Wheat stem rust (Puccinia graminis f. sp. tritici) and race analysis

Stem rust (black rust), caused by a fungus Puccinia graminis f.sp tritici of wheat, belongs to the class Basidiomycetes, order Uredinales, family Pucciniaceae and the genus Puccinia. Stem rust has been one of the most significant wheat diseases worldwide, causing famines, economic and even political crises. It is highly specialized biotrophic wheat pathogen with narrow host ranges (Singh et. al., 2012). Stem rust attacks all the above-ground parts of the wheat plant. It is mainly found on the stems, but, at times, on leaves, sheaths, glumes, awns and seeds (Marsalis and Goldberg, 2006). Raised, long and narrow, orange-red pustules occur in early stages of the disease on the stems and leaves of susceptible cultivars (Fig. 1). A crop that appears healthy three weeks before harvest can be devastated by explosive build-up of stem rust if sufficient inoculum arrives from a heavily infected wheat crop in some distant region (Leonard and Szabo, 2005).

Fig. 1: Puccinia graminis under field conditions at Ambo, Ethiopia, 2014.

Fig. 1: Puccinia graminis under field conditions at Ambo, Ethiopia, 2014.

Furthermore, the productivity of wheat in Ethiopia has remained extremely low (2.54 t/ha) due to biotic factors like Puccinia graminis f.sp. tritici (FAOSTAT, 2015). At times of epidemics, yield losses due to stem rust can reach 100%. Several stem rust epidemics have been recorded in different parts of Ethiopia in recent history that have caused great losses: stem rust epidemics in 1975 on variety Laketch; in 1992/93, on variety Enkoy; in 1994, on variety Kubsa; and, in 2013, on variety Digelu. The epidemics occurred due to the appearance of new races, probably as a result of mutation and/or sexual recombination. So, to minimize the threat of future epidemics, it is important to characterize the race composition of stem rust pathogens and the appearance of new races in the country. The high yield losses caused by this pathogen and frequent appearance of new races demands full-fledged research on wheat stem rust in this country.
This document is a guide for technicians, junior researchers, students and trainees on how to process stem rust races in the greenhouse and how to multiply the races and store the races for use in the future. It also emphasizes the precautions that should be taken during the process.
Race analysis requires organized human resources as well as materials. The activities involved in race analysis are intensive and demand excellent techniques and precautions. Greenhouse and greenhouse related activities for wheat stem rust race analysis are organized in a number of distinct major steps outlined in this document.