Evaluation of Bread Wheat Genotypes for Adult Plant Resistance to Stem Rust

Wheat stem rust ( Puccinia graminis f. sp tritici ) also known as 'polio of agriculture' is a ravaging disease of wheat ( Triticum aestivum L.) making it a foremost confront to wheat production in Ethiopia and other wheat producing countries of the world due to the frequently evolving of virulent pathogen races. Because of this, searching novel genes in wheat genotypes is a critical issue. Twenty eight advanced bread wheat genotypes were evaluated for their response against stem rust caused by Puccinia graminise under field conditions at Kulumsa and Asassa during 2018 main growing season. Evaluation was carried out through disease assessment including terminal stem rust severity (TRS) and coefficient of infection (ACI) under natural infection. Twenty two wheat genotypes viz; ETBW-8858, ETBW-8870, ETBW-8583, ETBW-8684, ETBW-9548, ETBW-9549, ETBW-9554, ETBW-9558, ETBW-9559, ETBW-9560, ETBW-8751, ETBW-8862, ETBW-8804, ETBW-8896, ETBW-8991, ETBW-9560, ETBW-9556, ETBW-9486, ETBW-9561, ETBW-9550, ETBW-9553 and ETBW-9555 out of twenty-eight evaluated genotypes were discovered to be adult plant resistant (TRS < 30 and ACI < 20 ) to stem rust disease at both screening sites. Therefore, these genotypes with high stem rust resistance could be backcrossed to widely acclimatized and high yielding but susceptible Ethiopian wheat varieties to prevent further wheat yield declines. However, may need to evaluate for seedling response to confirm whether the genotypes are true adult plant resistant.


Introduction
Wheat ranks first as a cultivated cereal in the world (>200 mha annually) and is the most important crop with respect to sustaining food security (Shiferaw et al 2013). Ethiopia is the most important wheat-growing country in sub-Saharan Africa, with 1.6 million hectares and annual grain production of 4.5 million tons at 2.67 t grain yield per hectare (Central Statistical Agency 2017). Wheat and wheat products represent 14 percent of the total caloric intake in Ethiopia, making wheat the second-most important food, behind maize (19 percent) and ahead of teff, sorghum, and enset (10-12 percent each) (FAO, 2014). However, enhancing the production in the face of changing climate inter alia requires protection against biotic stresses (Singh et al 2011;) that cause huge yield loss. Among various biotic stresses, three rust diseases (stem rust, leaf rust and stripe rust) are the major threats to wheat production globally (Murray,2009).
All three rust pathogens belong to the genus Puccinia are host-specific viz., P. graminis f. sp. tritici Erik. & E. Henn. for stem rust, P. triticina Erik. for leaf rust and P. striiformis West. for the stripe rust. Of the three, stem and yellow rust are the most devastating in Ethiopia (Olivera et al., 2015).
Under favorable environmental conditions, stem rust can cause yield losses of up to 100% in susceptible wheat varieties (Roelfs, 1985b). The yield loss due to this disease is usually greatest if the disease becomes severe before the grain is completely formed, but yield losses are generally influenced by the resistance level of the cultivar grown, the weather conditions and the onset of the disease (Luig, 1985;Roelfs, 1985a).
The first devastating effects of stem rust epidemics was recorded in East Africa (in Kenya in the early 1990s and Ethiopia in 1972) because of a breakdown of the resistance gene Sr36 in cultivar Enkoy (Kebede et al. 1995). In Ethiopia, an epidemic of stem rust of wheat occurred in 1972, due to the loss of resistance in cultivar Lakech, which was grown on large area. Similarly, the cultivar Enkoy went out of production in the country after the epidemics in 1972. In Ethiopia, yield losses due to stem rust have been reported to be in the range of 61-100% depending on the susceptibility of the variety and environmental conditions (Eshetu, 1985;Shank, 1994).
In order to reduce losses associated with wheat stem rust, planting resistant varieties is eco-friend and cost effective option. However, breeding for wheat rust resistance always requires constant novel sources of resistance genes, due to the appearance of new virulent pathogen races (Singh et al., 2011) In most cases of country breeding programs wheat cultivars were replaced by new resistant cultivars due to susceptibility to rusts (Admassu et al., 2009). However, the only effective way to eliminate crop yield losses due to leaf rust infection (other than the use of fungicides rarely a cost effective option) are through planting stem rust resistant wheat varieties. Therefore, the objective of this investigation was to evaluate 28 advanced bread wheat genotypes against stem rust under Ethiopian wheat belt zone rust screeing sites at field conditions to select the resistant genotype to be included in wheat breeding programs.

Descriptions of study area
Arsi, with diverse agro-ecological zones, main wheat belt of east Africa and prevalence of pathogens, affords a chance to screen wheat germplasm at various disease hotspots on a large scale.
Asassa, major wheat producer district in south East Ethiopia located at 39 o 11' 43'' E and 07 o 05' 57''N altitude of 2371m above sea level. It has a mean annual temperature of 18 0 C and 930mm rainfall. The economy of the districts is dominated by agriculture. The farming system is mainly characterized by producing annual cereals mainly wheat in the absence of crop rotation in the main meher growing season. This screening site is a hotspot for leaf and stem rusts and is the natural epicenter of initial inoculum of these rusts. Therefore, infection occurs naturally in susceptible cultivars.
Kulumsa research center is located at 08 o 01' 10'' N, 39 o 09' 11'' E and at 2200 meters above sea level (m.a.s.l). It receives mean annual rainfall of 820 mm representing highland and high rainfall agro ecology. The monthly mean minimum and maximum temperature is 10.5 and 22.8°C respectively. The sites dominant soil type is loam type, which is fertile.

Planting materials and experimental procedure
Twenty-eight advanced bread wheat lines used in the present study obtained from Kulumsa Agricultural Research center, Ethiopian national wheat research excellence. Their pedigree sources were used in this study (Table 1).This experiments was carried out at two wheat rust screening sites i.e. Kulumsa agricultural research center and Asassa agricultural research site Ethiopia during 2018 growing season. These experiments were planted in randomized complete block design (RCBD) with 3 replicates. The tested wheat genotypes were planted in plots containing two rows of 1.2m × 1m width and length respectively. In order to engage uniform spread of inoculums and for adequate disease development during the trial period, a universal susceptible wheat cultivar 'Morocco' was planted a week earlier as infector row between blocks perpendicular to entries. The infector rows were sprayed and injected with active uredinospores collected and maintained at Kulumsa wheat rust laboratory, south east Ethiopia. To maintain crop stand/vigor normal agronomic practices including recommended fertilization dose and irrigation schedule were applied.  et al., 1948), where Tr = less than 5 % and 5 = 5 % up to 100 = 100 %, and host response (infection type) based on scale described by Stakman et al. (1962), which was expressed in five types as follows: immune (0) et al., 1986). Average coefficient of infection (ACI) was derived from the sum of CI values of each line divided by number of scoring times.

Results and Discussion
A total of 28 advanced bread wheat genotypes were tested for adult plant resistance to stem rust disease at Asassa agricultural research experimental site and Kulumsa agricultural research center south East Ethiopia, results showed below in Table 2.  Table (2) showed that, final stem rust severity of the tested genotypes ranged from 0-70 to 0 to 60% at Kulumsa and Asassa screening sites respectively. Out of 28 tested genotypes, 23 genotypes showed admirable slow rusting resistance (0-30%) to terminal stem rust severity at both locations. These genotypes were  According to Ali et al., (2008) genotypes with ACI values of 0-20, 21-40, 41-100 were regarded as high, moderate and low levels of adult plant resistance, respectively. In this study, 88% and 92.8% of the tested genotype namely,  showed ACI values between 0 and 20 at Kulumsa and Asassa experimental locations and were designated as having a high level of slow rusting genes. On the other hand, the susceptible check Morocco and four ETBW-8802,ETBW 8668, ETBW-8595 and ETBW-9557, and two ETBW-8668 and ETBW-9557 tested genotypes displayed ACI above 41 Table 2   .These wheat lines were found to be resistant to stem rust disease and can be used in breeding programs to release commercial cultivars as safely production under Ethiopian conditions. The result of this study is in agreement with Bekele Hundie et al. (2018), who reported advanced bread wheat lines extracted from wheat breeding trials against stem rust at adult plat stage under stem rust hot spot sites and at seedling stages in the greenhouse. In addition, the result of this study is in lined with the works of Nzuve F. (2012) who reported as evaluating wheat genotypes for both field and seedling resistance to stem rust and genotypes showed diverse seedling and adult plant resistance responses.

Conclusion
Disease tolerant wheat cultivars are believed the main issues in wheat breeding agriculture research programs to protect wheat plants from disease epidemics and thus from yield loss. In this study 28 wheat genotypes were grown at two wheat rust diseases screening hotspot sites, Kulumsa and Asassa, south East Ethiopia. The grown wheat genotypes were evaluated in two major diseases assessment features; terminal rust severity and average coffient of infection. Thus, among the 28 genotypes evaluated, 78.5% have showed high slow rusting therefore could be suggested either for release to production and/or also could be used for durable stripe rust resistance breeding in Ethiopia.