SCREENING OF CIMMYT WHEAT GENOTYPES AGAINST YELLOW RUST IN EGYPT

Article history Received: January 07, 2020 Revised: March 11, 2020 Accepted: April 21, 2020 Yellow (stripe) rust caused by Puccinia striiformis f. sp. tritici, is a serious problem of wheat (Triticum aestivum) production in many parts of the world including Egypt. The pathogen is capable to produce new physiological races that attack resistant varieties and develop epidemic under optimal environmental conditions which results in a serious yield loss. Host resistance is the most economical way to manage wheat stripe rust. Therefore, the present study was conducted to evaluate the reaction of 53 wheat genotypes, delivered to Egypt by International Maize and Wheat Improvement Center (CIMMYT) by artificial inoculation against the major virulent races at adult plant stage at two locations; Itay El-Baroud and Sakha Agricultural Research Stations; during three growing seasons i.e. 2016/17, 2017/18 and 2018/19. Results of the current study showed that 34 wheat genotypes; No. 2, 3, 4, 5, 6, 8, 10, 11, 12, 13, 14, 15, 16, 17, 21, 22, 24, 25, 26, 27, 28, 30, 31, 32, 33, 34, 35, 36, 40, 41, 42, 44, 45 and 48 were resistant and had the lowest values of FRS, ACI, and AUDPC. Therefore, we can select these genotypes as resistant lines in the breeding program for resistance to yellow rust. As for 1000 kernel weight, 10 wheat genotypes i.e. 4, 6, 11, 14, 17, 28, 33, 34, 41 and 48 showed the highest values of 1000 kernel weight and were also resistant to yellow rust. Correlation analysis of different parameters also showed a high correlation between FRS, ACI, RRI and AUDPC with 1000 kernel weight of the tested wheat genotypes. Intensive genetic and molecular studies are useful for developing high yielding and disease resistant wheat cultivars in Egypt.


INTRODUCTION
Wheat is now the most widely cultivated cereal in the world with more than 220 million ha planted annually under wide ranges of climatic conditions and in many geographic regions (Shiferaw et al., 2013). However, enhancing the production is facing many factors i.e. changing of climatic factors requires and biotic stresses Singh et al., 2011) that cause significant yield loss. Among various biotic stresses, three rust diseases i.e. leaf, stem and stripe caused by Puccinia triticina Eriks., P. graminis Pers. f. sp. tritici Eriks. & E. Henn. and P. striiformis Westend. f. sp. tritici) are still the major threats to wheat production globally (McIntosh et al., 1995;Murray and Brennan, 2009). Yield losses due to stripe rust range from 10-70% (Chen, 2005;Ashmawy and Ragab, 2016), moreover, stripe rust can cause 100% yield loss if infection occurs at very early growth stage and the disease continues to develop during the growing season (Afzal et al., 2007). Although the application of recommended fungicides against rust diseases can manage the disease to some extent their use adds to the production costs. Breeding for resistance remains the most effective and efficient management strategy as it does not add input costs to farmers and is environmentally safe (Yang and Daqun, 2004). To date, 80 yellow rust resistance (Yr , s) genes have been permanently named in wheat, including the recently mapped Yr 79 (Feng et al., 2018) and Yr 80 (Nsabiyera et al., 2018), and 67 stripe rust resistance genes have been temporarily designated, including all-stage resistance (also termed seedling resistance) and adult-plant resistance (APR) (Wang and Chen, 2017). Although these Yr genes have been identified in diverse wheat accessions, the race specificity of seedling resistance genes limits their efficacy against pathotypes (Kankwatsa et al., 2017). In contrast, APR is generally considered to be durable, but APR genes represent a minority of known resistance genes (Kankwatsa et al., 2017;Yuan et al., 2018). Therefore, enhancing the resistance of adult plants to cope with evolving races of Pst is the preferred strategy to breeding for resistance. The identification and knowledge of the resistance genes in commonly used parental germplasm and released cultivars is very important for utilizing the genetic resistance to manage yellow rust in full potential. The long term and economical strategy could thus be resistance breeding through the deployment of effective rust resistance genes over space and time (Zeng et al., 2014). The genes expressing at adult plant stage have special significance because the cultivars having such genes have shown partial resistance that has remained effective for longer durations (Khan and Saini, 2009). Resistant wheat germplasm to rust diseases enables the plant breeder to identify broadly adapted genotypes that offer stable performance across a wide range of sites, as well as under specific conditions such as high disease pressure (Yan and Tinker, 2005). This could aid in the development of an optimum breeding strategy for releasing varieties adapted to a target environment (Ahmad et al., 1996). Consequently, the development of resistant varieties will reduce the cost of production and frequency of serious epidemics; this will enhance wheat production in Egypt and other countries. The objectives of this study were to evaluate 53 CIMMYT wheat genotypes for yellow rust resistance at adult plant stage under artificial epiphytotic conditions and therefore, the resistance genotypes can be used for further manipulation in the wheat breeding program by incorporation into adapted cultivars to assess the variability to yellow rust resistance.

MATERIALS AND METHODS Plant Materials Source of wheat genotypes:
A total of 53 wheat genotypes (Table 1) in two sets were provided to Egypt by International Maize and Wheat Improvement Center (CIMMYT), Mexico, through the website (http://www.cimmyt.org/seedrequest/#wheat) including the two wheat varieties; Misr 3 (Egyptian commercial cultivar) and Morocco (check highly susceptible). The two sets of germplasm evaluated included (1) Elite Spring Wheat Yield Trial (ESWYT) and (2) Stress Adaptive Trait Yield Nursery (SATYN), consisting of 100 and 30 entries, respectively. A total of 53 wheat genotypes i.e. 38 genotypes from (ESWYT) and 15 (SATYN) wheat germplasm that was selected from 130 tested wheat genotypes which were selected according to their response to yellow rust. ATTILA*2/ABW65*2/KACHU CMSS06Y00258 2T-099TOPM-099Y-099ZTM-099Y-099M-10WGY-0B-0EGY Morocco -  2016/17, 2017/18 and 2018/19. These experiments were planted in a randomized complete block design (RCBD) with three replicates. The tested wheat genotypes were planted in rows of 3 m long. The experiments were surrounded by a spreader area planted with a mixture of highly susceptible wheat genotypes to yellow rust disease. These genotypes were Triticum spelta sahariensis and Morocco to spread yellow rust inoculum. For field inoculation with yellow rust, the spreader plants were sprayed with a mist of water and dusted with urediniospores of a mixture of most prevalent and aggressive pathotypes i.e. 4E16, 70E20, 70E32 and 192E192 (Ashmawy et al., 2019) mixed with a talcum powder at a ratio of 1 : 20 (v/v) (spores: talcum powder). Plants were dusted in the early evening (at sunset) before dew point formation on the leaves. The inoculation of plants was carried out at the booting stage according to the method of Tervet and Cassell (1951). The urediniospores of yellow rust received from Wheat Diseases Research Department, Plant Pathology Research Institute, Agricultural Research Center, Egypt. To maintain crop vigor normal agronomic practices including recommended fertilization dose and irrigation schedules were followed.

Disease assessment
Yellow rust response of the tested wheat genotypes was characterized using the four epidemiological parameters; final rust severity (FRS %), Average coefficient of infection (ACI), relative resistance index (RRI), and area under disease progress curve (AUDPC). Yellow rust severity (%) which estimated as a percentage of leaf area covered by yellow rust (0% to 100%) (Peterson et al., 1948). Final yellow rust severities were recorded for each genotype when the highly susceptible (check) variety; Morocco was severely rusted and the disease rate reached its maximum level of severity (Das et al., 1993). Plant reaction (infection type) was expressed in five types (Stakman et al., 1962); immune (0), resistant (R), moderately resistant (MR), moderately susceptible (MS) and susceptible (S). The coefficient of infection (CI) was calculated by multiplying rust severity with constant values of infection type (IT). The constant values for infection types were used based on; R = 0.2, MR = 0.4, MS = 0.8 and S = 1 (Stubbs et al., 1986). The average coefficient of infection (ACI) was derived from the sum of CI values of each line divided by the number of locations. After some modifications, a rating scale for disease resistance was adopted in 1982 for use with cereals (Aslam, 1982) based on the scale by Doling (1965) for selecting wheat varieties to powdery mildew. The highest ACI of a candidate line is set at 100 and all other lines are adjusted accordingly. This gives the country an average relative percentage attack (CARPA). Using 0 to 9 scale previously designated as resistance index (RI) has been re-designated as a relative resistance index (RRI). From CARPA the value of RRI is calculated on 0 to 9 scale, where 0 denote most susceptible and 9 highly resistant (Akhtar et al., 2002). The relative resistance index is calculated according to the following formula: The desirable index and acceptable index number for rusts are as below (Aslam, 1982).

Disease
Desirable index Acceptable index Stripe and stem rust 7 and above 6 The area under the disease progress curve (AUDPC) was calculated for each of the tested genotypes by using the equation of Pandey et al. (1989). AUDPC = D [½ (Y1 + Yk) + Y2 + Y3 + …. + Yk-1] Where: D = days between two consecutive records (time intervals) Y1 + Yk = Sum of the first and last disease scores. Y2 + Y3 + …….. + Yk-1 = Sum of all in between disease scores.

Yield assessment
Grain yield expressed as 1000 kernel weight (g) was determined for all of the tested wheat genotypes and calculated following Hassan (2004) in the three growing seasons at the two locations. Randomly selected thousand kernels from each genotype were counted with a seed counter and weighed with an electronic balance to calculate 1000-kernel weight.

Statistical analysis
A combined analysis of variance over the three growing seasons was also carried out ( Table 2). The significance of difference among the studied genotypes was tested by the analysis of variance (ANOVA) test as outlined by Snedecor and Cochran (1967). Mean comparisons for variables were made among genotypes using least significant differences (LSD at 5%) tests.

Association between the four epidemiological parameters and 1000 kernel weight (g)
The association between each of the four Table 7 Continued… epidemiological parameters i.e. FRS (%), ACI, AUDPC and RRI with 1000 kernel weight of the tested wheat genotypes was determined through regression analysis during 2016/17, 2017/18 and 2018/19 growing seasons at the two locations. The significant negative correlation between each of the three epidemiological parameters i.e. FRS (%), ACI and AUDPC with 1000 kernel weight during the three growing seasons. While, significant positive correlation between only RRI with 1000 kernel weight during the three growing seasons (Figure 1, 2, and 3). Regression analysis revealed a significant negative linear relationship between FRS (%) and 1000

DISCUSSION
It is clear that the increased number of infection cycles may lead not only to changes in the intensity and virulence of the extant rust pathotypes but also accelerated the evolution of new rust pathotypes (Chakraborty et al., 2010). The rust pathogens with a high reproductive rate and the ability to spread quickly and evolve new pathotypes rapidly are a major threat to food security (Duveiller et al., 2007). The emergence of the stem rust race Ug99 to which 90% of the wheat varieties grown worldwide are susceptible (Singh et al., 2011) and spread of races of stripe rust virulent on varieties carrying the Yr 27 gene in West and Central Asia (Shiferaw et al., 2013) have accelerated research investment in the identification and transfer of new sources of resistance (Singh et al., 2011;Joshi et al., 2010;James et al., 2008). In Egypt, Yr27 was attacked by race Pst2, v27 (Shahin and Abou, 2015). Past experience of screening and deployment of genes (Singh et al., 2011) warrants searching for additional genes, which confer race non-specific resistance to provide durable control. The horizontal resistance (Van der Plank, 1968) also known as partial resistance or slow rusting (Parlevliet, 1985) has been attributed to minor genes (Browder, 1973). The Lr34/Yr18/Sr57/Pm38 resistance gene, provides adult-plant resistance (APR), slow rusting resistance or partial resistance (PR) to leaf rust, yellow rust, and several other diseases of wheat (Singh et al., 2000;Lagudah et al., 2006;Fahmi et al., 2015;Shahin et al., 2018;Elbasyoni et al., 2019;El-Orabey et al., 2019a;El-Orabey et al., 2019b). Gene banks, conserving a large number of landraces, germplasm and wild relatives Relative resistance index (RRI) D collected from different agro-ecological regions at different points of time provide an opportunity to bioprospect for such genes. Genetic resources fortunately conserved in gene banks around the world carry an assortment of alleles needed for resistance/tolerance to diseases, pests and harsh environments (Hoisington et al., 1999). Conservation of a resource only becomes important if the resource has or acquires recognized value.
We conducted an unprecedented experiment; the first such exercise carried out by any gene bank in the world where the entire germplasm collection of cultivated wheat was evaluated at multiple hotspots to identify potential new sources of rust diseases resistance. Such efforts can aid the ongoing efforts of wheat breeders to develop new varieties or transfer new sources of resistance to broadly-adapted high yielding wheat germplasm lines (for instance the efforts of the Borlaug Global Rust Initiative). The ambitious venture of evaluating nearly 20K wheat germplasm is significant not just by its sheer scale or that it exhibited the utility of the gene banks but it could successfully identify many sources of rusts resistance individually or in the combination that may lead to the development of multiple disease resistant cultivars in the future.
In the present study, 11 wheat genotypes i.e. 28,17,6,33,14,Misr 3,41,4,34,48 and 11 showed the highest values of 1000 kernel weight and were also resistant for yellow rust. These 11 wheat genotypes should be tested for grain yield and other agronomic characters i.e. Days to heading and maturity, plant height (cm), biological yield (kg), straw yield and also flour extraction (%) and rheological properties to be registered as a new commercial cultivar, also, it must identify the yellow rust resistance genes present in these lines by the molecular marker to know the yellow rust resistance genes and the number of genes present in these lines. Finally, the obtained results gave evidence to the presence of positive relation coefficient during the two seasons between ACI and the rest of the tested parameters i.e. least reading AUDPC, RRI and the 1000kernel weight, similar results run in parallel lines with the present one in Egyptian wheat varieties (Shahin, 2014). Degrees of resistance within the tested entries can be used for future manipulation in wheat improvement program in Egypt. These 11 wheat genotypes are considered new sources of resistance under the Egyptian conditions.