Resistance has been an environmentally friendly and proven means of controlling stem rust for more than a century; the problem is that resistance has not been durable. A collection of 100 wheat landrace accessions from the Institute of Agri-Biotechnology and Genetic Resources, NARC-Islamabad, was tested at the seedling stage for response to 11 of Pgt races with multiple virulences, including TTKSK (from Kenya), TRTTF (Yemen), TTTTF (USA), and RRTTF (Pakistan). Six accessions were resistant (IT 0; to 2) to race TTKSK, 11 to race RRTTF, and 9 to races TRTTF and TTTTF. Further tests with US races QTHJC and TPMKC indicated that the majority of these landraces were susceptible. The resistant landraces could be used as donor parents in crossing programs to broaden the genetic base for stem rust resistance in Pakistani wheat varieties.
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The Lr34 resistance gene from Triticum aestivum encodes a putative ABC transporter protein that confers broad spectrum, partial adult plant resistance to all three rusts species and powdery mildew. It has remained a durable source of resistance for over 100 years in which time no increased virulence towards Lr34 has been observed. This gene is located on chromosome 7D and consequently cannot be readily transferred to durum wheat by traditional breeding. A transgenic approach was used to transfer Lr34 to durum wheat cultivar Stewart by Agrobacterium transformation. Homozygous progeny from a number of independent Stewart lines expressing Lr34 under regulatory control of its endogenous promoter showed high levels of rust resistance at the seedling stage. A correlation between seedling resistance and transgene expression levels was observed in these plants. In contrast seedlings from a near isogenic line of hexaploid wheat cultivar Thatcher containing Lr34 showed only a minor difference in rust growth when compared with Thatcher seedlings, typical of this adult plant resistance gene in hexaploid wheat. Little is known about how the Lr34 gene product functions; however, the seedling resistance of these durum transgenics enables functional assays to be readily undertaken without the need for adult plant material. By infecting seedlings we have shown that day length has an effect on Lr34 resistance to leaf rust, with higher levels of resistance observed under long days (16 h light) compared with short days (8 h light). This study demonstrates that Lr34 provides strong and presumably durable seedling resistance to rust in durum plants that can be used to further understand how this gene confers resistance.
Resistance is an effective and economic method of controlling wheat stem rust provided the resistance gene(s) remains effective. We studied the stem rust responses of 372 synthetic hexaploid wheats (SHW) originally from CIMMYT under natural field infection conditions at Kelardasht, a stem rust hotspot. Stem rust severities were recorded using the modified Cobb scale prior to harvest ripeness. Seventy six and 17% of the genotypes (93% in total) were scored susceptible or moderately susceptible, whereas only 5 and 2% (7% in total) were moderately resistant or resistant, respectively. Based on our results, 26% of the genotypes were considered to have resistance, displaying field responses of 40MS to 20MSS. Those SHWs resistant to local Pgt races at Kelardasht could be used as sources of resistance in Iranian wheat breeding programs. Race analysis of Pgt isolates from the trials and seedling response tests on the entire SHW set are in progress.
Wheat is an economically important food crop in South Africa and production is influenced by a number of diseases caused by fungal pathogens, especially leaf rust, stem rust, stripe rust and Fusarium head blight. The aim of the study is to combine durable rust and FHB resistances into a single wheat line with a high percentage of cv. Krokodil genetic background. Two sets of wheat lines respectively resistant to the three rusts or FHB were developed from different breeding programs at the University of the Free State. These lines were used as parents to combine durable rust and FHB resistance genes/QTL into a single line. Three of the best rust resistant lines were selected as female parents containing rust resistance genes/QTL Lr19, Lr34/Yr18/Sr57, Sr2, Sr26, Sr39 and QYr.sgi-2B.1. FHB resistant BC2F2 and BC2F6 lines were selected as male parents; these lines contained different combinations of Fhb1, Qfhs.ifa-5A-1 and Qfhs.ifa-5A-2. All parental lines were evaluated using molecular markers to confirm the presence of the expected genes/QTL. More than 100 crosses were made between the rust and FHB resistant parents. Since the parents were not homozygous for all markers, leaf material from six-week-old F1 seedlings was collected for marker-assisted selection and to identify the best plants with combined rust and FHB resistances. The best selected lines will be use to develop a backcross population using cv. Krokodil as the recurrent parent. Lines with different combinations of resistance genes/QTL are currently being evaluated in the field to confirm the presence of these genes/QTL.
Landraces and wild relatives of wheat are rich repositories of new rust resistance genes. Landraces are preferred over wild relatives for the absence of deleterious effects associated with large alien segments. A common wheat landrace, AUS 27969 (ex Portugal), from the Watkins Collection was resistant under field conditions and produced seedling infection type (IT) 2C against the widely virulent Australian Puccinia striiformis f. sp. tritici (Pst) pathotype 134 E16 A+ Yr17+ Yr27+. AUS 27969 was crossed with the susceptible genotype Avocet S (AvS) and the distribution of F3 lines conformed to monogenic segregation [40 non-segregating resistant (NSR), 93 segregating (Seg), and 37 non-segregating susceptible (NSS); ?2 = 1.61, P2d.f. >0.05] when tested with the same pathotype at the seedling stage. The population is currently being selfed to F6. DNA from NSR and NSS lines will be sent for high throughput analysis to identify the genomic region carrying the resistance gene. Resistance-linked SNPs will be mapped on the F6 RIL population. The resistance gene will be backcrossed into modern Australian wheat backgrounds.
Yellow rust is the most dangerous of the wheat rusts worldwide. Disease management involves breeding and fungicide application, with the former being more cost effective and environmentally acceptable. Despite the release of numerous yellow rust resistant cultivars in many countries, new aggressive strains inevitably overcome the resistances in a zigzag or ‘boom and bust’manner. For example, Chamran (Attila-50-Y), released in 1997 in Iran, immediately became the most popular cultivar nationally. In 2012-2013, a new aggressive Pst strain overcame the resistance in Chamran as well as Vee/Nac (an early maturity line suitable for the wheat-maize cropping system) in Khuzestan, a major wheat-producing region in the southwest of the country. Evaluations of wheat germplasm at the Safiabad Agricultural Research Center (North Khuzestan) identified 17 completely or partially resistant lines. Pedigree analyses of resistant lines identified Batavia, Genaro 81, Opata, Pastor, Trap and Yaco as possible sources of resistance. Genotype information of these cultivars obtained from the http://wheatpedigree.net/ database indicated the presence of Yr33, Yr30+Yr18 and Yr31 in Batavia, Opata and Pastor, respectively. Genaro 81, Trap and Yaco carry Yr18. Currently, F2 populations of 34 crosses of 17 resistant lines to locally adapted cultivars Chamran and Vee/Nac are undergoing field selection in a nursery inoculated with the 2012-2013 aggressive race. The progenies of selected plants will undergo further testing and selected homozygous F3 lines will be genotyped for markers associated with Yr18 (Xgwm295-7D), Yr30 (flanking markers Xgwm533.1 - Xgwm493-3B), Yr31 (Xgwm630/Xgwm374-2B (Lr13/Lr23)) and Yr33 (flanking markers Xgwm111 - Xgwm437-7D).
High-temperature adult-plant (HTAP) resistance to stripe rust in wheat has proven to be durable. Molecular markers tightly linked to HTAP resistance offer an alternative to phenotypic selection and are useful for pyramiding HTAP resistance genes with other types of resistance. This study assessed HTAP resistance in six diverse mapping populations derived from four types of crosses: 1. Resistant × Susceptible, IDO444 × Rio Blanco (winter wheat RILs) and UI Silver × Shaan 89150 (winter wheat DH lines); 2. Moderately Resistant × Resistant, UI Platinum × SY Capstone (spring wheat DH lines) and UI Stone × IDO686 (spring wheat RILs); 3. Moderately Resistant × Moderately Resistant, UI Stone × Alturas (spring wheat RILs); and 4. Moderately Susceptible × Moderately Susceptible, IDO835 × Moreland (winter wheat, DHLs). Eight QTL significantly associated with HTAP resistance were earlier identified in the IDO444 x Rio Blanco RIL population, including three major QTL (QYrid.ui-2B.1, QYrid.ui-2B.2, and QYrid.ui-4A) and five minor QTL (QYrid.ui-1A, QYrid.ui-3B.1, QYrid.ui-3B.2, QYrid.ui-4B, and QYrid.ui-5B) (Chen et al. 2012, Mol Breeding 29:791–800). These QTL are being validated and novel QTL are being identified in the other five populations. The current study used elite × elite crosses; therefore, the identified QTL may have application in selecting lines with combinations of stripe rust resistance and other superior agronomic traits and perhaps for release as new cultivars.
Yellow rust is a widely distributed wheat disease, that is more damaging in cooler, temperate regions. Epidemics have increased worldwide due to spread of aggressive high temperature tolerant strains PstS1/S2 that reached North Africa and southern France in 2004 and the widely virulent exotic Warrior race that spread in Western Europe in 2011. Resistant varieties are effective solutions to reduce the use of pesticides. However, races of the pathogen quickly overcome resistance genes. Therefore, selection of varieties with durable resistance to yellow rust is paramount for protection of both bread wheat and durum. To conduct a genetic control strategy, it is essential to study the pathotype dynamics and the resistance genes in wheat. We identified the pathotypes using the European and world differential sets that discriminate between 23 avirulence/virulence factors as well as simple sequence repeat (SSR) diversity among 20 Pst isolates collected in Tunisia in 2014. In addition, we postulated resistance genes in 28 Tunisian varieties and accessions at the seedling stage in order to identify the resistance diversity. Race 239 E175V17 was involved in the 2014 epidemic in Tunisia. Genetic analysis revealed that this race is exotic and distinct from the Northwestern European and Mediterranean groups, previously present in Tunisia. Resistance gene postulation indicated the presence of Yr3, Yr6, Yr7, Yr9+Yr4, and Yr25 in Tunisian varieties and accessions. Durum varieties Khiar and Salim, and bread wheat variety Tahent, were resistant to the local Northwestern European and Western Mediterranean pathotypes as well as the Warrior race. These varieties are thus short-term measures to address the yellow rust problem in Tunisia. Gene identifications will be confirmed by molecular and pedigree analyses of the accessions.Yellow rust is a widely distributed wheat disease, that is more damaging in cooler, temperate regions. Epidemics have increased worldwide due to spread of aggressive high temperature tolerant strains PstS1/S2 that reached North Africa and southern France in 2004 and the widely virulent exotic Warrior race that spread in Western Europe in 2011. Resistant varieties are effective solutions to reduce the use of pesticides. However, races of the pathogen quickly overcome resistance genes. Therefore, selection of varieties with durable resistance to yellow rust is paramount for protection of both bread wheat and durum. To conduct a genetic control strategy, it is essential to study the pathotype dynamics and the resistance genes in wheat. We identified the pathotypes using the European and world differential sets that discriminate between 23 avirulence/virulence factors as well as simple sequence repeat (SSR) diversity among 20 Pst isolates collected in Tunisia in 2014. In addition, we postulated resistance genes in 28 Tunisian varieties and accessions at the seedling stage in order to identify the resistance diversity. Race 239 E175V17 was involved in the 2014 epidemic in Tunisia. Genetic analysis revealed that this race is exotic and distinct from the Northwestern European and Mediterranean groups, previously present in Tunisia. Resistance gene postulation indicated the presence of Yr3, Yr6, Yr7, Yr9+Yr4, and Yr25 in Tunisian varieties and accessions. Durum varieties Khiar and Salim, and bread wheat variety Tahent, were resistant to the local Northwestern European and Western Mediterranean pathotypes as well as the Warrior race. These varieties are thus short-term measures to address the yellow rust problem in Tunisia. Gene identifications will be confirmed by molecular and pedigree analyses of the accessions.
Stem rust resistance genes Sr39 (RL6082) and Sr36 (Cook) were transferred from Aegilops speltoides and Triticum timopheevi to chromosome 2B of wheat. Both genes are located on large translocated segments. Genotypes carrying Sr36 and Sr39 produce infection types (ITs) 0; and 2, respectively, against avirulent pathotypes. This investigation was planned to study the genetic relationship between these genes with the aim of combining them in a single genotype. Seedling tests on RL6082/Cook F3 lines showed complete repulsion linkage [25 Sr39Sr39sr36sr36 (IT2-) : 53 Sr39sr39Sr36sr36 (IT2-, IT0;) : 13 sr39sr39Sr36Sr36 (IT 0;)], and preferential transmission of the Ae. speltoides segment over the T. timopheevi segment was evident from the segregation ratio. The Sr39-carrying translocation was shortened by Niu et al. (2011; Genetics 187: 1011-1021) and the genetic stock carrying the shortest segment was named RWG1. Based on the reported location of Sr39 in the smaller alien segment in RWG1, we predicted that it should recombine with Sr36. F3 lines derived RWG1/Cook were phenotyped for stem rust response at the two-leaf stage and again complete repulsion linkage between Sr39 and Sr36 was observed [23 Sr39Sr39sr36sr36 (IT2-) : 78 Sr39sr39Sr36sr36 (IT0;, IT2-) : 68 sr39sr39Sr36Sr36 (IT 0;)]. In contrast to the cross involving the large Sr39 translocation, preferential transmission of the T. timopheevi segment was observed. These results indicated that a genetic determinant of meiotic drive had been deleted in the shortened Ae. speltoides segment. Genotyping with the co-dominant STS marker rwgs28 matched the phenotypic classification of F3 families. Marker rwgs28 was diagnostic for the Ae. speltoides segment, but the rwgs28 allele amplified in Cook was not T. timopheevi-specific.
China is the largest stripe rust epidemic area in the world. Central Shaanxi, as an important stripe rust overwintering zone for the disease serves as a “bridge” for the pathogen, where early sown wheat infected during the previous autumn provides inoculum for spring epidemics in more eastern regions. Studies of resistance and Yr-gene distribution among local candidate cultivars provide valuable insights into the influence of host genotype on selection of the rust pathogen population. A total of 183 local advanced lines from 2009 to 2011 were tested for seedling resistance with 12 Pst races in the greenhouse, and with mixed races at Tianshui in Gansu province. Gene postulations were based on the seedling response data and molecular markers. Four (2.2%) entries were resistant at all growth stages; 15 (8.2%) were resistant as adult plants; 164 (89.6 %) were susceptible to one or more races at the seedling or adult stages; and 40 were resistant to the currently prevalent races CYR32 and/or CYR33, but susceptible to at least one of the potentially important races Su11-4, Su11-5 and Su11-7, V26/CM42 and V26/Gui22. All entries showed seedling stage susceptibility at Tianshui. Postulated genes included Yr7, Yr9, Yr10, Yr17, Yr18, and Yr24/Yr26. Yr5, Yr15 and Yr61, currently effective against all Chinese races, were not present. Although advanced wheat lines bred in Shaanxi may be diverse our results show that most of them are highly susceptible to one or more prevalent or low frequency races in Shaanxi or adjacent Gansu. This situation indicates that Shaanxi farmers should be using partial adult plant resistances to reduce inoculum levels and hence reduce the amount of primary inoculum spread to more easterly wheat growing areas.