Stem rust: its history in Kenya and research to combat a global wheat threat

Abstract

Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a major disease of wheat. In Kenya, Pgt has caused sporadic but serious losses to farmers since large-scale wheat production began in the early twentieth Century. Breeding for stem rust resistance in Kenya has been conducted since 1910. Mutants of Pgt are common in Kenya as large pathogen populations survive on wheat crops planted throughout the year, with virulence to effective major genes developing shortly after release of resistant cultivars. Gene Sr31 was first deployed in Kenya in ‘Kenya Pa’a’ in 1982 and subsequently in ‘Duma’ in 1993, the latter grown on large acreage. Virulence to Sr31 was first detected in Uganda in 1998, in what became known as Pgt-Ug99 (race TTKSK). Virulence to Sr31 may have occurred earlier in Kenya, but Ug99 was first reported in 2001. Race TTKSK has migrated across East Africa and to Yemen and Iran, spreading to 13 countries with 13 race variants. Other Pgt races (TKTTF, TTRTF, TTTTF) with broad virulence were recently detected in Kenya, likely originating from central Asia. A ‘Sounding the Alarm’ message from Norman Borlaug in 2005 triggered extensive research on worldwide virulence in Pgt, and on finding, characterizing, and developing molecular markers for Sr genes effective to Ug99-lineage pathotypes of Pgt. While fungicides can control Pgt, the best strategy uses host resistance. The best gene stewardship practice to provide enduring resistance combines effective major and minor adult plant resistance (APR) genes prior to release of new wheat cultivars.