ICARDA News

International Center for Agricultural Research in the Dry Areas

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Phone: (963-21) 2213433, 2213477, 2225112, 2225012
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E-mail: ICARDA@CGIAR.ORG
Website: www.icarda.org
29 May 2008
Media contact: icarda-media@cgiar.org
 
Wheat Rusts: A Global Challenge
Wheat plants can be attacked by a variety of diseases. So far, the development and widespread use of resistant varieties has succeeded in limiting global epidemic outbreaks. But conditions change and pathogens evolve continuously, sometimes with devastating effects. One group of wheat pathogens is causing particular concern. These are the fungi that cause rust. Crop losses due to the rusts can be as high as 70%, leaving little for the farmer to harvest, while the seed will be mostly shriveled and of lower quality.

There are three rust diseases that affect wheat - stem, leaf and stripe (yellow) rust - caused by different species of the fungus Puccinia. Stem rust is caused by Puccinia graminis f.sp. tritici, leaf rust by P. triticina, stripe rust by P. striiformis. Each causal agent can exist in the form of many 'races' that vary in pathogenicity, aggressiveness and virulence. New races may develop due to mutation or recombination between different races within the same rust group. Existing races may spread faster, or become more virulent, if environmental conditions change. Consequently, specific genes that confer resistance could lose their effectiveness. In the 1980s, for example, a new pathogen race in the CWANA region overcame the Yr9 gene, a key source of resistance to stripe rust. A disease epidemic followed, causing losses of millions of dollars. In South Asia, Yr27, an important gene for stripe rust resistance there, has been overcome by a more virulent stripe rust race, which is now spreading.

Stripe rust resistance is an important component of ICARDA's wheat breeding program. One key aim is to find resistance that is more durable. Each year, over 15,000 wheat accessions are screened under artificially created local epidemics, to identify potential sources of resistance to different pathogen races and under varying conditions. As a result of these efforts, new varieties resistant to stripe rust have been released in almost every country in Central Asia and the Caucasus (CAC). One problem is that stripe rust populations are becoming better adapted to warmer temperatures - some resistant varieties are becoming susceptible, and stripe rust, formerly restricted to bread wheat, is now found on durum as well. This is now being studied.

All three types of rust cause severe problems in the CAC region, and several epidemics have been reported in recent years. An ongoing ICARDA study is examining the identity and diversity of resistance genes in cultivars grown in CAC countries. The results will help breeders remove susceptible cultivars from the cultivated genepool, determine the identity of currently deployed resistance genes, pyramid several genes together in one advanced line and increase genetic diversity in breeding germplasm. Thirty-two major wheat cultivars are being tested for resistance to the three rust diseases at the seedling growth stage.

An even bigger threat is stem rust. Ug99, a new biotype of the stem rust pathogen first identified in Uganda, has already spread from Kenya to Ethiopia, Yemen, Sudan and Iran. The wind-borne spores are expected to spread further, into South Asia, the CAC region, perhaps even parts of Europe and the Americas. In the CWANA region alone, 50 million hectares may well become infected and associated grain losses may exceed 15 billion US dollars.

ICARDA plays a key role in the Borlaug Global Rust Initiative that is co-funded by a grant from Cornell University, we are working with CIMMYT, NARS partners in the CWANA region, and advanced research centers in the US, Canada and Australia, to find ways to halt the spread of Ug99 by positioning resistant varieties in its path. Activities include surveillance, breeding for durable resistance, seed multiplication and training. A 'shuttle breeding' program, involving ICARDA and three national research stations in Ethiopia (Kulumsa, Debra Zeit and Ambo), tests over 10,000 lines of bread and durum wheat each year. At another testing site (Njoro, Kenya), over 5000 lines are being screened for resistance not only to Ug99 but also to new variants that are not yet found in Ethiopia.

Some resistant lines are already being identified by ICARDA breeders (Osman Abdalla, Miloudi Nachit) in collaboration with our cereal pathologists (Amor Yahyaoui and Kumarse Nazari).
 

About ICARDA: Established in 1977, ICARDA (www.icarda.org) is one of the 15 international research centers supported by the Consultative Group on International Agricultural Research (CGIAR). ICARDA serves the entire developing world for the improvement of barley, lentil, and faba bean; and dry-area developing countries for the on-farm management of water, improvement of nutrition and productivity of small ruminants (sheep and goats), and rehabilitation and management of rangelands. In the Central and West Asia and North Africa (CWANA) region, ICARDA is responsible for the improvement of durum and bread wheats, chickpea, pasture and forage legumes and farming systems; and for the protection and enhancement of the natural resource base of water, land, and biodiversity.

The Consultative Group on International Agricultural Research (CGIAR) (www.cgiar.org) is a strategic alliance of countries, international and regional organizations, and private foundations supporting15 international research centers that mobilizes cutting-edge science to promote sustainable development by reducing hunger and poverty, improving human nutrition and health, and protecting the environment.

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