Puccinia graminis f. sp. tritici (Pgt) has significantly impacted worldwide wheat (Triticum aestivum L.) production throughout human history. The most recent wheat stem rust outbreak in the United States was reported around 1953 - 1954, with an estimated 40% yield loss . The loss in yield is attributable to several factors associated with colonization of Pgt on its poaceous hosts. These include competition with the fungus for nutrients, lodging due to weakened stems, and reduced metabolic efficiency.
Telial host range includes wheat, (Triticum aestivum), barley (Hordeum vulgare), goatgrass (Aegilops spp.) and wild rye (Elymus spp.). Aecial host range includes common barberry (Berberis vulgaris), additional Berberis spp., Mahoberberis spp., and Mahonia spp.
Prolific spore generation, wind-borne dispersal, and viability of spores over thousands of miles are characteristics that have made this an efficiently destructive pathogen . Until the late 1990s, successful mitigation of the disease was largely attributable to deployment of wheat resistance genes ) and alternate host eradication programs In 1998, observations revealed a virulent form of Pgt on wheat lines armed with the Sr31 resistance gene in Uganda. In 1999, further analyses confirmed existence of a new race of the pathogen, which was subsequently termed “Ug99” in reference to the country and year in which it was first reported. In accordance with the nomenclature system of North America, the new virulent race was designated TTKSK . It may be noted that although often referred to as such, Ug99 is not a race but rather an individual of the TTKSK race.