path that Ug99 is predicted to follow. Over 90 percent of the world's commercial wheat varieties are susceptible to the new variant, including almost all the wheat on the predicted path between East Africa and South Asia. Without swift intervention, stem rust will re-establish itself as a devastating cause of lost production in vast tracts of wheat in the developing world.

In January 2007, it was confirmed that Ug99 had migrated from eastern Africa and was infecting wheat in Yemen in the Arabian Peninsula. Scientists are convinced that the Ug99 lineage will reach the Middle East in the immediate future. More recently, Ug99 was found in Iran. A summary of the status and prospects of this threat was recently published (Singh et al., 2008).

Potential Impact of Ug99 on the World's Poor

Wheat represents approximately 30% of the world's production of grain crops. The FAO predicts that 598 million tons of wheat will be harvested this year on 220 million hectares of land. Nearly half of that production will be harvested in developing countries. On average, each person in the world consumes 68.2 kilograms of wheat each year. That equates to about 630 calories per day per person, or 1/2 to 1/3 of the minimal energy requirements of most adults. In North Africa, West and Central Asia, wheat provides more calories than all other grains combined. The Middle East and North African countries consume over 150% of their own wheat production and are heavily dependent on imports. In Sub-Saharan Africa wheat is the number one urban food staple.

Once Ug99 and its derivatives have established themselves in North Africa, the Middle East, and South Asia, annual losses could reach US$3 billion in a given year. The effects on rural livelihoods and geopolitical stability would be incalculable. Large populations of poor wheat- farming families would be seriously affected and few have alternative livelihoods. The impact on landless laborers dependent on agricultural jobs would also be severe, and one could anticipate an increase in the rural-urban migration of landless laborers and small farmers. Moreover, such large production losses would have significant implications for national economic growth rates in seriously affected countries and could even affect global wheat markets.

Durable Rust Resistance in Wheat - Phase II

In early 2008, the Bill & Melinda Gates Foundation awarded a three-year grant, "Durable Rust Resistance in Wheat" (DRRW), to a consortium of institutions led by Cornell University. Phase II is a five-year continuation of DRRW Phase I, where year 1 of Phase II is coincident with year 3 of Phase I.

A significant addition to the Phase II proposal is Objective 21: Improved testing, multiplication, and adoption of replacement varieties. The majority of outputs of Phase I have contributed to the rapid development of locally adapted and acceptable replacement varieties resistant to prevailing stem rust races in the targeted countries threatened by Ug99. The Project's goals, however, will be realized only when farmers are planting seed of durably resistant and agronomically improved varieties. Although the belief that "good varieties sell themselves" may be true, variety turnover in most developing countries is very slow, and the preponderance of a single variety over a long period of time is common in large wheat growing areas. Both of these features suggest that the systems that link the outputs of breeding programs with resource poor farmers are not inherently configured for the kind of response that is required to replace current susceptible varieties in a short period of time when under the threat of an epiphytotic. Objective 21 identifies the components of a "seed system," and proposes coordination of multilateral interventions to improve the delivery of resistant varieties to resource poor farmers.

Another significant feature of the Phase II is an integrated Gender Strategy embedded in the Activities of the Objectives. We acknowledge in Phase II the need to address gender equity at both the level of professional development of women wheat scientists, as well as a moral imperative to ensure food security at the household level in developing countries.

We also recognize the need for a comprehensive review and appraisal of gene deployment strategies that improve the durability of deployed resistance genes or gene combinations. This is reflected in the greater emphasis on minor gene adult plant resistance (APR) and combinations of major genes where APR is not yet attainable. The "type" of resistance, and the number of genes in a given cultivar is one consideration in gene deployment to improve durability. Mixtures (blends), coordinated deployment of varieties or genes to avoid uniformity and disrupt the pathosystem, and an array of other approaches are in need of consideration. Promoting international discourse and requisite research on questions relevant to rational gene deployment strategies remains part of our portfolio and will be the subject of an upcoming DRRW Gap Analysis.

In addition, we have established project-wide metrics that will aid us in monitoring the delivery of outputs and measure their impact.

Cornell University