Emergence and spread of new races of wheat stem rust fungus: Continued threat to food security and prospects of genetic control

Authors: SINGH, RAVI - International Maize & Wheat Improvement Center (CIMMYT); HODSON, DAVID - International Maize & Wheat Improvement Center (CIMMYT); Jin, Yue; LAGUDAH, EVANS - Commonwealth Scientific And Industrial Research Organisation (CSIRO); AYLIFFE, MICHAEL - Commonwealth Scientific And Industrial Research Organisation (CSIRO); BHAVANI, SRIDHAR - International Maize & Wheat Improvement Center (CIMMYT); Rouse, Matthew; Szabo, Les; PRETORIUS, ZACHARIAS - University Of The Free State; HUERTA-ESPINO, JULIO - Instituto Nacional De Investigaciones Forestales Y Agropecuarias (INIFAP); BASNET, BHOJA - International Maize & Wheat Improvement Center (CIMMYT); LAN, CAIXIA - International Maize & Wheat Improvement Center (CIMMYT); HOVMØLLER, MOGENS - Aarhus University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2015
Publication Date: 3/6/2015
Citation: Singh, R., Hodson, D., Jin, Y., Lagudah, E., Ayliffe, M.A., Bhavani, S., Rouse, M.N., Szabo, L.J., Pretorius, Z.A., Huerta-Espino, J., Basnet, B.R., Lan, C., Hovmøller, M.S. 2015. Emergence and spread of new races of wheat stem rust fungus: Continued threat to food security and prospects of genetic control. Phytopathology. 105(7):872-884.

Interpretive Summary: Wheat stem rust race Ug99 (TTKSK), detected in Uganda in 1998, has been recognized as a serious threat to food security because it possesses a combined virulences to a large number of resistance genes found in current widely-grown wheat varieties and germplasm, leading to its potential for rapid spread and evolution. Since its initial detection, variants of the Ug99 lineage of stem rust have been discovered in Eastern and Southern African countries, Yemen, Iran, and Egypt. To date, eight races belonging to the Ug99 lineage are known. Increased pathogen monitoring activities have led to the identification of other races in Africa and Asia with additional virulences to commercially important resistance genes. This has led to localized but severe stem rust epidemics becoming common once again in East Africa. Enhanced research in the last decade under the umbrella of the Borlaug Global Rust Initiative has identified various race-specific resistance genes that can be utilized, preferably in combinations, to develop resistant varieties. Research and development of improved wheat germplasm with complex adult plant resistance (APR) based on multiple slow rusting genes has also progressed. Several APR resistance genes have been characterized and additional quantitative trait loci identified. Recent cloning of several rust resistance genes opens new perspectives on rust control in the future through the development of multiple resistance gene cassettes. However, at present, disease surveillance, short-term chemical control, large-scale deployment of new varieties with multiple race-specific genes, or high to adequate levels of APR, and reducing the cultivation of susceptible varieties in rust hot-spot areas remains the best stem rust management strategy. This review ariticle provides the latest updates on the pathogen variation and host resistance that will be useful to scientists in global surveillance of the disease and breeding for stem rust resistance.

Technical Abstract: Race Ug99 (TTKSK) of Puccinia graminis f. sp. tritici, detected in Uganda in 1998, has been recognized as a serious threat to food security because it possesses a combined virulences to a large number of resistance genes found in current widely-grown wheat (Triticum aestivum) varieties and germplasm, leading to its potential for rapid spread and evolution. Since its initial detection, variants of the Ug99 lineage of stem rust have been discovered in Eastern and Southern African countries, Yemen, Iran, and Egypt. To date, eight races belonging to the Ug99 lineage are known. Increased pathogen monitoring activities have led to the identification of other races in Africa and Asia with additional virulences to commercially important resistance genes. This has led to localized but severe stem rust epidemics becoming common once again in East Africa due to the break-down of race-specific resistance gene SrTmp, which was deployed recently in the ‘Digalu’ and ‘Robin’ varieties in Ethiopia and Kenya, respectively. Enhanced research in the last decade under the umbrella of the Borlaug Global Rust Initiative has identified various race-specific resistance genes that can be utilized, preferably in combinations, to develop resistant varieties. Research and development of improved wheat germplasm with complex adult plant resistance (APR) based on multiple slow rusting genes has also progressed. Once only the Sr2 gene was known to confer slow rusting APR; now four more genes – Sr55, Sr56, Sr57, and Sr58 – have been characterized and additional quantitative trait loci identified. Cloning of some rust resistance genes opens new perspectives on rust control in the future through the development of multiple resistance gene cassettes. However, at present, disease surveillance based chemical control, large-scale deployment of new varieties with multiple race-specific genes, or high to adequate levels of APR, and reducing the cultivation of susceptible varieties in rust hot-spot areas remains the best stem rust management strategy.