Wheat Stem Rust Race Research Articles

Wheat Stem Rust Detection and Race Characterization in Tunisia.

Climate changes over the past 25 years have led to conducive conditions for invasive and transboundary fungal disease occurrence, including the re-emergence of wheat stem rust disease, caused by Puccinia graminis f.sp. tritici (Pgt) in East Africa, Europe, and the Mediterranean basin. Since 2018, sporadic infections have been observed in Tunisia. In this study, we investigated Pgt occurrence at major Tunisian wheat growing areas. Pgt monitoring, assessment, and sampling from planted trap nurseries at five different locations over two years (2021 and 2022) revealed the predominance of three races, namely TTRTF (Clade III-B), TKKTF (Clade IV-F), and TKTTF (Clade IV-B). Clade III-B was the most prevalent in 2021 as it was detected at all locations, while in 2022 Pgt was only reported at Beja and Jendouba, with the prevalence of Clade IV-B. The low levels of disease incidence during these two years and Pgt population diversity suggest that this fungus most likely originated from exotic incursions and that climate factors could have caused disease establishment in Tunisia. Further evaluation under the artificial disease pressure of Tunisian wheat varieties and weather-based modeling for early disease detection in the Mediterranean area could be helpful in monitoring and predicting wheat stem rust emergence and epidemics.

Genetic characterization and genome-wide association mapping for stem rust resistance in spring bread wheat

BackgroundEmerging wheat stem rust races have become a major threat to global wheat production. Finding additional loci responsible for resistance to these races and incorporating them into currently cultivated varieties is the most economic and environmentally sound strategy to combat this problem. Thus, this study was aimed at characterizing the genetic diversity and identifying the genetic loci conferring resistance to the stem rust of wheat. To accomplish this, 245 elite lines introduced from the International Center for Agricultural Research in the Dry Areas (ICARDA) were evaluated under natural stem rust pressure in the field at the Debre Zeit Agricultural Research Center, Ethiopia. The single nucleotide polymorphisms (SNP) marker data was retrieved from a 15 K SNP wheat array. A mixed linear model was used to investigate the association between SNP markers and the best linear unbiased prediction (BLUP) values of the stem rust coefficient of infection (CI).ResultsPhenotypic analysis revealed that 46% of the lines had a coefficient of infection (CI) in a range of 0 to 19. Genome-wide average values of 0.38, 0.20, and 0.71 were recorded for Nei’s gene diversity, polymorphism information content, and major allele frequency, respectively. A total of 46 marker-trait associations (MTAs) encompassed within eleven quantitative trait loci (QTL) were detected on chromosomes 1B, 3A, 3B, 4A, 4B, and 5A for CI. Two major QTLs with –log10 (p) ≥ 4 (EWYP1B.1 and EWYP1B.2) were discovered on chromosome 1B.ConclusionsThis study identified several novel markers associated with stem rust resistance in wheat with the potential to facilitate durable rust resistance development through marker-assisted selection. It is recommended that the resistant wheat genotypes identified in this study be used in the national wheat breeding programs to improve stem rust resistance.

Virulence analysis of wheat stem rust (Puccinia graminis f. sp. tritici) in Horro Guduru Wollega, Western Ethiopia

AbstractStem rust caused by Puccinia graminis f. sp. tritici is one of Ethiopia's most serious wheat diseases. Because of mutation and genetic recombination, stem rust can rapidly evolve new virulence to resistant genes. The Ethiopian highlands are well‐known hotspots for the rapid emergence and spread of new wheat stem rust races. The objective of this study was to determine the virulence diversity and spatial distribution of Pgt races in the main wheat‐growing districts, Horro Guduru Wollega. Wheat stem rust samples were collected from Abay Chomen, Amuru, Guduru, Hababo Guduru, Horro, Jardaga Jarte, Jimma Ganati and Jimma Rare districts during the main cropping seasons of 2015 and 2016. In the study, 147 stem rust isolates were derived from 121 viable samples. The physiological races of stem rust were determined on seedlings of standard wheat stem rust differentials using the International System of nomenclature. Twelve (12) physiological races of stem rust were identified. TKTTF, TTKSK, JRCQC, TRTTF and TTTTF were the most common in all districts with a frequency of 53.1%, 15.6%, 14.3%, 6.8% and 2.7%, respectively. TKTTF and TTKSK, two highly virulent races, were found in all districts. Four stem rust resistance genes (Sr11, Sr31, Sr36 and SrTmp) confer resistance to the majority of the virulent races identified. Sr24 was effective against all virulent races in this study but ineffective against the Ug99 variant (race TTKTT) detected in the country. The findings could be helpful for breeders to select many effective resistance gene combinations to provide more durable stem rust resistance.

Distribution, dynamics, and physiological races of wheat stem rust (Puccinia graminis f.sp. tritici) on irrigated wheat in the Awash River Basin of Ethiopia.

Wheat is one of the high-value major crops in the world. However, wheat stem rust is considered one of the determinant threats to wheat production in Ethiopia and the world. So this study was conducted to assess the disease intensity, seasonal distribution dynamics pattern, the genetic variability of Puccinia graminis f. sp. tritici, and to determine the virulence spectrum in the irrigated ecology of the Awash River Basin. Totally 137 wheat farms were evaluated, from 2014/15-2019/20 in six districts representing the Upper, Middle, and Lower Awash River Basin. Farm plots were assessed, in every 5-10 km intervals, with 'X' fashion, and data on disease incidence, severity, healthy plants were counted and recorded. Diseased samples were collected from the diseased wheat stem by Puccinia graminis physiological and genetic race analysis. The seasonal trend of stem rust disease progress showed its importance to infer the future progresses of the disease for the country's potential production plan of irrigated wheat. The result revealed that the disease prevalence, disease incidence, and severity were significantly varied; among the different districts and seasons in the two regions. The survey results also indicated that about 71.7% of the wheat fields were affected by stem rust during the 2018/19 growing period. The disease's overall incidence and mean severity during the same season were 49.02% and 29.27%, respectively. In 2019/20, about 63.7% of the wheat fields were affected by stem rust, disease incidence 30.97%, and severity 17.22% were lower than the previous season. In 2019/20, even though seasonal disease distribution decreased, the spatial distribution was expanding in Afambo and Dubti districts. Four, stem rust dominant races were identified (TTTTF, TKTTF TKKTF, and TTKTF) by physiological and genetic race analysis during 2018/19 and one additional race (TKPTF) in 2019/20, production year. The result indicated that the races are highly virulent and affect most Sr genes except Sr31 and Sr24. From the race analysis result, TTTTF, and TKKTF have the broadest virulence spectrum race, which affects 90% of the Sr genes. Generally, we can conclude that the spatial and seasonal distribution of the disease is expanding. Most of the races in the irrigated areas in the Basin were similar to that of rain-fed wheat production belts in Ethiopia, so care must be given, to effective management of the diseases, in both production ecologies towards controlling the spore pressure than race variability. Therefore, these findings provide inputs for wheat producers to reduce the spread and disease' damage in the irrigated ecologies of Ethiopia. Also, it gives an insight for breeders to think about the breeding program in their crossing lines.

Wheat Stem Rust Races in Sinop, Turkey

Stem rust caused by Puccinia graminis f. sp. tritici is an important disease of wheat in the world. This disease is also common in the Sinop province of Turkey. The pathogen forms new races and these races may overcome the resistance present in wheat cultivars. For sustainable wheat production, identification of the stem races is necessary. During 2016 and 2017, surveys were conducted in wheat-growing areas of Sinop province and from diseased plants, 42 single uredospore pustule isolates were obtained. Using North American differential genotypes stem rust races were identified. The most common race was TTTTF followed by TTKTF, RTKTF, TTKTC, and RTTTF.

Distribution and Physiological Races of Wheat Stem Rust (Puccinia graminis F. Sp tritici) in North and East Shoa Zones of Ethiopia

Wheat (Triticum aestivumL.) is the most important staple crop in temperate zones and is in increasing demand in countries undergoing urbanization and industrialization. However, its production is affected by many biotic and abiotic factors. Among biotic production constraints; wheat stem rust ( Puccinia graminis f.sp tritici) is the most important one. This study was (i) to assess the importance of wheat stem rust in North and East Shoa zones of central Ethiopia and (ii) to identify physiological races. Purposive multistage sampling was used to select major wheat growing districts and farmers associations from each zone. Wheat stem rust race identification was carried out via inoculation of isolates on susceptible line (McNair); single pustule isolation; inoculation on standard differential sets and infection type evaluation of each line fourteen days after inoculation. One hundred fifty wheat fields (75 from each zone) were assessed. Wheat stem rust was observed in 71 (94.7%) and 52 (73.3%) of wheat fields in East and North Shoa zones, respectively. Disease incidence and severity were significantly different (p < 0.0001) between the two zones. Six physiological races of Puccinia graminis f.sp tritici (pgt) namely; TKTTF, TTTTF, TKKTF, TTKTT, TTKTF and TTTTT were identified. TKKTF was the dominant race which was detected from 40 (48.2%) samples followed by TKTTF (Digelu race) which was identified from 28 (33.7%) samples. But, TTTTT, TTKTT and TTTTF were less frequent races. They were identified from 1 (1.2%), 2 (2.4%) and 4 (4.8%) samples, respectively. The majority of resistance genes in differential host lines (80-100%) were defeated with the races. Resistance genes Sr24 and Sr31 were effective to majority of races identified. Hence, they can be used as source of resistance in breeding program.

Evaluation of Wheat Genotypes for a Single Stem Rust Race TTTTF in Ethiopia

Stem rust caused by <i>Puccinia graminis</i> is one of the most destructive diseases of wheat which causing considerable yield losses in wheat-growing areas in Ethiopia. Disease-resistant cultivars provide one of the best means for controlling stem rust. The current study aimed to evaluate wheat genotypes for seedling and adult plant stage resistance for the predominant wheat stem rust races; TTTTF. Both seedling and adult plant stage evaluations were conducted on 203 wheat genotypes under greenhouse and field conditions. Seedling and adult plant stage evaluations were conducted using artificial inoculation of the race TTTTF. Augmented and completely randomized designs were used for field and seedling tests, respectively. Three susceptible and two resistance check genotypes were used for field evaluation for each block. Seedling evaluation results showed 85% of the genotypes exhibited resistance infection type (<2). Field phenotyping results revealed 72%, 22% and 6% of the genotypes showed resistance (<15%), moderately resistance (20-35%) and susceptible (>40) reaction for stem rust race TTTTF. From the ANOVA table, highly significant variation between test and check genotypes was observed for different disease parameters and TKW. High and positive correlations existed between different disease parameters and a negative correlation was observed between TKW and disease parameters. Based on field and seedling evaluation, a total of 151 genotypes were selected for future breeding pipelines. Therefore, resistant wheat genotypes identified in the current study will be a potential source of genotypes for further wheat breeding and pathological research activities.

Assessments and perspectives on stem rust resistance in South African malting barley

The production of malting barley under dryland and irrigated environments in South Africa (SA) has increased recently, making it the second most important winter crop after wheat. To assess the potential vulnerability of adapted barleys from SA to wheat stem rust (caused by Puccinia graminis f. sp. tritici, Pgt, race PTKST) and rye stem rust (Puccinia graminis f. sp. secalis, Pgs, isolate UVPgs1), 50 accessions were evaluated as seedlings and as adult plants. The same barley germplasm was tested against Pgt and Pgs cultures from the USA that have proven useful for detecting the stem rust resistance genes Rpg1 and the rpg4/Rpg5 complex. The possible presence of these resistance genes was assayed using molecular markers and sequence analysis. Barley accessions exhibited mostly resistant infection types (ITs) to the South African cultures, whilst field data revealed that several of the barley varieties displayed high severities in combination with more compatible host response types to race PTKST. The barley accessions exhibited mostly high ITs to cultures from the USA. None of the seedling IT data was sufficiently distinct to reliably postulate the presence of possible resistance genes. This result was corroborated from the molecular assays for Rpg1 and rpg4/Rpg5. Except for one line that tested heterozygous for the Rpg1 gene, none of the test entries contained this gene or a functional rpg4/Rpg5 complex due to the prevalence of mutations in the rpg4/Rpg5 + carriers. Susceptibility among barley varieties to wheat stem rust race PTKST under field conditions in the present study is of concern.

Dissection of Cell Death Induction by Wheat Stem Rust Resistance Protein Sr35 and Its Matching Effector AvrSr35.

Nucleotide-binding leucine-rich repeat receptors (NLRs) are the most abundant type of immune receptors in plants and can trigger a rapid cell-death (hypersensitive) response upon sensing pathogens. We previously cloned the wheat NLR Sr35, which encodes a coiled-coil (CC) NLR that confers resistance to the virulent wheat stem rust race Ug99. Here, we investigated Sr35 signaling after Agrobacterium-mediated transient expression in Nicotiana benthamiana. Expression of Sr35 in N. benthamiana leaves triggered a mild cell-death response, which is enhanced in the autoactive mutant Sr35 D503V. The N-terminal tagging of Sr35 with green fluorescent protein (GFP) blocked the induction of cell death, whereas a C-terminal GFP tag did not. No domain truncations of Sr35 generated cell-death responses as strong as the wild type, but a truncation including the NB-ARC (nucleotide binding adaptor) shared by APAF-1, R proteins, and CED-4 domains in combination with the D503V autoactive mutation triggered cell death. In addition, coexpression of Sr35 with the matching pathogen effector protein AvrSr35 resulted in robust cell death and electrolyte leakage levels that were similar to autoactive Sr35 and significantly higher than Sr35 alone. Coexpression of Sr35-CC-NB-ARC and AvrSr35 did not induce cell death, confirming the importance of the leucine-rich repeat (LRR) domain for AvrSr35 recognition. These findings were confirmed through Agrobacterium-mediated transient expression in barley. Taken together, these results implicate the CC-NB-ARC domains of Sr35 in inducing cell death and the LRR domain in AvrSr35 recognition.

First Report of Puccinia graminis f. sp. tritici Race PTKSK, a Variant of Wheat Stem Rust Race Ug99, in South Africa

First Report of <i>Puccinia graminis</i> f. sp. <i>tritici</i> Race PTKSK, a Variant of Wheat Stem Rust Race Ug99, in South Africa

Sr33 and Sr35 gene homolog identification in genomes of cereals related to Aegilops tauschii and Triticum monococcum

Using bioinformatics analysis, the homologs of genes Sr33 and Sr35 were identified in the genomes of Triticum aestivum, Hordeum vulgare, and Triticum urartu. It is known that these genes confer resistance to highly virulent wheat stem rust races (Ug99). To identify amino acid sites important for this resistance, the found homologs were compared with the Sr33 and Sr35 protein sequences. It was found that sequences S5DMA6 and E9P785 are the closest homologs of protein RGAle, a Sr33 gene product, and sequences M7YFA9 (CNL-C) and F2E9R2 are homologs of protein CNL9, a Sr35 gene product. It is assumed that the homologs of genes Sr33 and Sr35, which were obtained from the wild relatives of wheat and barley, can confer resistance to various forms of stem rust and can be used in the future breeding programs aimed at improvement of national wheat varieties.

Genetic Characterization of Resistance to Wheat Stem Rust Race TTKSK in Landrace and Wild Barley Accessions Identifies the rpg4/Rpg5 Locus.

Race TTKSK of the wheat stem rust pathogen (Puccinia graminis f. sp. tritici) threatens the production of wheat and barley worldwide because of its broad-spectrum virulence on many widely grown cultivars. Sources of resistance against race TTKSK were recently identified in several barley landraces (Hordeum vulgare subsp. vulgare) and wild barley accessions (H. vulgare subsp. spontaneum). The objectives of this study were to characterize the inheritance of resistance to wheat stem rust race TTKSK in four barley landraces (Hv501, Hv545, Hv602, and Hv612) and two wild barley (WBDC213 and WBDC345) accessions, map the resistance genes, and determine the allelic relationships among the genes in these accessions and the previously described rpg4/Rpg5 locus. Resistant accessions were crossed with the susceptible cv. Steptoe and resulting F3 populations were evaluated for resistance to race TTKSK at the seedling stage. Segregation of F3 families in populations involving the resistance sources of Hv501, Hv545, Hv612, WBDC213, and WBDC345 fit a 1:2:1 ratio for homozygous resistant (HR)/segregating (SEG)/homozygous susceptible (HS) progenies (with χ2=2.27 to 5.87 and P=0.053 to 0.321), indicating that a single gene confers resistance to race TTKSK. Segregation of F3 families in cross Steptoe/Hv602 did not fit a 1:2:1 ratio (HR/SEG/HS of 20:47:43 with χ2=11.95 and P=0.003), indicating that more than one gene is involved in imparting resistance to race TTKSK. Bulked segregant analysis using >1,500 single-nucleotide polymorphism markers positioned a resistance locus in all six populations on chromosome 5HL in very close proximity to the known location of the rpg4/Rpg5 complex locus. Allelism tests were conducted by making crosses among resistant accessions Hv501, Hv545, and Hv612 and also Q21861 with the rpg4/Rpg5 complex. No segregation was observed in F2 families inoculated with race TTKSK, demonstrating that all Hv lines carry the same allele for resistance and that it resides at or very near the rpg4/Rpg5 locus. Phenotype evaluations of the six barley accessions with wheat stem rust race QCCJ revealed resistant infection types (ITs) at a low incubation temperature and susceptible ITs at a high incubation temperature, similar to Q21861, which carries the temperature-sensitive gene rpg4. The accessions also exhibited low ITs against the rye stem rust isolate 92-MN-90, suggesting that they also carry Rpg5. This result was confirmed through molecular analysis, which revealed that all six barley accessions contain the serine threonine protein kinase domain that confers Rpg5 resistance. These results indicate that cultivated barley is extremely vulnerable to African stem rust races such as TTKSK because even these diverse selections of landrace and wild barley accessions carry only one locus for resistance.

Assessment of Ethiopian Wheat Lines for Slow Rusting Resistance to Stem Rust of Wheat Caused by Puccinia graminis f.sp. tritici

AbstractThe emergence and rapid spread of virulent races of wheat stem rust has driven a search for sources of resistance for durable resistance breeding. This study was carried out to identify possible sources of stem rust resistance between Ethiopian wheat lines. Two hundred and fifty‐two wheat accessions and a universal suscept, cultivar Morocco were evaluated for their resistance at the seedling stage to the stem rust isolate Ug99 in a controlled environment. Ninety‐one lines that exhibited intermediate and susceptible seedling reactions were further field tested in 2012 main season for their slow rusting characteristics. Among the 91, 38 genotypes that had high to moderate level of slow rusting were advanced to a 2013 off season field evaluation. Slow rusting resistance at the adult‐plant stage was assessed through the determination of final disease severity (FRS), coefficient of infection (CI), and relative area under disease progressive curve (rAUDPC). The results revealed that wheat lines H04‐2, 204408‐3, 214551‐1, 231545‐1, 7041‐1, 7514‐1, 226385‐1, 226815‐1, 7579‐1 and 222495‐1 had low values of FRS, CI and rAUDPC and were regarded as good slow rusting lines. Of these 231545‐1, 7041‐1, 226815‐1 and 7579‐1 exhibited complete susceptibility at the seedling stage, with infection types ranging from 3− to 3+, which suggests that they possess true slow rusting resistance. Lines 237886‐1, 227059‐1, 203763‐1, 226275‐1, 227068‐2, 226278‐1 and 7994‐1 had moderate values for the stem rust resistance parameters and were identified as possessing a moderate level of slow rusting. High correlations were observed between different parameters of slow rusting. Among the slow rusting lines 231545‐1, H04‐2 and 222495‐1 had high yields and kernel weight in both seasons. The slow rusting lines identified from this study can be used to breed for stem rust resistance in wheat.

The rpg4-Mediated Resistance to Wheat Stem Rust (Puccinia graminis) in Barley (Hordeum vulgare) Requires Rpg5, a Second NBS-LRR Gene, and an Actin Depolymerization Factor

The rpg4 gene confers recessive resistance to several races of wheat stem rust (Puccinia graminis f. sp. tritici) and Rpg5 provides dominant resistance against isolates of the rye stem rust (P. graminis f. sp. secalis) in barley. The rpg4 and Rpg5 genes are tightly linked on chromosome 5H, and positional cloning using high-resolution populations clearly separated the genes, unambiguously identifying Rpg5; however, the identity of rpg4 remained unclear. High-resolution genotyping of critical recombinants at the rpg4/Rpg5 locus, designated here as rpg4-mediated resistance locus (RMRL) delimited two distinct yet tightly linked loci required for resistance, designated as RMRL1 and RMRL2. Utilizing virus-induced gene silencing, each gene at RMRL1, i.e., HvRga1 (a nucleotide-binding site leucine-rich repeat [NBS-LRR] domain gene), Rpg5 (an NBS-LRR-protein kinase domain gene), and HvAdf3 (an actin depolymerizing factor-like gene), was individually silenced followed by inoculation with P. graminis f. sp. tritici race QCCJ. Silencing each gene changed the reaction type from incompatible to compatible, indicating that all three genes are required for rpg4-mediated resistance. This stem rust resistance mechanism in barley follows the emerging theme of unrelated pairs of genetically linked NBS-LRR genes required for specific pathogen recognition and resistance. It also appears that actin cytoskeleton dynamics may play an important role in determining resistance against several races of stem rust in barley.

Sr31-Virulent Races (TTKSK, TTKST, and TTTSK) of the Wheat Stem Rust Pathogen Puccinia graminis f. sp. tritici are Present in Tanzania.

Since the first detection of race TTKSK (syn. Ug99) in Uganda in 1999 (2), the migration and evolution of Sr31-virulent races of Puccinia graminis f. sp. tritici [Pgt] have been closely monitored, particularly in Kenya and countries north, along the likely trajectory of migration to major wheat-producing regions of Asia. More recently, surveillance efforts have been undertaken to the south as well, and Ug99-related races have been detected in South Africa and Zimbabwe (3,4). Here we report for the first time results of a survey conducted in Tanzania. Systematic race surveillance provides data not only on the current distribution of the Ug99 race group, but also on the possible points of origin as well as the pace and probable paths of dispersal of future races from the region. In this context, the presence or absence of the Ug99 group of wheat stem rust races in adjacent countries like Tanzania assumes regional, and possibly global, relevance. A preliminary survey conducted in September 2006 indicated the presence of Sr31-virulent races of Pgt outside Slahhamo Village (3°15'S, 35°48'E) in the Ngorongoro highlands of northern Tanzania, based on compatible reactions with cv. K-Mamba (a.k.a. Mwamba), a cultivar whose pedigree indicates the presence of Sr31. A broader survey was conducted in August 2009, during which infected tissue was collected from currently-grown cultivars in research plots and on large estates, as well as from the mixes of older cultivars common on smallholder farms. In all, Pgt-infected samples were collected from one site in the Arusha region [Monduli (3°16' S, 36°24'E)], three sites in the Ngorongoro highlands [Karatu (3°20' S, 35°40' E), Upper Kitete (3°14' S, 35°53' E), and Slahhamo], one site in the Manyara region [Hanang (4°43' S, 35°40' E)], and one site in the Mbeya region [southern highlands (8°87' S, 33°40' E)], thereby giving representation to all four major wheat growing areas in the country. Sample storage, inoculation, incubation, disease assessment, and derivation of single-pustule cultures were all performed according to the methods described by Jin et al. (1). In addition to the 20 differentials in the expanded Pgt differential set of North America, we included two supplemental tester lines: Siouxland (Sr24 + Sr31) and Sisson (Sr31 + Sr36). Each single-pustule-derived isolate was evaluated for virulence on the differential and supplemental lines at least twice. A total of 39 single-pustule isolates were derived from the six collection sites. All 39 isolates were identified as belonging to the Ug99 race group, with six identified as TTKSK (all four regions), 30 identified as TTKST (Sr31 + Sr24 virulence; Arusha region and the Ngorongoro highlands), and three identified as TTTSK (Sr31 + Sr36 virulence; Manyara region and the Ngorongoro highlands). The results of this study suggest that, to more precisely locate the "hot spots" and thereby gain a better understanding of the mechanisms of novel race emergence in East Africa, it would be prudent to include Tanzania, heretofore a blank area on the wheat rust surveillance map, in future systematic race monitoring efforts.

Yield Evaluation of a Wheat Line with Combined Resistance to Russian Wheat Aphid and Stem Rust Race “Ug99” in Kenya

InKenya, Russian wheat aphid (RWA) and stem rust race TTKS (“Ug99”) are the most devastating pests of wheat. Severe infestations by RWA result in yield losses of up to 90% while epidemics of “Ug99”can cause up to 100% loss. The two pests combined have seriously affected farmer incomes forcing them to rely heavily on pesticides and increasing the cost of production. This study sought to evaluate a wheat line that has been developed to be resistant to both RWA and “Ug99”by pyramiding two major resistance genes. Three varieties were used in this study: “Kwale”, a Kenyan high yielding commercial variety but susceptible to both RWA and “Ug99”; “Cook”, an Australian variety carrying stem rust resistance gene Sr36 conferring immunity to “Ug99”; and “KRWA9”, a Kenyan line resistant to RWA but with poor agronomic attributes. The F1 of the double cross (DC F1) was obtained by crossing the F1 of “Kwale × Cook” and the F1 of “Kwale × KRWA9”. The DC F1 population was subjected to sequential screening for both RWA and “Ug99”resistance. The surviving DC F1 progenies were left to self pollinate in the field to obtain the DC F2. The DC F2 progenies were sequentially screened against RWA and “Ug99”to obtain a resistant population to both RWA and “Ug99”. The yield and yield components of the new resistant line were compared with the three parents. Results showed that the DC F2:3 had higher yields than the three parents based on 1000 kernel weight, weight of kernel per spike, and the actual yield in tons/ha, indicating that the genes were successfully introgressed. It is concluded that though races with virulence for Sr36 have been reported, the gene provides immunity to race “Ug99”and can be used as a component for “Ug99”resistance breeding together with other Sr genes.

Development and Characterization of Wheat Germplasm with Combined Resistance to Russian Wheat Aphid and Stem Rust (Race “Ug99”) in Kenya

Wheat is the second most important cereal in Kenya. However, production is severely constrained by both abiotic and biotic stresses. Of the biotic stresses a devastating pest (Russian wheat aphid (RWA)) and a serious disease (stem rust race TTKS (“Ug99”)) are currently the biggest problem for wheat producers in Kenya. Severe infestations by RWA may result in yield losses of up to 90% while “Ug99” infected fields may suffer 100% crop loss. The two pests combined are seriously affecting wheat farmers’ incomes because of the heavy reliance on pesticides that increase the cost of production. This study attempted to develop and characterize wheat lines that are resistant to both RWA and “Ug99” by pyramiding two major resistance genes. Three wheat varieties: “Kwale”, a Kenyan high yielding variety but susceptible to both RWA and “Ug99”; “Cook”, an Australian variety carrying stem rust resistance gene Sr36 conferring immunity to “Ug99”; and “KRWA9”, a Kenyan line with resistance to RWA but of poor agronomic attributes were used. A double cross F1 (DC F1) was obtained by crossing the F1 of “Kwale × Cook” and the F1 of “Kwale × KRWA9”. The DC F1 population was subjected to sequential screening for both RWA and “Ug99” resistance. Surviving DC F1 progenies were left to self pollinate to obtain the F2 of the double cross (DC F2). The DC F2 progenies were sequentially screened against RWA and “Ug99” to yield a population that was resistant to both RWA and “Ug99”. Genotyping of the DC F2:3 families were conducted to select homozygous resistant plants. Data indicated that the RWA and “Ug99” resistance genes were successfully pyramided. Though races with virulence for Sr36 have been reported, the gene provides immunity to race “Ug99” and can still be effectively used as a component for “Ug99” resistance breeding together with other Sr genes.

Haplotype analysis of molecular markers linked to stem rust resistance genes in Ethiopian improved durum wheat varieties and tetraploid wheat landraces

The recent emergence of wheat stem rust race Ug99 (TTKSK) and related strains threaten Ethiopian as well as world wheat production because they overcome widely used resistance genes that had been effective for many years. The major cause which aggravates the ineffectiveness of Ethiopian wheat varieties against stem rust is the narrow genetic base on which the breeding for resistance has been founded, however, little is known about the resistance genotypes of Ethiopian durum wheat varieties and tetraploid wheat landraces. The objective of the study was to identify stem rust resistance genes that are present in the Ethiopian tetraploid wheat landraces and improved durum wheat varieties using molecular markers and assess which genes are effective for current Ethiopian stem rust races of Puccinia graminis f. sp. tritici including Ug99. The investigated 58 tetraploid wheat accessions consisted of 32 (Triticum durum s.l. incl. Triticum aethiopicum Jakubz., Triticum polonicum) landraces and 22 registered T. durum varieties released in Ethiopia between 1966 and 2009 and four T. durum varieties from ICARDA. A total of 17 molecular markers (SSR, EST and InDel) linked or diagnostic for stem rust resistance genes Sr2, Sr13, Sr22 and Sr35 were used for genotyping. Haplotype analysis indicated that only few of the Ethiopian durum wheat varieties carried Sr13. The resistant variety ‘Sebatel’ showed a haplotype for Sr2 and Sr22 and variety ‘Boohai’ for Sr22, however further evaluation is needed for the diagnostic value of these haplotypes. This study is the first report on the presence of stem rust resistance (Sr) genes in Ethiopian durum wheat varieties and tetraploid wheat landraces based on linked or associated molecular markers. Thus it might help in the identification of varieties carrying resistant alleles that provide valuable genetic material for the development of new improved varieties in further breeding programmes.

Distribution and Physiologic Races of Wheat Stem Rust in Tigray, Ethiopia

Wheat stem rust caused by Puccinia graminis f. sp. tritici is amongst the biotic catastrophic diseases because of its ability to cause complete annihilation of wheat crops during epidemic years. The highland of Ethiopia is considered as a hot spot for the development of stem rust complex. Hence, this study was conducted to investigate the distribution and intensity; to detect the virulence diversity of the pathogen in South Tigray. The findings of this paper were based on stem rust surveys to compute the intensity; race analysis by inoculating isolates on to the 20 differential hosts. During the survey, 66 wheat fields were examined in 2010, of which 33.3% were affected. The overall mean incidence and severity of the disease were 15.6 and 8.5%, respectively. A total of 20 races were identified from 32 isolates, which included the most prevalent races TTSNK, RRJJC and HRJJC. Most of the the genes possessed by the differentials were ineffective against one or more of the tested isolates except Sr24. Sr genes 24 and Tmp were effective in 100 and 90% of the races, respectively. In contrast, SrMcN and Sr9b were ineffective to 96.9 and 93.8% of the isolates tested, respectively. Thus, use of effective Sr genes singly or incombination with other genes through gene pyramiding has paramount importance as the additive effects of several genes offer the cultivar a wider base stem rust resistance along with periodic race survey to track further virulence evolution.

Sources of Resistance to Stem Rust Race Ug99 in Spring Wheat Germplasm.

Wheat stem rust (Puccinia graminis f. sp. tritici) race TTKSK (Ug99), with virulence to the majority of the world's wheat (Triticum aestivum) cultivars, has spread from Uganda throughout eastern Africa, Yemen, and Iran. The identification and spread of variants of race TTKSK with virulence to additional stem rust resistance genes has reminded breeders and pathologists of the danger of deploying major resistance genes alone. In order to protect wheat from this rapidly spreading and adapting pathogen, multiple resistance genes are needed, preferably from improved germplasm. Preliminary screening of over 700 spring wheat breeding lines and cultivars developed at least 20 years ago identified 88 accessions with field resistance to Ug99. We included these resistant accessions in the stem rust screening nursery in Njoro, Kenya for two additional seasons. The accessions were also screened with a bulk of North American isolates of P. graminis f. sp. tritici in the field in St. Paul, MN. In order to further characterize the resistance in these accessions, we obtained seedling phenotypes for 10 races of P. graminis f. sp. tritici, including two races from the race TTKSK complex. This phenotyping led to the identification of accessions with either adult-plant or all-stage resistance to race TTKSK, and often North American races of P. graminis f. sp. tritici as well. These Ug99 resistant accessions can be obtained by breeders and introgressed into current breeding germplasm.