Impact of Stem and Yellow Rusts on Grain Yield of Bread Wheat (Triticum Aestivum L) Genotypes Under Rainfed Conditions of Ethiopia

: Plant diseases are among the major factors affecting the yield of wheat, especially rust diseases have historically been one of principal biotic production constraints in the world. Among the three main rusts affecting wheat, yellow rust, caused by Puccinia striiformis f. sp. tritici, and Stem rust caused by Puccinia graminis f. sp tritici are the most important disease in most wheat growing areas of Ethiopia. There are a limited number of resistant varieties available and new pathotypes that overcome the most widely deployed genes have arisen. The development of improved varieties of bread wheat (Triticum aestivum L.) has always remained a focal point for wheat breeders. Therefore, the purpose of this study was to select genotypes with good agronomic performance that have high grain yield and yield component with better rust resistance especially for stripe and stem rusts which are the major diseases in Ethiopia and to recommend the best genotypes to be released as new varieties and as an initial material in breeding. Twenty-Eight genotypes with two checks were evaluated in consecutive two years. From the twenty-eight tested genotypes almost all genotypes except one (ETBW9589) showed higher grain yield than the two standard checks (Kingbird and Ogolcho). But for the case of both rust diseases as AUDPC and CI showed that tested genotypes were exhibited different reaction responses, if we see one genotypes as an example ETBW9578 had the highest grain yield and good for yellow rust but as AUDPC showed it is very susceptible reaction response for stem rust. Generally phenotypic variation was observed for infection types, level of severity and reaction response for both diseases of the 28 tested elite bread wheat genotypes and the two standard checks. Reaction response for stem rust exhibited from susceptible (S) to Moderately resistance-moderately susceptible (M) and from immunity (0) to moderately resistance (MR) for the yellow rust. Around nine genotypes had good performance for all parameters; for grain yield and yield components and also for both rust diseases. The results of current study indicated that the genotypes had diversity regarding resistance reaction, ranging from complete resistance to susceptible. Most of the evaluated genotypes exhibited moderate resistance (MR) to moderately susceptible (MS) reactions under high disease pressure.


Introduction
Wheat (Triticum aestivum L.) has a prominent position among the cereals that supplement nearly one-third of the total world population's diet by providing half of the dietary protein and more than half of the calories [8]. With the rising global population and decreasing arable land, wheat production and yield improvement became crucial. Therefore, to fulfill the food demands of an ever-growing population, the food produced in developing countries has to be enhanced by 70 per cent till 2050 [19].
Ethiopia is the largest wheat producer country in Sub-Saharan Africa [6]. About 5 million Ethiopian farmers produce 5.3 million tons of wheat across 1.8 million hectares of land under rain-fed conditions [5]. Its popularity comes from the versatility of its use in the production of a wide range of food products, such as injera, breads, cakes, pastas, etc. Wheat ranks third in area coverage and total production after teff and maize. Although the productivity of wheat has increased in the last few years in Ethiopia; it is still very low as compared to other wheat producing countries. The national average productivity is estimated to be 2.97 t ha-1 [5]; which is by far below experimental yields of over 5 tons ha-1 [11]. So, for national productivity reduction there are different constraints from those, Plant diseases are among the major factors affecting the yield of wheat, especially rust diseases have historically been one of principal biotic production constraints in the world.
Among the three main rusts affecting wheat, stripe rust, caused by Puccinia striiformis f. sp. tritici, and Stem rust caused by Puccinia graminis f. sp tritici are the most important disease in most wheat growing areas of Ethiopia. There are a limited number of resistant varieties available and new pathotypes that overcome the most widely deployed genes have arisen. Due to stripe rust grain yield losses of 10 to 70% have been reported depending upon the cultivar grown and the environmental conditions [17]. Great losses of wheat production have been associated with stripe rust, when epiphytotics occurred under favorable conditions [18]. Under favorable conditions, stem rust can also cause yield losses of up to 100% in susceptible varieties [16].
The development of improved varieties of wheat (Triticum aestivum L.) has always remained a key point for wheat breeders all over the world [4]. Therefore, the purpose of this study was to select genotypes with good agronomic performance that have high grain yield and yield component with better rust resistance especially for stripe and stem rusts which are the major diseases in Ethiopia and to recommend the best genotypes to be released as new varieties and as an initial material in breeding.

Experimental Plot Design for Yield Assessment
The experimental materials consisted of 28 genotypes, in addition, two released varieties (Kingbird and Ogolcho) were used as a check. The genotypes were evaluated in alpha lattice design with three replications at four environments (Kulumsa, Asasa, Dhere and Melkasa Research sites) for two consecutive main cropping seasons (2017/18-2018/19). In both years each genotype was sown with six rows of 2.5m length with 0.2m space between the rows, being plot size of 3m 2 . Six rows were harvested and the net harvested plot was 3m 2 (2.5m x 1.2m). Field management and agronomic practices were carried out as recommended for each location. The seed rate was maintained at 150 Kg ha-1. Urea and DAP fertilizer were applied at the rate of 50kg/ha and 100kg/ha, respectively. The N fertilizer in the form of Urea was applied at planting and tillering time (top dressing). Locations are the main variety testing site for wheat regional center Excellence and were fall in the Midland to Lowland zone (2200-1550 meter above sea level). To estimate significant differences among genotypes the data were subjected to statistical analysis by using R software.

Disease Assessment
To evaluate these genotypes for yellow and stem rust diseases have been planted at two hot spot areas Meraro and Arsi Robe for yellow and stem rust respectively. The data have been collected from hot spot areas for yellow rust from Meraro site and Stem rust from Arsi Robe by observing the spore severity on the leaves surfaces of each genotype. Host response to both rusts was recorded based on the modified Cobb scale [14]. This scale combines several infection types; resistant (R), moderately resistant (MR), moderately susceptible (MS), combination of MR and MS (M), and susceptible (S). Severity was recorded on 0-100% scale where 0% was considered as immunity while 100% was completely susceptible. The severity and field response were converted to coefficient of infection (CI) by multiplying the severity with the arbitrary constant value for field response [20,16], where R=0.2, MR=0.4, M=0.6, MS=0.8, and S=1. Field response was recorded 3 times at every 12 days interval for the case of stripe rust starting from mid of september and for stem rust starting from october upto the disease development and the crop response to the disease stop/at maturity stage.
The Area under Disease Progress Curve (AUDPC) was calculated following the method used by Wilcoxson et al. (1975).
Where, xi=the average coefficient of infection of i th record, Xi+1=the average coefficient of infection of i+1 th record and ti+1 -ti=Number of days between the ith record and i+1 th record, and n=number of observations  From the twenty-eight tested genotypes almost all genotypes except one (ETBW9589) showed higher grain yield than the two standard checks (Kingbird and Ogolcho).

Result and Discussion
But for the case of both rust diseases as AUDPC and CI showed some lines were exhibited susceptible reaction responses, if we see one genotypes as an example Yewubdar Shewaye et al.: Impact of Stem and Yellow Rusts on Grain Yield of Bread Wheat (Triticum Aestivum L) Genotypes Under Rainfed Conditions of Ethiopia ETBW9578 had the highest grain yield and good for yellow rust but as AUDPC showed it is very susceptible reaction response for stem rust (Table 2).

Figure 1. Grain yield and yellow rust diseases correlation.
Around nine genotypes had good performance for all parameters; for grain yield and yield components and also for both rust diseases. Most of the tested genotypes grain yield and the two rust diseases had a negative correlation except one genotype (ETBW9578), Which had highest grain yield even it had high AUDPC and CI with susceptible reaction reapnse for stem rust disease. Generally for both rust diseases more than 50% of the tested genotyepes had similar (low) cofficient of infection and AUDPC with good resistance reaction response for both diseases (Figures 1 and 2).
The AUDPC and CI values ranged from 15 to 2400 and 0.4 to 90 respectively for stem rust. While for yellow rust, AUDPC and CI values ranged from 0 to 637.5 and 0 to 40 respectively ( Table 2). Data of this study revealed that, nine of the tested wheat genotypes i.e. ETBW9565, ETBW9568, ETBW9573, ETBW9575, ETBW9579, ETBW9581, ETBW9583, ETBW9585 and ETBW9587 displayed the lowest values of CI and AUDPC (less than 300 for both rust diseases). So these nine genotypes after further evaluation for other traits will be released as a new wheat varieties for the end users (commercial and/or smallholder farmers).

Figure 2. Grain yield and yellow rust diseases correlation.
Phenotypic variation was observed for infection types, level of severity and reaction response for both diseases of the 28 tested elite bread wheat genotypes and for the two standard checks. Reaction response for stem rust exhibited from susceptible (S) to Moderately resistance-moderately susceptible (M) and from immunity (0) to moderately resistance (MR) for the yellow rust diseases. Terminal score for stem and yellow rust ranged from 5MRR (resistant) to 90 S (highly susceptible), from (immune) to 40s (susceptible) respectively. The higher AUDPC observed on ETBW9589 (2400) followed by Ogollcho (1965) for stem rust. Higher AUDPC for yellow rust was 637.5 for ETBW9580 genotype.
Generally, the tested genotypes with high AUDPC and CI showed low grain yield. Whereas, wheat genotypes with the lowest values of AUDPC and CI had better grain yield.
Many researchers found that the wheat genotypes with lower values of AUDPC mostly showed the lowest yield loss; while, higher values of AUDPC caused higher grain yield loss [1]. The genotypes that have high level of resistance for both rust diseases will be used for future crossing / breeding in wheat improvement program in Ethiopia as a diseases resistance sources and the other genotypes that are high yielding, resistant to both diseases and good for other important traits will be released as a new varieties, so these nine genotypes are considered as new sources of resistance and after further test best performed genotypes with good disease resistance will be released as a new variety. So the Knowledge of the genetic basis of rust resistance is very essential because it will facilitate the incorporation of resistance genes into high yielding and locally adapted bread wheat cultivars and release new rust resistant varieties for large scale production by end users/ farmers.
This study was undertaken with the objectives of testing the impact of major rust diseases on grain yield of bread wheat genotypes under rainfed conditions. The results of current study indicated that the genotypes had diversity regarding resistance reaction, ranging from complete resistance to susceptible. Most of the evaluated genotypes exhibited moderate resistance (MR) to moderately susceptible (MS) reactions.
Historically, stem rust epidemics have occurred throughout major wheat-producing areas, and the need to control this disease served as a cornerstone to the Green Revolution which led to the introduction of stem rust-resistant wheat varieties. Although stem rust has been well controlled in many parts of the world, forecasting models assuming the absence of durable resistance estimate that global losses would average 6.2 million metric tons per year or higher under severe epidemics The 'Digalu' race caused a devastating epidemic in Ethiopia in 2014 and a similar race has been reported in Germany [10].
For decades, stem rust has been under effective control through the use of genetic resistance. The occurrence and spread of Sr31-virulence races in the Ug99 race group in East Africa and other virulent races causing epidemics and localized outbreaks in Ethiopia, Europe and Central Asia, indicates that the disease is re-emerging as a threat to wheat production. Races in the Ug99 group have been detected across South, East and northern Africa, and the Middle East have the potential to reach critical wheat growing regions in the world [12].
Yellow rust is currently the most economically important wheat rust disease with yield losses reaching 100% in susceptible cultivars. Approximately 88% of the world's wheat varieties are susceptible to yellow rust and global losses inflicted by the disease are nearly US$ 1 billion annually [3,21]. Wheat yellow rust has been reported in more than 60 countries and evidence suggests a significant global geographical expansion of yellow rust in the last 50 years [3].
Many race-specific rust-resistance genes have been defined genetically in wheat and are now being cloned in increasing numbers. As implied, non-race-specific resistance is defined as operating against all races of a pathogen species and is sometimes effective against multiple pathogens. Such resistance is generally quantitative, involving a partial resistance phenotype in which the pathogen growth is slowed without an obvious immune response. In wheat, this resistance is often manifested only at later stages of development and is therefore referred to as adult plant resistance (APR) [13]. More than 150 wheat rust resistance genes have been genetically defined in wheat or wild relatives, most conferring race-specific resistance [9]. At least 50 of these genes are designated Stem rust (Sr) resistance genes that are responsible for reactions to Stem rust [9]. Developing resistant varieties combining both allstage resistance and partial resistance determined by race specific and minor genes, respectively, is a priority research area for breeders in Ethiopia. Varieties with combination of resistance genes could be more durable and more effective than varieties with sole all-stage or adult plant resistance types [2]. As a general from this study variation for yield, yield component and disease resistance for both rusts have been observed so the genotypes that are best for all parameters (yield and yield component disease resistance) will be release as new varieties after further test, but genotypes that exhibited resistance reaction response for disease and showed low grain yield will be used for developing resistance line through crossing.

Conclusion
Durable rust resistance mechanism in wheat is accomplished through incorporation of partially resistant minor genes which seems to be more appropriate and important solution for sustainable wheat production. Response of wheat germplasms along with grain yield and yield components indicated the presence of inverse relation with the disease level. Breeding disease resistance genotypes is a continuous and key process for the plant breeder through pyramiding/adding new effective genes to their breeding materials. The present research deals new sources of resistance that can be incorporated into wheat to escape heavy yield losses wreaked by the two important rust diseases (yellow and stem).
This research` results clearly show that grain yield potential with diseases resistance continues to increase through breeding, High grain yield potential was also successfully combined with high levels of adult plant resistance to wheat rusts. So, the major strategy for the management of rust diseases in Ethiopia would remain focused on the development of resistant varieties. Besides, plant breeder's cooperation with pathologists should be encouraged, appreciated as well as accounted for to continuously monitor rust situation and evolve resistant varieties to ensure food security.