Because new differentials were developed, the nomenclature of P. striiformis f. sp. tritici races was modified. First, the number system, similar to the previous PST series (3,12), was used with a “PSTv” prefix which designates a specific race of virulence or avirulence response on the differential set. For example, PSTv-1 is given to a race which is virulent to Yr1 and avirulent to the remaining 17 Yr gene lines in the new differential set. The prefix “PSTv” stands for P striiformis f. sp. tritici virulence. The “v” is included in the prefix is to distinguish the new race names from the previous “PST” races.
Second, an octal system, which is similar to the letter-coding systems for naming races of P. graminis f. sp. tritici (15) and P. triticina (18), was used for coding P. striiformis f. sp. tritici races. The 18 Yr single-gene differentials were grouped into six sets, each with 3 differentials (Table 2). Numbers 0 to 7 were used to code all eight possible virulence or avirulence patterns for each set of three differentials. For each race (virulence patterns on the 18 differential lines), a code of six digits can be obtained.1
For example, PSTv-41 has an octal code of 3757441 which indicates that PSTv-41 has an octal number of 3 when tested with the first differential set (AAV to Yr1, Yr5, and Yr6 respectively), an octal number of 7 when tested with the second differential set (VVV to Yr7, Yr8, and Yr9 respectively), and so on.