Several different systems exist for race nomenclature of stem rust and there is no single standard unified system. This meets the need of individual countries, but makes international comparison a difficult task. In a recent international survey of rust labs undertaking race analysis for stem rust, the North American nomenclature system emerged as the most widely used.
The North American system was developed by Roelfs and Martens (1988). It uses a “letter code” system for naming races, based on sets of 4 single-gene differential lines. A corresponding letter is assigned, based on the sequence of infection responses (L=Low, H=High). Three sets were initially described, and five sets are currently used. The North American nomenclature system, using five sets of 4 gene differential lines, results in Ug99 being given the letter code TTKSK.
To understand how this nomenclature system works, refer to the code sheet and the illustrated example for race TTKSP (the Sr24 variant of the presumed progenitor of Ug99, found in South Africa – example courtesy of Prof. Z.A. Pretorius, University of Free State, South Africa).