Virulence Spectrum of Stem Rust (Puccinia graminis f.sp. tritici) and Reactions of Wheat Varieties to Dominant Races in Tigray Region, Northern Ethiopia

Abstract

Wheat (Triticum spp.) is one of the most important staple cereal crops cultivated in the world particularly, Ethiopia. However, wheat stem rust (black rust) is one of the most important airborne diseases of wheat caused by Puccinia graminis f.sp. tritici remains a constraint to wheat production. It is an obligate biotrophic fungus only grow and multiply on living hosts. Since 1975, multiple wheat stem rust epidemics that caused significant losses have been occurred in different wheat growing regions of Ethiopia. Therefore, this study was aimed to assess the current distribution of wheat stem rust, identification of physiological race and reaction of bread wheat varieties against dominant races. The purposive multi-stage sampling procedure was used to select major wheat growing zones, districts and peasant associations. The current distribution of wheat stem rust was intensively assessed and examined in 95 sample wheat farms from 7 districts across the major wheat growing zones of Tigray region. The study showed that wheat stem rust was prevalent with significantly (p < 0.01) varied in incidence and severity of damage among fields, districts, and zones. The mean prevalence of wheat stem rust was 85.55% in Southern, 62.22% in Eastern and 53.33% in Southeast zones. The highest disease incidence of 78.67%, 66.50%, and 47.00% was recorded in Kilte Awulaelo, Raya Azebo, and Ofla districts, with corresponding severity of 43.67%, 35% and 20.67%, respectively which was higher than the previous reports from the region indicated the present distribution of the disease is remarkably on increasing trend. Race identification through inoculation of stem rust isolates, multiplication of single-pustule of the pathogen and race designation by inoculating on a set of wheat differential lines were done in the greenhouse. Forty-seven stem rust isolates were analyzed on the twenty stem rust differentials and resulted in the identification of six races namely; TTTTF, TKTTF, TRTTF, TTRTF, RRTTF and TKPTF. In this study, race TTRTF was detected for the first time in Ethiopia and throughout the world. Out of the six races identified, TTTTF was detected from 25 (53.19%) isolates and TKTTF from 15 (31.91%) isolates. The most virulent race that made 18 stem rust resistant genes non-effective was TTTTF, which virulent on 90% of stem rust resistance genes. Differential hosts carrying Sr24 and Sr31 were effective genes which confers resistance to all of the races identified. Hence, the stem rust resistance gene Sr24 and Sr31 can be used as sources of resistance in the wheat breeding program. Evaluation of bread wheat varieties was done by artificial inoculation at seedling stage against dominant stem rust races in the greenhouse. Thirty-nine bread wheat varieties evaluated for their reactions against the dominant races TTTTF, TKTTF, TTKSK, TRTTF, RRTTF and JRCQC. Seven varieties, namely Honqolo, Huluka, Millennium, ETBW-9017, ETBW-9042, Dilfiker and Wabe showed resistance reactions (fleck (;) to 2). However, to come up with a precise and reliable recommendation, the experiment should be done at adult stage resistance. Therefore, further research should be conducted with the same varieties against those dominant races at adult stage resistance.