Evaluation of Ethiopian Wheat Germplasm Against Yellow Rust (Puccinia striiformis) Disease Under Field Condition

Wheat rust outbreak especially yellow rust is the major production constraint in Ethiopia. Arsi Zone is the hot spot zone for yellow rust (Puccinia striiformis (Pst)) particularly Meraro and Bekoji sites are frequently yellow rust epidemic was occur due favorable environmental and higher pressure of inoculums in the areas. Therefore, the objective the study was to identify sources of resistance against yellow rust at adult plant stage under hot spot sites. In this study, Three hundred seventy five elite breeding and advanced lines were planted at Meraro, Arsi-Robe and Bekoji. The results show that 34, 10 and 54% of the lines at Meraro were found resistant to moderately resistant, intermediate and moderately susceptible to susceptible. At Arsi-Robe 61, 23.5 and 15.5% of lines were showed that resistant to moderately resistant, intermediate and moderately susceptible to susceptible. Finally, at Bekoji 35.5, 21.3 and 43.2% lines at were resistant to moderately resistant, intermediate and moderately susceptible to susceptible category of yellow rust. Average coefficient of infection (ACI) less than 20% showed at Meraro (34%), Bekoji (35.5%) and Arsi-Robe (61%) was good percent of resistance to the current yellow rust pressure respectively, indicating higher disease pressure at Meraro and Bekoji than that of Arsi-Robe. Among the lines, 43.7% showed a good level of resistance (<30% severity) in all locations. Generally, most of the lines, which showed better resistance at Arsi-Robe, were susceptible at Meraro and Bekoji.


Introduction
In Ethiopia wheat (Triticum aestivum) is one of the staple food crops cultivated by 5million small scale farmers and it is covered 1.7 million hectares and it accounts for 13.49% of the cropland, with an annual production of 4.5 million metric tons in the country [6]. Wheat contributes about 15.63% of the grain production in the country. In terms of area, wheat ranks fourth after teff, maize and sorghum and the mean wheat yield in Ethiopia is estimated to be 2.67 t ha -1 [6], which is well below the world mean of 3.0 t ha -1 [9]. This is due to losses caused by biotic and a biotic constraints [1,10]. Among biotic constraints wheat rusts are the main restrictive factors for wheat production in Ethiopia [1]. From rusts especially, yellow rust can cause yield losses of up to 100% on susceptible cultivars if infection was occurs at early stage and the disease continues to develop during all growing season [3,11]. The impact of yellow rust varies depending upon the susceptibility of cultivar, earliness of the initial infection, rate of disease development and duration of the disease [5]. Yellow rust epidemics are becoming more frequent in Ethiopia. According to Mozgovoy [12], epidemics of yellow rust occurred in 1977, 1980-83 and 1986, resulting 30-40% yield losses In another report, grain yield losses up to 96% were impose on susceptible wheat cultivars [18]. The Epidemics occurred from in  have resulted in 58% of yield loss [4]. Therefore, the past two decades, frequent epidemics of yellow rust have been a major challenge to wheat production in the country. Releasing of genetically resistant cultivars and chemical option are the two major management of yellow rusts disease in Ethiopia as well as in worldwide. According to Chen [5] research report increasing resistant cultivars is preferred approach for control of yellow rust because it is an effective, easy-to-use, economic and environmentally friendly strategy. on the other hand, resistant cultivars become susceptible due to dynamics of pathogen within the population. Therefore, searching for resistance sources to the current yellow rusts diseases is becoming crucial. The objective of this study was to identify yellow rust resistance genotype/elite lines at adult plant stage (APR) under hot spot sites.

Materials and Methods
A total of 375 elite bread wheat lines that were either selected from international nurseries including resistant and susceptible check were used for study. From resistant cultivars (Danda'a, Wane, and Kingbird Daka) and susceptible check varieties (Kekeba and Lemu) were repeatedly planted at certain intervals of test lines. Natural yellow rust epidemics were enhanced by planting a mixture of spreader rows of PBW343, Kubsa, Digalu and Morocco in perpendicular to the test entries in 2019 main cropping season. Plots were kept weed free by two times hand weeding. When the susceptible check started to show yellow rust development, scoring of severity was carried out using the modified Cobb scale [13]. Average Coefficients of Infection (ACI) were calculated by taking into account the severity of yellow rust of the lines and cultivars, and their field response [14] in addition, genotypes were compared for resistance using final rusts severity score, field reaction and average coefficient of infection. The scores were converted into average coefficients of infection, by multiplying severity by an assigned value for the field response, as suggested by Stubbs et al. [14] ( Table 1). The constant value was further modified to include infection responses of resistant to moderately resistant (RMR=0.3) and moderately susceptible to susceptible (MSS=0.9).

Reactions of the Evaluated Genotype
From 375 elite breeding and advanced lines at Meraro, Arsi Robe and Bekoji 15, 1 and 31 were immune against all the races of yellow rust prevailing in nature respectively (Table 2)

Severity of Yellow Rust
Majority of the taste lines (131 and 133) tested at Meraro, and Bekoji ( Figure 2) showed a severity < 20%; whereas at Arsi Robe, almost two-third of the line (229) showed the same severity levels ( Figure 2). 41, 88 and 80 display a severity range between 21-30% at Meraro, Arsi robe and Bekoji respectively. 203, 58 and 162 genotype/lines show severity range > 30%. Terminal yellow rust severity less than or equal to ten percent and coefficient of infection The selection of elite bread wheat lines was made based on the occurs of the highest yellow rust pressers site from the three yellow rust hot spot areas. In 2019 main cropping season the highs yellow rust pressers was occurrence at Meraro than the other hot spot location. Therefore, we are select the terminal severity of the yellow rust leas than or equal to ten percent by correcting of the other hot pot location. The selection was made by considering the three parameters, the highest score was recorded for elite line KU18BWOL_339_9 (Table 3). The least score was recorded for elite bread wheat line KU18BWPL1_29 (Table 3). Two elite lines show an average coefficient of infection (ACI) value of less than or equal two. These included lines KU18BWOE_134 and KU18BWOE_69. Generally 39 (79%) of the test genotypes displayed an ACI value of less than or equals to ten percent of the coefficient of infection. On the other hand 10 (20%) The test genotype show as greater than or equals to ten.

Response of the Check Cultivars Against Yellow Rust in Hot Spot Areas
When see the average disease severity in check cultivars and it is response almost all check cultivars was moderately susceptible to susceptible reactions except wane at Meraro, Arsi-Robe and Bekoji. The responses of check cultivars against yellow rust was score 24, 34, 46.8 and 46.5% on Wane, Lemu, King-bird and Kekeba in all location respectively ( Figure 3). From the chekek cultivars the highest disease severity was recorded on King-bird and Kekeba.

Discussion
According to Admassu et al. [2] reported that indigenous germplasm evaluations against yellow rust in field condition indicated that most of the lines were moderately resistant after the resistance status which is suggested to use in the wheat breeding program because resistant varieties are the best option for successful wheat production. The field data indicated that the variability of yellow rust in severity and infection response of genotype/elite lines, and check cultivars at all testing site, Meraro, Bekoji and Arsi-robe respectively (Tables 2 and 3), indicated that the yellow rust pathogen populations have differences in their virulence profiles. More genotype/lines were found to be more susceptible at Meraro and Bekoji than Arsi-robe. The results clearly indicate the broad range of yellow rust in reaction from resistance to susceptible for yellow rust in breeder's germplasm. Therefore, it is concluded that some of the wheat germplasm had a great potential to be used against yellow rust. this result similar with Ayele, 2002 Some of the advanced bread wheat lines and commercial cultivars, which showed susceptible reaction at seedling stage exhibited low terminal yellow rust severities under field conditions in Ethiopia (Ayele, 2002). Finally, this study indicated that most of the bread wheat genotype/lines contained different resistance range when compared to the commercial cultivars. Therefore, identifying resistant bread wheat genotype/lines have been vital as sources of resistance to the prevailing races of yellow rust in Ethiopia.

Conclusion
The genotype/lines that confirm low level of yellow rust severities from R to MR reactions to yellow rust in all test locations, can be utilized as resistant parental lines in the wheat breeding programmed or they can be advanced to yield trials for the development of high yielding and yellow rust resistant cultivars to fight for new evolving races of yellow rust.