Infection types of isolates and race identification

Race identification of stem rust of wheat can be done in three ways:

  1. Bring rust samples and inoculated McNair to make pustule isolation, multiply the isolated pustule and then inoculate the differentials;
  2. Bring one fresh rust sample and directly inoculate the differential set; and
  3. Bring rust samples and inoculate the susceptible host McNair to revitalize the sample and then identify the race.

Fourteen days after inoculation, the differentials are evaluated using 0-4 scoring scale (Stackman et al. 1962), where 0-2+ scores are considered avirulent (resistant reactions or effective genes) and 3-4 scores are considered virulent reactions (susceptible or non-effective genes) (Fig. 19 and Appendix A).

After reading the infection types and recording on the wheat stem rust differential scoring sheet (Table 2 and Appendix B), the race is identified using the North American nomenclature system of Roelfs and Martens 1988, Jin et al., 2008 (Appendix C).

For instance, low infection type (IT) on all four hosts in a set is assigned the letter B, while high IT on the four hosts is assigned T. Hence, if an isolate produces low IT (resistant reaction) on each of the 20 differential lines, the race will be designated with a five letter race code BBBBB. In the same way, an isolate that produces a high IT (susceptible reaction) on the 20 differential lines will have a race code TTTTT. If an isolate produces a low IT on Sr36, SrTmp, and Sr24, but a high infection type on the remaining 17 differential lines, the race will be designated as TTKSK (Ug99). See Fig. 20 for an illustrated example for TTKSP.

Fig. 20: Illustrated example for race TTKSP.  Source: Z.A. Pretorius, University of Free State, South Africa
Fig. 20: Illustrated example for race TTKSP.
 Source: Z.A. Pretorius, University of Free State, South Africa