Examining Ethiopian bread wheat genotypes for the novel sources of resistance to strip rust (Puccinia striformis f.sp.Tritici)

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Introduction

Wheat is the most important strategic and food security cereal crop in Ethiopia, ranks 3^rd in the area coverage after teff (Eragrostis tef) and maize and 2^nd in volume of production next to maize. It is cultivated on a total area of 2.1 million hectares annually with a total production of 6.7 million tons (Tadesse, W., et al., 2003). The national average yield is 3.1 t ha⁻¹ rain fed and 4t ha⁻¹ irrigated which is below the average of 3.5 tha^-1 world yield production and potential yield of 7 and 8 t ha⁻¹ under rain fed and irrigated conditions respectively (Ulrik, G., et al., 2021; CSA, 2021). This low productivity is attributed to several scenarios of climate change, evolution of new virulent pathogen races, increased intensity of weeds and insect pests, increasing of soli acidity and salinity, increment of fertilizers cost and demand of biofuels and slow rate of adoption to new agricultural technologies (Dixon, J., et al., 2009).

Yellow rust is repeatedly production constraint in potential wheat growing areas of world. Regarding to rust epidemiology, Ethiopia and Yemen form ecological unit in playing important role in inoculum spread and evolving of new virulence pathotypes across Central and West Asia and North Africa (Badebo, A., et al., 1992). It is dynamic in nature and a serious problem to wheat production in all cropping season due to favorable climatic conditions, continuous planting of susceptible cultivars in hotspot areas of yellow rust and genetic uniformity of current cultivating mega cultivars favoring chance of evolving new virulent races in Ethiopia (Solh, M., et al., 2012).

Extensive surveys revealed that wheat rusts (stem and yellow) caused, 10s of millions of USD annually in in the country. This is due to evolution of new virulent strains which are cold tolerant stem rust races and warm temperature adapted yellow rust races. The rapid emergence of virulent races of PSt11 and PSt16 have overcome most of currently released cultivars and known stripe rust resistant genes of wheat in Ethiopia (Nazari, K., et al., 2008). To date, for the past five decades more than 88 bread wheat varieties of local cross, CIMMYT and ICARDA origin have been released with continuous progress in yield, acceptable end use qualities and improving disease resistance but, less than 20 varieties cover country’s wheat growing areas and unfortunately most of mega cultivars are becoming out of production due to recurrent rust epidemics. Thus, taking into account on periodic outbreaks of rust epidemics and economic invasion, continuous work needs in adequate monitoring and searching of novel sources of durable resistant wheat genotypes against yellow (stripe) rust. The objective of the study was to identify novel sources of resistance to stripe (yellow) rust among one thousand four (1004) Ethiopian bread wheat genotypes exhibited 157 local cross, 663 CIMMYT introductions, 173 ICARDA sources and 11 commercial cultivars.

Materials and Methods

Description of the study area

The experiment was conducted at kulumsa research center substations of Bekoji and Meraro which is located at 7°32'37"N, 39°15'21"E and 2780 meters and 7°24'27"N, 39°14'56"E and 2990 meters above sea level respectively. Monthly maximum and minimum temperatures of Bekoji and Meraro have 7.9 and 18.6, and 5.7 and 18.1°C with annual rain fall of 102 mm and 1196 mm respectively. Both locations were characterized bimodal receiving extended rainfall and represents yellow rust host spot areas and major wheat production potential agro ecologies of Arsi, Ethiopia.

Planting materials

A total of one thousand four (1004) genotypes exhibited 157 local cross, 663 CIMMYT introductions, 173 ICARDA sources and 11 commercial cultivars were screened for the resistance to yellow rust. The genotypes were tested at preliminary yield trials and observation nurseries for different traits at quarantine site of kulumsa research center which is national wheat research coordinating center of Ethiopia and advanced lines were selected to test at severely affected hot spot areas to yellow rust in field condition.

Experimental design

The trail was implemented with partially replicated design consisted of 1508 entries. The spacing of each entry was planted 0.5 m length with 0.2 m row spacing in single row with 26 blocks which was consisted 58 entries in each blocks. Mixtures of different highly susceptible varieties namely Morocco, PBW343, Kubsa, Digalu and newly susceptible poplar variety Ogolcho were planted in each block as to receive uniform inoculum to the entries. All agronomic and weed management practice were applied as per recommendations for all entries.

Disease assessment

Disease assessment was performed three times at Bekoji and Meraro experimental sites at fourteen day’s interval, started when susceptible spreader row morocco reached 40 percent yellow rust severity according to modified Cobb scale (8). Response of wheat genotypes were assessed through final rust severity (FRS) and coefficient of infection (CI). The host plant response to infection was scored according to Roelfs, A.P., et al., (1992), and average coefficient of infection (CI) was calculated by multiplying the percentage severity and the constant value assigned to each reaction type (Saari, EE., et al., 1974). The constant values were considered as Immune=0, R=0.2, R-MR=0.3, MR=0.4, MRMS=0.6, MS=0.8, MSS=0.9 and S=1.

Results and Discussion

Among one thousand four (1004) tested genotypes 993 and 11 were advanced bread wheat lines and released commercial cultivars respectively. Thus, genotypes exhibited, 157 local cross, 663 CIMMYT introductions and 173 ICARDA sources were screened for the resistance to yellow rust at Bekoji and Meraro which are nationally yellow rust prone areas. The final yellow rust severity and response of genotypes were presented in Fig. 1. Fortunately, the season was conducive to yellow rust disease epidemics result revealed, various field reactions ranging from immune to susceptible(s) response were assessed at both experimental sites. From 1004 tested advanced genotypes 2.2%, 7%, 31.4%, 8.5%, 13.5% and 37.4% showed immune, moderately resistant, intermediate reaction, moderately susceptible, moderately susceptible to susceptible, and susceptible diseases reaction and none of the tested genotypes showed resistant and resistant to moderately resistant disease reaction at Bekoji whereas, 1.6%, 1.9%, 8.5%, 8.1%, 11.7%, 7.7% and 60.5% of the 1004 screened genotypes showed immune, resistant to moderately resistant, moderately resistant, intermediate reaction, moderately susceptible, moderately susceptible to susceptible and susceptible disease reaction and none of the tested genotypes showed resistant response respectively at Meraro experimental site (Fig. 1).

Fig 1. Response of bread wheat genotypes to yellow rust at Bekoji and Meraro in 2022.

Despite highest yellow rust epidemics at Bekoji and Meraro 55 genotypes selected which 50 of them exhibiting final rust severities ranging from 5 to 20% with compatible RMR and MR disease reaction, are great importance to attain current breeding for long-lasting resistance (Parlevliet, JE., 1988), whereas five genotypes showed immune type response with zero disease severity (Table 1). On the other hand sixty two genotypes were also selected which showed intermediate response to yellow rust at both locations (Table 2).

Table 1. Response of selected to 55 wheat genotypes showed best sources of resistance to yellow rust at in 2022.

Table 2. Wheat genotypes showed intermediate resistance to yellow rust at Bekoji and Meraro.

The available resistant genes in these selected genotypes overcome yellow rust virulence in the field and led to statistically low yellow rust severities in spite of well-suited host pathogen reactions (Nzuve, FM, et al.,). According to van der Plank’s (1968) and Robinson’s (1979) attempts, Horizontal, uniform, race-non-specific or stable resistance can be distinguished from vertical, differential, race-specific or unstable resistance by a test in which a number of host genotypes are tested against a number of pathogen genotypes. Horizontal resistance should be built up in crops as a primary objective and as the foundation of disease management, with vertical plant pathology resistance being added as necessary, along with cultural control measures and targeted use of pesticides, as part of an IPM strategy. So Van der Plank's test for horizontal resistance appears to be a simple and sound way to test for polygenic inheritance of resistance (Parlevliet, JE., 1976) (Tables 1-3).

Table 3. Response of check varieties to yellow rust at both locations.

Conclusion

Forecasting of upcoming transformations and developing an operational research tactics with use of new breeding tackles, needs a corresponding strength in creating communication net-works and teamwork’s. Timely monitoring and information exchange between stake holders, creating new capacities and skills in ministries and extension services to develop effective management tactics, and continued research in searching of durable resistant genotypes to virulent races, planning to rapidly deliver appropriate improved seeds and fungicides to halt spread of wheat rust should be strategies to address the current existing problems of wheat rust disease management. Out of 1004 tested genotypes only 5.5% genotypes exhibited 1.2%, 0.7% and 3.6% exhibiting having RMR and MR field response, local cross, ICARDA and CIMMYT introductions performed best and selected to yellow rust resistance respectively, While 6.2% identified genotypes also showed intermediate (MRMS) field response, could be suggested to for further breeding purpose.

Acknowledgement

Accelerating Genetic Gain in wheat and Bill and Melinda Gates Foundation (BMGF), projects are genuinely acknowledged for technical and financial support of the Experiment. Ethiopian Institute of Agricultural Research is recognized for accommodating the field research. The all-round provision by the wheat research team of kulumsa Agricultural research Center of Ethiopia is greatly valued.

Conflict of Interest

The authors declare no conflict of interest.

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