Disease resistance. Stem Rust Resistance.

Background information

Stem rust is one of the most devastating diseases affecting wheat worldwide. In North America the last epidemics were recorded in 1955, since then stem rust has been under control by the use of resistant cultivars and eradication of alternate hosts. Resistant cultivars have always been effective control of this disease worldwide. But in 1999 a new race of Puccinia graminis f. sp. tritici, the causative agent of stem rust, was detected in Uganda that was virulent against the traditional wheat resistance genes, Sr31 and Sr38 (1). This race was denominated TTKS, or alternatively, Ug99. In 2003 race Ug99 was found in Kenya and in 2007 in Yemen (1-3). This virulent race could soon migrate to the Middle East and Central Asia and can attack many of the current cultivars in use worldwide (4, 5).

A survey of US cultivars (6) indicated that out of 84 hard red spring cultivars analyzed, only 16% were resistant or heterogeneous (resistant and susceptible individuals within the same line), even worse, most of the new lines bred for the Northern Great Plains are susceptible to Ug99. The authors postulated that the intense breeding for Fusarium head blight resistance during the last years could have diluted the stem rust resistance genes. The same study analyzed 200 hard red winter cultivars and breeding lines, and found that 48% of them were resistant to Ug99.

Due to the widespread susceptibility to Ug99 and the potential perspectives of new widespread stem rust epidemics, the Global Rust Initiative (www.globalrust.org) set as one of its top priorities the monitoring and deployment of new sources of resistance to stem rust.

Genes that are currently effective against Ug99 (5,7) are listed below,

¹ Virulence for the gene is known to occur in other races.

² Level of resistance conferred in the field usually not enough.

Sr24 and Sr36, which showed resistance to the initial forms of Ug99 are no longer effective against some more virulent forms (8), so their use is no longer recommended, unless combined with other genes. A new race of UG99 has been recently found that is virulent on Sr36 (TTTSK=UG99+Sr36 virulence). The Sr2 complex, responsible for slow rusting, provides partial resistance against Ug99 but pyramiding with other genes is recommended to attain effective protection. Sr25 is another useful gene against Ug99 present in wheat varieties carrying the distal 7EL translocation from Lophopyrum ponticum. This translocation also carries the leaf rust resistance gene Lr19 and a gene designated Y that confers yellow color to the flour. Sr39 provides resistance to all currently known pathotypes of Puccinia graminis f. sp. tritici (Pgt) including Ug99 (TTKSK) and its variants TTKST and TTTSK. In any case, in order to attain effective protection a pyramiding strategy is strongly recommended.

Methods

The links on the left panel of any Sr protocol page, including this one, will redirect you to pages with detailed nformation on markers for several stem rust resistnace genes.

Available germplasm

See reference 3 for lists of US cultivars and breeding lines containing different stem rust resistance genes and profiles of resistance against several races of P. graminis f. sp. tritici. Tsilo et al. (10) surveyed 76 wheat lines from 12 countries for the presence of Sr36. For those breeding programs that want to transfer the Sr25 resistance gene without the associated yellow pigment, they can use the variety 'Wheatear' that has an EMS mutation in the yellow pigment gene. Also check the individual method pages for other germplasm sources.

Other stem resources in the web

The web sites of the Borlaug Global Rust Initiative (BGRI), CIMMYT and USDA among others, provide news, resources and other documents on stem rust. Below we highlight some of them:

Ug99-RustMapper: CIMMYT and BGRI provides a Google Earth visualization that tracks Ug99 status world-wide.

BGRI Wheat Rust Surveillance Manual: A publication that provides guidelines for field surveys, use of GPS units and protocols on handling field samples.

Ug-99 field and greenhouse evaluations: wheat and barley susceptibility data from nurseries in Kenya and the US. Data collected by the Cereal Disease Lab of the USDA-ARS.

Plant Disease Lessons - Stem Rust: information produced by the American Phytopathological Society on pathogen biology, epidemiology, disease detection and management.

References

1.Detection of virulence to wheat stem rust resistance gene Sr31 in Puccinia graminis f. sp. tritici in Uganda. Pretorius ZA, Singh RP, Wagoire WW, Payne TS. In: Plant Disease, 2000, 84:203. DOI: 10.1094/PDIS.2000.84.2.203B

2. Deadly wheat fungus threatens world's breadbaskets. Stokstad E. In: Science, 2007, 315:1786-1787. DOI: 10.1126/science.315.5820.1786

3. The spread of stem rust caused by Puccinia graminis f. sp. tritici, with virulence on Sr31 in wheat in Eastern Africa. Wanyera R, Kinyua MG, Jin Y, Singh R. In: Plant Disease, 2006, 90:113. DOI: 10.1094/PD-90-0113A

4. Tracking wheat rust on a continental scale. Kolmer JA. In: Current Opinion in Plant Biology, 2005, 8:441-449. DOI: 10.1016/j.pbi.2005.05.001

5. Current status, likely migration and strategies to mitigate the threat to wheat production from race Ug99 (TTKS) of stem rust pathogen. Singh RP, Hodson DP, Jin Y, Huerta-Espino J, Kinyua MG, Wanyera R, Njau P, Ward RW. In: CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 2006, 1, No.054 DOI: 10.1079/PAVSNNR20061054

6. Resistance in U.S. wheat to recent Eastern African isolates of Puccinia graminis f. sp. tritici with virulence to resistance gene Sr31. Yin Y. In: Plant Disease, 2006, 90(4):476-480. DOI: 10.1094/PD-90-0476

7. Characterization of seedling infection types and adult plant infection responses of monogenic Sr gene lines to race TTKS of Puccinia graminis f. sp. tritici. Jin Y, Singh RP, Ward RW, Wanyera R, Kinyua MG, Njau P, Fetch Jr, T, Pretorius ZA, Yahyaoui A. In: Plant Disease, 2007, 91:1096-1099. DOI:10.1094/PDIS-91-9-1096

8. New virulence within race TTKS (Ug99) of the stem rust pathogen and effective resistance genes. Jin Y, Pretorius ZA, Singh RP. In: Phytopathology, 2007, 97:S137.