Near Isogenic lines for assessment of pathogenic variation in the wheat stripe (yellow) rust pathogen

Global

Dr Ravi Singh
Phone: 52 55 58042004
Fax: 52 55 58047558
Email: r.singh@cgiar.org

• International Center for Agricultural Research in the Dry Areas, Syria
• University of Sydney, Australia

$223,885

01/02/1997 - 31/01/2002

Dr Tony Fischer

In recent years wheat stripe rust, also known by the common name yellow rust (YR) and caused by the fungus Puccinia striiformis f.sp. tritici (Pst), has been a recurrent disease problem in many of the significant cereal producing regions of the world, causing yield loss and leading to increasing fungicide usage. There have been stripe rust epidemics in countries of East Africa, the Middle East, China and Central and West Asia
Unpredicted change in the pathogenic capabilities of the rust fungus population has been an important factor, among others, for these disease epidemics. The major strategy for disease control in these regions is the deployment of resistant wheat cultivars. However, the absence of any clear understanding of pathogen variability has placed regional breeding programs and wheat variety deployment schemes at serious disadvantage. Indeed, the absence of these data at an early stage has been a primary limiting factor for an adequate and timely response to the recent national disasters experienced in these regions.
It became evident that a reliable, field-based methodology was required to monitor pathogenic change. Researchers determined that Near Isogenic Lines (NILs) of wheat would be needed for this program.

The objectives of the project was to develop, and assess as widely as possible, a set of NILs to serve as differential testers to pathogenic variability in yellow rust fungus in both greenhouse and field experiments.

The scientists developed and assessed NILs at The University of Sydney, Plant Breeding Institute (PBI) Cobbitty. The NILs were based on the Australian cultivar Avocet and represented a set of wheat lines that were identical except for the presence of single genes for resistance. As part of the trial of these lines they were distributed through the networks established by the partner institutes CIMMYT and ICARDA, extending to National Agricultural Research Services (NARSs) in developing countries and enabling them to be grown and assessed in key regional locations. Project staff visited certain testing locations to provide direct feedback with field cooperators, and made direct observations of NILs adaptability and value in disease monitoring.

Data presented in the final report indicated that field-based testing of near isogenic lines provided a convenient and effective monitor of pathogenicity characters in Puccinia striiformis tritici.

A set of more than 17 NILs in Avocet background and produced by six backcrosses each was verified then deployed globally at 149 site-year combinations in developing countries. The choice of Avocet as recurrent parent, as broadly adapted widely susceptible variety was vindicated. Useful field assessments (i.e. Avocet check showing clear susceptibility) were obtained from 96 of these environments, and useful information derived on the distribution of yellow rust virulence genes, and hence on the potential value of wheat resistance genes in different parts of the world. A strong network of collaborators, especially in Asia, was set up.
During the project many NARSs, especially new ones from Central Asia, came on board as collaborators in dire need of help on yellow rust. Only the IARCs, with the permanent presence of their people in these regions and visiting NARSs regularly, can keep this sort of collaborative network going. Yet such a testing network is invaluable because yellow rust knows few natural boundaries, being widely spread from one country to another on the wind. PBI Cobbity added the resources, rigour and skills needed to develop the NILs.
The impacts are subtle and long term. The results will influence what YR resistance genes breeders work with and the YR races they use in screening. It will permit scientists to stay ahead of yellow rust using major gene resistance. Such a tool as the Avocet NILs will be needed until such time as reliable durable minor gene resistance becomes widely deployed. This is not likely to happen soon, so the continuing use of the tool will be a measure of the importance attached to it, and hence its impact. As new resistance genes are discovered they will need to be incorporated into the NIL set. PBI Cobbity and CIMMYT in Mexico will continue to develop the near isogenic set and maintain the integrity of its basic seed.