Asian Journal of Applied Sciences

Year: 2020 | Volume: 13 | Issue: 2 | Page No.: 76-83
DOI: 10.3923/ajaps.2020.76.83

Assessment of Sources of Adult Plant Resistance Genes to Stem Rust in Ethiopian Durum Wheat Genotypes

Abebech Desalegn Ayalew, Mesfin Kebede Gessese and Gobeze Loha Yada

Abstract: Background and Objective: Stem rust is a devastating disease of bread wheat and durum wheat in the major wheat-growing regions of the world. Particularly, the stem rust race identified as Ug99 and its mutants initially emerged in Uganda in 1999 had crossed borders of neighboring countries in Africa, Middle East and Asia has become a major threat to the world wheat industry. Therefore, the present study was conducted in the greenhouse and field to assess sources of durable resistance to stem rust. Materials and Methods: Fifteen durum wheat genotypes and a susceptible cultivar ‘Morocco’ were evaluated in the greenhouse and field at Debre Zeit Agricultural Research Center, Ethiopia, in order to detect the presence of effective stem rust resistance genes. A mixture of three dominant races of Puccinia graminis f. sp. tritici (TKTTF, TTKSK and JRCQC) was used for inoculation. The field experiment was conducted using Random Complete Block (RCB) design with three replications at two different locations. Results: Phenotyping of the genotypes at seedling stage in the greenhouse showed four genotypes (Ginchi, Quami, DW-#3 and DW-#11) that carried effective All Stage Resistance (ASR) genes; however, the rest 11 genotypes showed susceptible reaction. On the other hand, the field assessment of the genotypes for stem rust resistance showed presence of varied levels of field resistance. The combined results from both seedling reaction test and field experiments indicated that the 11 genotypes might possess one or more Adult Plant Resistance (APR) genes to stem rust of wheat. Conclusion: The 11 genotypes that possessed APR genes can be good sources of durable stem rust resistance genes to be incorporated in the Ethiopian durum wheat improvement program.

Keywords: stem rust, Adult plant resistance, seedling resistance, durum wheat and physiological races

INTRODUCTION

Wheat (Triticum spp.) is considered as the earliest domesticated cereal crop and currently the most important agricultural product of the world. It is one of the most important cereal crops in the world in terms of cultivation area and amount of produce. According to FAOSTAT¹, 729 million Mt of wheat produced in the world. The major producing countries in the world were European Union, China, India, USA and France in that order. In 2015/16 cropping season during main and minor cropping seasons the average yield of wheat in Ethiopia² was 2608 kg ha^–1.

There are many known wild and cultivated species of wheat in the genus Triticum. However, the principal wheats of commercial importance are T. aestivum L. and T. durum Desf.³. Durum wheat (T. durum Desf.) is the predominant tetraploid species that constitutes nearly 10% of wheat production in the world and approximately 30% in Ethiopia^3-5.

In Ethiopia, the most suitable altitudes for wheat production are between 1900-2700 m above sea level⁶. Despite the large area of cultivation under wheat, average yield in Ethiopia is below the world average⁷. The low yield of wheat in Ethiopia is attributed to a number of factors which encompass soil fertility, weeds, moisture stress and pests of which disease is rapidly spreading fungal diseases causing epidemics that require urgent actions⁸. Stem rust is a potentially devastating fungal disease that can kill wheat plants and small grain cereals but more typically reduces foliage, root growth and grain yields⁹. Epidemics of stem rust could cause a loss^10,11 of up to 100%. Temesgen et al.¹² reported that an outbreak of stem rust epidemics which occurred in Arsi and Bale regions caused 67-100% loss on commercial durum wheat cultivars. The main reason for such a disaster was the continuous release of cultivars with major gene (race-specific) resistance¹³. Since race-specific resistance usually overcome through emergence of new races of virulence in the pathogen population, durable resistance is of great interest to wheat breeders^14,15.

Stem rust of wheat caused by the pathogen Puccinia graminis f. sp. tritici (Pgt) has become an important disease of wheat in the major wheat producing regions of Ethiopia. Hence, use of resistant cultivars particularly of Adult Plant Resistance (APR) genes is the most effective, sustainable and environment friendly way of managing rust diseases of wheat due to its durable nature of resistance¹⁶. Presence of a single or couple of APR genes in a cultivar may not provide sufficient resistance levels in a high disease pressure area. However, cultivars with high levels of resistance were developed by pyramiding 3-5 APR genes^17-21. In Ethiopia wheat varieties are becoming vulnerable to stem rust epidemics largely due to the use of varieties with race specific major gene resistance developed materials. A case in point could be the emergence of the new race Ug99 in Uganda in 1999²² which later appeared in Kenya and Ethiopia in 2005 that broke the resistance of the most widely deployed seedling resistance gene Sr31 after decades of control of the pathogen²⁰. The Ug99 has brought a major anxiety in the world wheat production as majority (>90%) of the world’s commercial wheat varieties became susceptible to it²³. Hence, this necessitates the need to identify sources of race non-specific adult plant resistance germplasm to be incorporated in the wheat breeding scheme. Thus, this study was initiated to assess sources of resistance to stem rust in the durum wheat genotypes.

MATERIALS AND METHODS

Description of experimental site: Field experiments were conducted during July-December, 2017 main cropping season at Debre Zeit Agricultural Research Center, Ethiopia in two different testing sites. The sites are located within the range of approximate geographical coordinates of 8° 44"N latitude and 38°57"E longitude with altitude range of 1900-1950 m above sea level. The average annual rainfall of the area is 851 mm and the soil type of the site is eutric vertisol (87.74%) and haplic andosols and vectric andosols constituting 5.94% each, respectively. The average minimum and maximum annual temperatures of the study sites are 11.23 and 25.19°C, respectively²⁴.

Plant materials: Fifteen durum wheat genotypes and four bread wheat genotypes including ‘Morocco’ were used as sources of plant materials in this study. The four bread wheat genotypes were used as spreaders or susceptible cultivars to facilitate infections and also as standard for susceptibility in scoring at greenhouse and field studies. Details of the plant materials are described in Table 1.

Pathogen materials: An equal proportion of the mixture of currently dominant stem rust races in the field (TKTTF, TTKSK and JRCQC) was used as source of inoculums to evaluate the durum wheat genotypes both in the greenhouse and field.

Greenhouse seedling evaluation for stem rust resistance: Phenotyping of the 15 durum wheat genotypes and one susceptible check (Morocco) was conducted to detect the presence of effective seedling resistance genes to stem rust.

Ten seeds of each wheat variety and a susceptible check were planted in plastic pots containing soil, compost and sand in the ratio of 2:1:1, respectively using RCBD (Randomized Complete Block Design) with 3 replications. After 7 days of planting, the seedlings were inoculated with mixture of the currently dominant stem rust races in the field (TKTTF, TTKSK and JRCQC) using standard procedure for inoculation of seedlings as described in previous studies²⁵. To create artificial dew and facilitate spore germination, water was sprayed using sprayer. Twenty minutes after inoculation they were placed in dew chamber in dark room (incubation room) covered with polythene plastic sheets for 24 h at 18-22°C. Upon removal from chamber, seedlings were exposed to 3 h of fluorescent light to dry dew on the leaves. Following this, the seedlings were transferred to the greenhouse microclimate rooms where conditions were regulated at 12 h photoperiod, at temperature range of ±25°C and RH of 60 -70%.

Scoring of the Infection Types (IT) commenced two weeks after inoculations (12-15 days) using 0-4 scale as described in previous literature²⁶. Where, ‘0’ = Immune, ‘.’ (flack) = Practically immune, ‘1’ = Very resistant, ‘2’ = Moderately resistant to resistant, ‘3’ = Moderately susceptible and ‘4’ = completely susceptible.

Field evaluation for adult plant resistance to stem rust: The fifteen durum wheat genotypes and a composite of 3 susceptible check cultivars; PBW343, Digelu and Arendato (used as a rust infector plants) were planted using randomized complete block design with three replications in two different experimental sites. The field plot size was 1.2×2.5 m where each experimental plot consisted of six rows. The spacing between blocks was 1 m and the spacing between plots and spreader row and within blocks was 0.5 m each, respectively and seeds were drilled in rows at spacing of 20 cm with seed rate of 150 kg ha^–1.

Experimental plots were fertilized with Diammonium phosphate (DAP) fertilizer at rate of 100 kg ha^–1 at planting as source of P (phosphorus). Urea was used as source of N (nitrogen) at rate of 150 kg ha^–1 and applied in splits where the first half at planting and the remaining half a month after planting. All crop management practices such as; cultivation, weeding etc., carried out as desired. Two rows of infector plants (susceptible varieties) were planted across the borders and between the replications. After 30 days of planting the infector plants were inoculated with the spore mixtures of the stem rust races.

Disease severity recording in the field commenced after establishment of the rust in the infector rows. Recording of rust severity was made using the modified Cobb’s scale^27-29 where, 0% = Immune and 100% = Completely susceptible. The field assessment of stem rust data recording done six times from each experimental plot randomly starting from booting stage until the crop attains its physiological maturity. The Average Coefficient of Infection (ACI) was calculated by multiplying the severity data obtained for each genotype and the constant value assigned for host response as described earlier²⁹. The area under disease progress curve was computed using the formula developed³⁰ as described:

Where:

Data analysis
Analysis of variance: The data for disease parameters at two sites were subjected to analysis of variance using GenStat 16th edition statistical software package (VSN International Ltd, London, UK) following the procedures described in previous studies³¹. The differences between treatment means was compared using Least Significant Difference (LSD) test at 5% level of significance when the ANOVA showed the presence of significant difference between genotypes.

RESULTS

Results from the experiments entailing assessment of slow rusting resistance genes for stem rust studies between durum wheat genotypes were conducted both in greenhouse and field conditions. The greenhouse seedling test and field assessment data for stem rust resistance has been subjected to Analysis of Variance (ANOVA) and demonstrated highly significant difference between genotypes for both (ASR and APR) types of genetic resistance.

Seedling reaction test: Results from the greenhouse experiment showed that the durum wheat genotypes varied in their reaction. To confirm the results of the seedling tests, the greenhouse experiment was repeated three times following the same procedure and the result was similar. The seedling test data ranged between infection types ‘1’ and ‘4’. Summary of the greenhouse experiment data is presented in Table 2.

Genotypes possessing only adult plant resistance character (genes) showed intermediate (3) or fully susceptible (4) reaction in the seedling tests. Based on the 0-4 scoring scale, only two cultivars (Ginchi and Quami) showed resistance reactions IT 1 consistently, the two promising genotypes (DW-NVT-OHMA-16/17-set-1-#11 and DW-NVT-OHMA-16/17-set-1-#3) showed resistance to moderately resistance reaction IT 2, five genotypes (Boohai, Denbi, Hitosa, Tesfaye and Yerer) showed moderately susceptible reaction IT 3, the rest six genotypes (Alemtena, Asasa, Mangudo, Mekuye, Ude and Utuba) showed susceptible reaction IT 4 to stem rust that is comparable to the standard susceptible check cultivar ‘Morocco’ (Table 2).

Field experiment: The field assessment data for stem rust resistance has been subjected to Analysis of Variance (ANOVA) and demonstrated highly significant difference between genotypes in both locations. The Analysis of Variance (ANOVA) for AUDPC showed highly significant (p<0.01) differences among genotypes and the Analysis of Variance (ANOVA) for all Average Coefficient of Infection (ACI) showed highly significant (p<0.01) differences and the disease severity data showed significant difference at both locations (Appendix 1).

This indicated the presence of sufficient genetic variability for the level of resistance/susceptibility among the genotypes investigated. Adult plant resistant genotypes were identified on the bases of their Area under Stem Rust Progress Curve (AUDPC), disease severity and average coefficient of infection at the two locations in addition to their susceptible disease reaction at seedling stage (IT 3+ to 4) and relatively resistance reactions observed at adult stage in the field (Table 2, Appendix 2, 3).

Area under stem rust progress curve and average coefficient of infection: At black soil the disease severity for the durum wheat genotypes showed moderately resistance disease reaction except two genotypes (Tesfaye and Ginchi) that showed moderately resistance to moderately susceptible (MR-MS) reaction (Appendix 3) whereas, the area under Disease Progress Curve (AUDPC) for all 15 genotypes showed moderately resistance reaction (Table 2).

At light soil, Three genotypes (Asasa, Tesfaye and DW-NVT-OHMA-16/17-set-1-#11) showed MR-MS reaction while the remaining genotypes demonstrated resistance to moderately resistance reaction (Appendix 4). On the other hand for disease severity, four of the genotypes (Boohai, Yerer, Tesfaye and DW-NVT-OHMA-16/17-set-1-#3) showed MR-MS reaction while majority of the genotypes showed resistance to moderately resistance reaction (Table 2). Average coefficient of infection showed moderately resistance reaction for all fifteen genotypes at both locations (Appendix 3 and 4).

The highest values of both AUDPC (83 in black soil and 69 in Light soil) and disease severity (100 in both black soil and 70 in light soil) was recorded on the spreader plots that were constituted from susceptible genotypes ‘Digelu’, ‘PBW343’ and ‘Arendato’ (Table 2, Appendix 3, 4). Whereas, disease severity of the experimental treatments (15 durum wheat genotypes) was less compared with the susceptible genotypes since the highest corresponding values of both AUDPC (33 in black soil and 38 in light soil) and disease severity (38 in black soil and 45 in light soil) were much less than half of the values observed for the susceptible genotypes (Table 2, Appendix 3, 4).

DISCUSSION

The present study was conducted to assess durable sources of adult plant resistance of durum wheat genotypes. The results from seedling reaction test revealed that four genotypes; Ginchi, Quami, DW-NVT-OHMA-16/17-set-I-#3 and DW-NVT-OHMA-16/17-set-I #11 carried effective ASR genes to Ug99 and its variants with ITs 1, 1+, 2 and 2, respectively. Five genotypes (Boohai, Denbi, Hitosa, Tesfaye and Yerer) showed moderately susceptible reaction IT 3; the rest six genotypes (Alemtena, Asasa, Mangudo, Mekuye, Ude and Utuba) showed susceptible reaction IT 4 to stem rust. The greenhouse and field evaluation data together showed that 11 genotypes (Alemtena, Yerer, Asasa, Denbi, Hitosa, Mangudo, Mekuye, Ude, Boohai, Tesfaye and Utuba) had source of only adult plant resistance character to stem rust since they showed intermediate to susceptible seedling reactions to stem rust races and comparably low AUDPC values. Debebe³² and Aida³³ reported that selection of genotypes having low AUDPC values with terminal disease score of less than 20S is normally accepted for practical purposes where the aim is to utilize slow rusting resistance as one of the durable resistance strategies. Therefore, these result indicated that all these 15 durum wheat genotypes carried resistance genes to stem rust effective under field conditions (Table 2).

Several studies showed that genotypes carrying only slow rusting resistance genes or APR genes are usually susceptible at the seedling stage (devoid of effective seedling resistance genes) but become resistant as the plant matures^34,35. Therefore, these 11 genotypes (Alemtena, Yerer, Asasa, Denbi, Hitosa, Mangudo, Mekuye, Ude, Boohai, Tesfaye and Utuba) may possess more than two adult plant resistance genes since their field assessment results confirmed their adult plant resistance character (Table 2, Appendix 3, 4). Significant number of findings^17-21 indicated that presence of a single or couple of APR genes in a cultivar may not provide sufficient resistance levels in high disease pressure areas, however, they mentioned that cultivars with high levels of resistance were developed by pyramiding 3-5 APR genes. Durable resistance can be explained that a consistent resistance reaction of a plant displayed across locations/environments for several years of cultivation under favorable condition to a disease development³⁶. Durable resistance to rusts can be achieved through a combination of both APR and ASR genes deployed to a single commercial cultivar^20,21. Since durable resistance is mostly associated with APR or slow rusting genes characterized by susceptible response to seedling tests, it is therefore, important to have seedling reaction test to identify adult plant resistance character.

In general, it is possible to surmise that those genotypes that exhibited field resistance to stem rust at both locations but with seedling reactions ranged from 3-4 lacks effective ASR genes^37,38, hence, they can be good sources of APR genes. Therefore, these genotypes have to be selected as donor parent for incorporating durable resistance in durum wheat improvement program. For effective and precise breeding outcome knowledge of identity of the APR genes present in these genotypes is essential; hence, genotyping/screening of these 11 genotypes with the already known molecular markers of the APR genes; Sr2, Sr55, Sr56, Sr57 and Sr58 is imperative. The outcome of these studies could be used as a preliminary source of information to develop high yielding stem rust resistant durum wheat cultivars for future breeding program particularly for durable resistance wheat breeding through gene pyramiding approaches using molecular marker assisted selection.

CONCLUSION

This study depicted that 11 genotypes; Alemtena, Yerer, Asasa, Denbi, Hitosa, Mangudo, Mekuye, Ude, Boohai, Tesfaye and Utuba had source of only adult plant resistance character to stem rust. Therefore, these genotypes have to be selected as donor parent for incorporating durable resistance in durum wheat improvement program.

SIGNIFICANCE STATEMENT

This study discovered that the 11 durum wheat genotypes possessed acceptable level of field resistance to stem rust composed of adult plant resistance genes. On the other hand, the known pleiotropic APR genes are originated in bread wheat, this study might indicate the presence of other (uncharacterized) APR genes in durum wheat germplasm because the level of resistance displayed showed possession of at least three APR genes in each of the cultivars investigated.

ACKNOWLEDGMENTS

The first author thanks Ministry of Education, Federal Government of Ethiopia for sponsoring her postgraduate study scholarship and Debre-Zeit agricultural Research Center for hosting the greenhouse and field experiments. We thank Mr. Wassihun and Mr. Ashenafi of Debre-Zeit agricultural research center for providing seeds of durum wheat genotypes.

REFERENCES