Yellow Rust Resistance in Advanced Lines and Commercial Cultivars of Bread Wheat from Ethiopia

Bread wheat (Triticum aestivum L.) cultivars often succumb to yellow rust (Puccinia striiformis f.sp. tritici Westend.) soon after their release for commercial production, especially in the highlands of south-eastern Ethiopia. Variety diversification may buffer the ever evolving new races of the yellow rust pathogen. The objective of this study was to characterize seedling yellow rust resistance in 21 advanced bread wheat lines and 20 cultivars from Ethiopia. Yellow rust infection types (ITs) produced on test wheat lines and cultivars from nine yellow rust races were compared with ITs produced on standard differential lines that differed for specific yellow rust resistance genes. The experiment was conducted at seedling stage under greenhouse conditions in Goettingen, Germany during 2001. The result indicated that most of the advanced bread wheat lines possess different yellow rust resistance genes when compared to the commercial cultivars. Seedling genes Yr1, Yr2, Yr3V, Yr4, Yr6 and Yr17 with or without Yr9 were postulated to be present in 11 advanced lines. However, only Yr7 and Yr9 were postulated to be present in five of the commercial bread wheat cultivars. The newly identified resistance sources could be of great importance for enhancing the genetic base of resistance of bread wheat to yellow rust in Ethiopia.


Introduction
Yellow or stripe rust (Puccinia striiformis f.sp.tritici Westend) is a major wheat disease in the highlands of Ethiopia.It was first reported in the early 1940s, but gained importance with the expansion of high-yielding, semi-dwarf bread wheat cultivars in the mid 1980s (Hailu, 1991).Yellow rust infects the leaf, leaf sheath and spikes of the wheat plant; it can cause yield losses of 96% depending on the susceptibility of the cultivars and environmental conditions (Eshetu, 1986).
Wheat researchers in Ethiopia have been continuously breeding for disease resistance, wide adaptability and high yield, which resulted in the release of many cultivars to farmers.However, most of these cultivars were abandoned from production due to their susceptibility mainly to yellow rust disease (Ayele and Stubbs, 1995).The main reasons for periodic outbreaks of yellow rust disease in Ethiopia are the scarce information on the genetic variation of host-pathogen interactions and unreliability of current sources of resistance to the prevailing race population (Ayele, 2002).Wheat cultivars with high levels of race-specific yellow rust resistance often select virulent races, which result in loss of resistance in these cultivars.
So far, more than 37 yellow rust resistance genes have been reported worldwide and most of these confer seedling resistances (http://www.wheat.pw.usda.gov.).However, virulence genes of the pathogen which overcome many of the seedling resistance genes had already been detected individually or in many combinations (Stubbs, 1988).
In general, research on yellow rust disease should supply wheat genotypes with combinations of effective genes for resistance to the prevailing races of the pathogen.
The gene-for-gene concept (Flor, 1971) is often applied to determine the probable identity of seedling rust resistance genes in wheat cultivars (Wellings et al., 1988;Ayele et al., 1990).Although the sexual stage of the yellow rust fungus has not yet been identified, the race specific yellow rust resistance genes apparently are assumed to conform to the gene-for-gene system (Zadoks, 1961).The low and high ITs produced by a diverse group of yellow rust races on lines under study are compared with ITs produced by the races on yellow rust differential lines that differ for specific resistance genes.Yellow rust races that produce distinct ITs on specific yellow rust resistance genes in the differential line will also produce similar ITs to those cultivars that have the same resistance genes.
In this paper, we report on the yellow rust resistance spectra of advanced bread wheat lines and commercial cultivars from Ethiopia with regard to nine different races at seedling stage.

Yellow Rust Isolates
Two yellow rust isolates from Ethiopia and seven from Germany were used in this study.The isolates from Germany were obtained from Biologische Bundesanstalt für Land und Forstwirtschaft, Braunschweig while the isolates from Ethiopia had originally been collected from southeastern Ethiopia during 1998/99.A mono-pustule was prepared for each isolate and multiplied on the susceptible bread wheat cultivar, 'Morocco', and analyzed on the standard differential sets according to Johnson et al. (1972).

Differential Lines
A total of 21 yellow rust standard differential lines obtained from IPO, Wageningen, The Netherlands were used to characterize the nine yellow rust isolates (Table 1).The 'world-set' and the 'European set' each comprised eight differential lines (Stubbs, 1985).Moreover, Kema and Lange (1992) proposed incorporating Yr15 in the differential set of cultivars through extension of the 'world set', and a similar suggestion for Yr5 by Wellings & McIntosh (1990) was adopted here.In addition, Stubbs (1988) suggested Kalyansona (Yr2) and Federation *4/Kavkaz (Yr9) and Bariana and McIntosh (1994) proposed another differential cultivar, VPM1 (Yr17) to be included as supplemental differential sets for yellow rust.
Table 1.Origin of nine yellow rust races and their resistance factors, which were used to study the resistance spectra of advanced bread wheat lines and commercial cultivars.

Origin
Race code a- World set European set Supplimental set Chinese 166 -Yr1 Johnson et al. (1972).b S= Virulence, R = Avirulence on differential lines

Wheat Genotypes
Twenty-one advanced bread wheat lines and 20 commercial cultivars were used in this study (Table 2).The advanced bread wheat lines were developed through a shuttle breeding program between the Institute of Plant Pathology and Plant Protection, Goettingen, Germany and Kulumsa Research Center, Ethiopia (Solomon, 2001).The commercial cultivars were obtained from the national bread wheat breeding program at Kulumsa.The wheat genotypes and yellow rust differential lines were simultaneously tested to nine individual yellow rust isolates.The experiment was conducted in the Institute of Plant Pathology and Plant Protection, Goettingen, Germany during 2001.

Testing Procedures
From each entry, 7-8 seeds were sown in 5cm 3 jiffy pots that contained a mixture of soil, sand, and compost at the ratio of 1:1:1 v/v/v/ in the greenhouse.Seedling tests were conducted according to Stubbs (1985).About a week old seedlings with fully expanded first leaves were sprayed with a suspension of yellow rust spores with a concentration of 10 5 -10 6 /ml in mineral oil (FC-40, 3M Fluorinert Electronic liquid, Saint Paul, USA) onto the leaves.The treated seedlings were incubated for 24 hours in plastic cages at ca. 10 o C and 100 % relative humidity.Thereafter, the seedlings were transferred to a growth chamber to allow symptom development.Inside the chamber, the day/night regime was 16 hrs of light (18,000 lx) and 8 hrs darkness at 16-17 o C, and the relative humidity was ca.70%.About a week after spraying, 2 g of fertilizer (N:P2O5:K2O=15:11:15) per 100 cc was added to each set of 24 pots, and the second leaves were cut once a week after inoculation to minimize light competition.

Disease Assessment and Data Analyses
Yellow rust assessment was made 16-17 days after spraying using a 0-9 disease-scoring scale (McNeal et al., 1971).Infection types 0-6 were classified as low or resistant while 7-9 scores were considered as high or susceptible infection types (Stubbs, 1985).A race number (code) was given to each isolate according to Johnson et al. (1972) which was based on their reaction on the 'world' sets and 'European sets' of yellow rust differential lines.
A matching technique based on the gene-for-gene concept was applied to characterize seedling yellow rust resistance genes in advanced bread wheat lines and commercial cultivars (Browder, 1971;Wellings et al., 1988).The reaction of each test genotype was compared with the reaction of the differential lines to nine individual races.Wheat genotypes which exhibited similar reaction patterns with that of specific differential lines to a range of isolates were being postulated to possess the same resistance gene(s).
The reaction of advanced bread wheat lines and commercial cultivars to nine yellow rust races is shown in Table 2. Almost all the advanced bread wheat lines exhibited different resistance spectra when compared with that of commercial cultivars.Eleven of the advanced bread wheat lines were resistant to races 230E158 from Ethiopia whereas all the commercial cultivars exhibited susceptible ITs.The aforementioned race was detected in Arsi and Bale during 1998 and overcame the resistance of most of the bread wheat cultivars including, 'Kubsa' (Ayele, 2002).Out of 41 tested entries only 8.3.8 (B) exhibited resistance while four bread wheat cultivars (K6295-4A, ET13, 'Dereselign' and K6290-B) showed susceptible ITs to all nine races employed in this study.However, with the exception 'Derselign', the rest often show low terminal severity when compared to other susceptible cultivars at adult plant growth stage (Ayele, 2002).The bread wheat cultivars HAR1685 ('Kubsa') and HAR604 ('Galama') were susceptible to race 6E22.This race was avirulent on Federation*4/Kavkaz (Yr9) and Clement (Yr9 + ) which indicated that the aforementioned resistance gene may be absent in the two widely grown bread wheat cultivars.
The yellow rust resistance genes were postulated in some of advanced bread wheat lines and commercial cultivars (Table 3).Two of the advanced lines 11.5.39-2 and 11.6.21were susceptible to 237E141, 232E137, 169E136, 109E141, 104E141 and 41E168 but resistant to the rest.The six races showed virulence for Vilmorin-23 which might indicate the presence of Yr3V in the above two genotypes.In East Africa, virulent races for Yr1 and Yr3V have rarely been reported (Stubbs, 1988;Danial and Stubbs, 1992).
This race was different from the others because of its combined virulence on the differential line VPM1 (Yr17).Thus, resistance gene Yr17 was proposed in the three bread wheat genotypes.Two other genotypes 11.4.16-2 and 11.5.26-2 were susceptible to races 237E141 and 232E137 while exhibited resistance to the rest.Therefore, Yr4 + (Hybrid 46) and Yr9 + (Clement) were postulated in the aforementioned genotypes.Virulence for Yr9 has commonly been reported but the two resistance genes Yr4 + and Yr17 are effective with regard to the prevailing races in East Africa (Danial and Stubbs, 1992;Ayele and Stubbs, 1995).Yr9 was first introduced into bread wheat from Rye through 1B/1R translocations (Zeller, 1973) and is common in CIMMYT-originated bread wheat cultivars (van Ginkel and Rajaram, 1993).However, virulence for Yr17 was detected recently in Europe (Bayles and Stigwood, 2001).The resistance gene was formerly introduced from Triticum ventricosa to bread wheat (Bariana and McIntosh, 1994).
Two bread wheat genotypes 8.2.7 and 11.2.23 showed susceptible reaction to 237E141, 169E136 and 109E141 but exhibited resistance to the rest.The three races had common virulences for Federation*4/KVZ (Yr9) and chinese166 (Yr1).Thus, Yr9 and Yr1 genes were postulated in the two genotypes.
Five commercial bread wheat cultivars, HAR416, TUSIE (HAR 1407), BATU, DASHEN and GARA, exhibited similar resistance spectra.They showed low infection types to all eight races but were susceptible to 230E158.The eight races lack combined virulences for Yr9 + (Clement) and Yr7 + (Reichersberg 42).Therefore, the above resistance genes might confer yellow rust resistance in the five bread wheat cultivars.
In general, it is not advisable to release bread wheat genotypes with Yr9 + , Yr7 + , Yr6 + , and Yr2 + unless they possess additional resistance genes conferring adult plant resistance.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).Such type of resistance, which slows disease development, has often been reported in the yellow rustwheat system (Ma and Singh, 1996).
Table 3. Postulated yellow rust resistance genes in 11 bread wheat lines and five commercial cultivars after inoculation with nine races at seedling stage.
This study revealed that most of the advanced bread wheat lines contained different resistance spectra when compared to the commercial cultivars which appeared to lack genetic variability for resistance to the current virulent yellow rust race from Ethiopia at seedling stage.To cope with the build up of virulences, the genetic bases of resistance to yellow rust should be broadened and diversified.The advanced bread wheat lines have been identified as important sources of resistance to the prevailing races of yellow rust in Ethiopia.

Acknowledgement
The study was financed by 'Bread for the World', Stuttgart, Germany.

Table 2 .
The reaction of advanced bread wheat lines and commercial cultivars to nine yellow rust races at seedling stage.