Ug99 race of stem rust pathogen: Challenges and current status of research to sustain wheat production in India

A highly virulent race of stem rust of wheat caused by Puccinia graminis Pers. f.sp. tritici Errikss. & Henn. was detected in 1999 in Uganda and may pose a major threat to wheat crop in many countries including India in coming years. This race is commonly known as Ug99 and designated as TTKSK based on North American system of nomenclature. The race Ug99 carries virulence to several genes commonly present in global wheat germplasm including Sr31, known to be present in several leading spring and winter wheat cultivars and germplasm. Emergence of a new variant TTKST that overcame resistance of Sr24 gene causing severe stem rust epidemic in Kenya has further aggravated the threat. The occurrence of Ug99 is now widespread in wheat growing areas of Kenya and Ethiopia and has rendered many popular cultivars susceptible. It crossed the red sea to Yemen and migrated to Sudan in 2006, and its presence was confirmed in Iran in 2007. Therefore, development and dissemination of suitable resistant cultivars in a relatively short time before rust migrates to South Asia is of utmost importance for food security. Indian wheat programme in collaboration with CIMMYT and Kenya has identified some wheat cultivars and genetic stocks that are resistant to Ug99. However, area coverage to these resistant cultivars must increase to occupy about 3–5% of the total area for enough seed procurement to replace the current popular susceptible cultivars, when Ug99 reaches Indian Subcontinent. Although, several alien genes can provide resistance, the long-term strategy should focus on achieving durable resistance by rebuilding the Sr2 complex.