Keywords

Dedicated nursery; Genotypes; Resistance rating reference set; Stem rust.

Introduction

Stem rust caused by Puccinia graminisi f. sp. tritici (Pgt) is the most extremely destructive rust diseases of wheat under favorable environmental conditions in the world including Ethiopia [1]. It is also called black rust of wheat due to abundant production of shiny black teliospores that form at the end of the growing seasons under unfavorable environmental conditions [2]. The Pgt can produce new virulent stem rust races that can overwhelmed resistant cultivars and cause sever epidemics under favorable environmental conditions that ensuing in higher yield losses [3]. Geographically, stem rust is widespread fungal disease affecting 50 to 70%, depending on susceptibility of varieties and environmental factors and has capable of destroying 100% yield losses in few weeks by hindering the global wheat production [4,5]. According to Sing et al. 80% of all African and Asian wheat varieties are susceptible to stem rust fungal disease in major wheat producing countries are in high alert [6]. In addition to yield losses; it also causes quality degradation resulting shriveling and reducing the number of kernels particularly in susceptible cultivars [7].

The development of Resistant (R) genes utilizes the selection pressure on the pathogen and the situation undertakes genetic variation that attributed to natural selection, mutation, recombination genetic drift, and gene flow in resultingthe breakdown of genetic resistance by wheat rusts [8]. Thus, development of the pathogen and occurrence of new Pgt races due to the nature of aggressiveness has resulted susceptibility of deployed mega cultivars [9]. Wheat stem rust has been acquainted in different countries of world including Ethiopia, Eretria, Kenya, Yemen, Sudan, Pakistan, South Africa, Zimbabwe, Tanzania, Iran, Egypt, India and USA have different familiar strains of stem rust races [10].

According to Mago, et al. and Jin, et al. significant resistant Stem Rust (SR) genes with Sr24 and Sr36 have been capitulated to stem rust races, a risk for wheat production globally with more than 80% of wheat cultivars are susceptible to the current virulent wheat stem rust races [11,12].

Exploiting stem rust resistant genes signifies an effective, economically viable and environmentally ecofriendly approach to wheat disease control [13]. Therefore, continuous monitoring, developing and deploying genetically diverse sources of resistant wheat genotypes are the most sustainable and welcoming approach to achieve durable resistance varieties and reducing yield losses caused by virulent stem rust races [14]. To combat the threat of wheat stem rust disease, evaluation of different nurseries from international, national and CGIAR centers of the breeding programmers were introduced. So the objective of this research work was to search and improve the novel sources of resistant wheat genotypes against stem (black) rust.

Materials and Methods

Dedicated stem rust nursery site

The trial was implemented at Debre-Zeit agricultural research center main-station which located at 08°44’ North (N), 38°58’ East (E) with altitude of 1900 meter above sea level. Monthly maximum and minimum temperature of the experimental site has received 9.5 and 24.1 with annual rainfall of 851 and 1275 millimeters. The location represents best screening and hotspot area of stem rust.

Planting materials

A total of 1164 wheat genotypes which were CIMMYT and ICARDA sources of introductions, local cross and commercial cultivars (checks) examined for the durable resistance to stem rust disease in dedicated hotspot area. These genotypes were tested the preliminary screening process of observation nurseries at Kulumsa agricultural research center main station of quarantine site. The advanced lines of the Preliminary Variety Trials (PVT), National Variety Trials (NVT) and Variety Verification Trials (VVT) that had been suggested to test in severely affected hotspot and dedicated disease nursery site for stem (black) rust disease in field condition.

Trial design

The experiment was conducted with partially replicated row column design consisted of 2028 entries. The genotype spacing of each entry was planted length of 0.5 meter and row spacing of 0.2 meter apart in single row in 26 blocks comprised 78 genotypes in each block. Mixtures of highly susceptible different varieties namely Morocco, Kubsa, Digalu, PBW343, Ogolcho and Hidassie were planted parallel to each block and perpendicular to each entry used as spreader (infector rows) to receive uniform inoculum to the entries. All other weed and agronomic management practices were implemented as per the recommendations of the disciplines.

Stem rust races spores

Eight stem rust races namely, TTTTF, TTRTF, JRCQC, TKTTF, TTKSK, TTKTT, TKKTF and TTKTT were used in mixing during inoculation.

Field inoculation

Ultra low volume spray inoculation was applied two times during early of tillering stages whereas syringe (point) inoculation techniques was done at stem elongation stage for field inoculation of stem rust isolates in dedicated screening site. In both methods the spores were collected in belg and early warning seasons of rust surveys from different fields of Ethiopia wheat belt areas. Maintenance of races spores and pathogenicity test was done in Ambo agricultural research center in controlled condition of greenhouse due to sensitivity of rust disease symptoms and environmental factors. The spores were mixed with mineral oil for uniform distribution and to facilitate penetration in to leaf. The solution was sprayed by aid of ultra-violate spray gun and point inoculated by aid of syringe inoculators to wheat plants.

Disease data assessment

Stem (black) rust disease scoring was done two times at Debre zeit agricultural research center dedicated disease screening nursery site at fourteen days’ interval, starting of susceptible spreader row 20% disease severity by both modified cob and 0 to 9 scales using electronic data capturing tablet [15]. Response of wheat genotypes were evaluated and scored through final rust severity and host plant response to infection according to Roelfs, et al. and Coefficient of Infection (CI) was intended by multiplying the severity percentage and constant values given to each reaction type [16]. The response values were given as; 0=immune, R (resistant)=0.2, Resistant to Moderately Resistant (RMR)=0.3, Moderately Resistant (MR)=0.4, Moderately Resistant to Moderately Susceptible (MRMS)=0.6, Moderately Susceptible (MS)=0.8, Moderately Susceptible to Susceptible (MSS) 0.9, Susceptible (S)=1.

Results and Discussion

Varied field response of genotypes to stem rust extending from Immune (0) to Very Susceptible (VS) reactions were captured at dedicated stem rust screening site of Debreziet agricultural main research station during the 2023 cropping season. The final rust severities and resistance categories of genotypes are shown in Figure 1. The percentage final rust severity exemplifies the cumulative results of all resistance factors during the progress of epidemics to the genotypes responses [17]. Based on response and resistance category of 1164 genotypes, three, one, seventeen, one hundred seventy, seventy-eight, nine, eight hundred sixty and twenty-six genotypes exhibited; Immune (0), Resistant (R), Moderately Resistant (MR), Moderately Resistant to Moderately Susceptible (MRMS), Moderately Susceptible (MS), Moderately Susceptible to Susceptible (MSS), Susceptible (S), and Very Susceptible (VS) disease reaction respectively (Figure 2). Despite the heavy stem rust disease pressure at dedicated disease screening site tested 1164 genotypes, 16.2 percent were exhibited 0.1%, 1.5% and 14.6% had with compatible R, MR and MRMS disease response selected for stem rust resistance because of prodigious significance of wheat breading program (Table 1). The available resistance genes in these genotypes overcome stem rust virulence in the field and statistically low disease severities even though, the compatible host pathogen reactions were regarded as possessing high levels of slow rusting resistance. The immune response of on these tested genotypes could be as a result of hypersensitive response; often breaks its resistance due to the development of new races of the pathogen. On the other hand, the rating reference sets of check varieties Shaki and Alidoro exhibited MRMS, Abay, Boru, Balcha, Daka, Danda’a, Dursa, Lemmu and Kingbird showed susceptible varietal response, Kakaba and Hidassie exhibited very susceptible response to stem rust (Table 2). According to van der Plank’s and Parleveli, et al. finds, Horizontal, uniform, race nonspecific, or stable resistance can be distinguished from vertical, differential, race specific, or unstable resistance by test in which number of host genotypes are tested against number of pathogen genotypes that horizontal resistance could be simple and sound way to test for polygenic inheritance of resistance [18,19]. In other way, Nzuve, et al. suggests available resistant genes overcome wheat rusts virulence in the field and low disease severities leads to the compatible genotype pathogen reactions [20]. Many scholars reported that marked final rust severities overcome adult plant resistance or slow rusting characters of wheat lines during field screening.

 

Figure 1: Number of tested genotypes with source of introductions.

 

Figure 2: Resistance category of 1164 tested wheat cultivars to mixed 7 stem rust races.

Note: 0: Immune; R: Resistant; MR: Moderately Resistant; MRMS: Intermediate Resistant; MS: Moderately Susceptible; MSS: Intermediate Susceptible; S: Susceptible; VS: Very Suscseptible

Table 1: Pre-planting soil chemical analysis of two planting seasons (late rains, 2015 and early rains, 2016).

Table 2: Response of released rating reference sets of check varieties to stem rust.

Conclusion

Most of the tested genotypes in dedicated stem rust screening site exhibited susceptible type of response. But one hundred eighty-eight genotypes showed better performance against stem rust resistance under high stem rust disease pressure and selected to be included and verified in national variety trial to examine yield and yield related parameters across locations in the coming season and better performed genotypes by both disease and yield will be released for the future.

In this study phenotypic field resistance was conducted, but Marker assisted selections and backcross breeding, genome location and modifying tools and conditions for PCR amplification of molecular markers will be supported and future required for the development of Pgt resistant wheat genotypes.

Acknowledgment

The authors recognized, Ethiopian Institute of Agricultural Research (EIAR) through its Modernizing Ethiopian Research on Crop Improvement (MERCI) and Accelerating Genetic Gain in Wheat (AGGW) projects are sincerely recognized for financial and technical support and Kulumsa Agricultural Research center for facilitating the field research. Entirely round delivery by national wheat research team of the center is really appreciated.

Competing Interest

The authors declare no competing interest.

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